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C – Site preparation and resistance to contaminants and moisture

Contents

The Approved Documents are ‘standard’ ways of getting Building Regulations approval. There are other ways. see more

If you follow the principles and rules given in the documents you can be sure that they will be approved. Of course you don’t have to use the Approved Documents, as the Regulations make clear:

Approved Documents are intended to provide guidance for some of the more common building situations. However, there may well be alternative ways of achieving compliance with the requirements. Thus there is no obligation to adopt any particular solution contained in an Approved Document if you prefer to meet the relevant requirement in some other way.

In fact the Approved Documents are a bit of a mishmash of traditional ‘rules of thumb’ and technical standards and they have gaping holes in them. Whereas, for instance, there are many pages on how to construct traditional masonry walls, there is nothing about timber frame construction except the odd reference to British standards.

They are also struggling to keep up with the times. If the Passivhaus Standard or zero energy new house building is introduced at some point then whole swathes of the Approved Documents will need rewriting because many of the constructional principles are based on quite low levels of insulation.

There are other ways of satisfying the regulations. For instance they make copious references to BS, BS EN and BS EN ISO standards which may be another way of fulfilling the criteria. It is even possible in some very rare cases to prove that something works by building it first and then testing it afterwards (though this is not for the faint hearted).

Although a self builder cannot be expected to understand all the building regulations, it often pays to have a grasp of what is involved, especially if last minutes changes need to be made to construction details.

The full official set of Approved Documents is available HERE.

The Approved document C relating to Site preparation and resistance to contaminants and moisture is available HERE

Below is an edited copy of Approved Document, C1 and C2 : Site preparation and resistance to contaminants and moisture with notes for self builders. The purpose is to draw attention to the main aspects of the document. If in doubt check the full Approved Document C.


Definitions

0.12   The following meanings apply to terms throughout this Approved Document:
Building and land associated with the building.The building and all the land forming the site subject to building operations which includes land under the building and the land around it which may have an effect on the building or its users (see also paragraph 2.11).
Contaminant. Any substance13 which is or may become harmful to persons or buildings, including substances which are corrosive, explosive, flammable, radioactive or toxic.
Floor. Lower horizontal surface of any space in a building including finishes that are laid as part of the permanent construction.
Groundwater. Water in liquid form, either as a static water table or flowing through the ground.
Interstitial condensation. Deposition of liquid water from a vapour, occurring within or between the layers of the building envelope.
Moisture. Water in liquid, solid or gaseous form.
Precipitation. Moisture in any form falling from the atmosphere, usually as rain, sleet, snow or hail.
Roof. Any part of the external envelope of a building that is at an angle of less than 70° to the horizontal.
Spray. Water droplets driven by the wind from the surface of the sea or other bodies of water adjacent to buildings. Sea spray can be especially hazardous to materials because of its salt content.
Surface condensation. Deposition of liquid water from a vapour, occurring on visible surfaces within the building.
Vapour control layer. Material of construction, usually a membrane, that substantially reduces the water vapour transfer through any building in which it is incorporated.
Wall. Any opaque part of the external envelope of a building that is at an angle of 70° or more to the horizontal.

Section 1: Clearance or treatment of unsuitable material


Site investigation

See our page on Ground Works

1.1    The preparation of the site will depend on the findings of the site investigation. The site investigation is relevant to Sections 1, 2 and 3 of this Approved Document and also to the requirements of Approved Document A with respect to foundations. The site investigation should consist of a number of well-defined stages:

  1. Planning stage. Clear objectives should be set for the investigation, including the scope and requirements, which enable the investigation to be planned and carried out efficiently and provide the required information;
  2. Desk study. A review of the historical, geological and environmental information about the site is essential;
  3. Site reconnaissance or walkover survey. This stage of the investigation facilitates the identification of actual and potential physical hazards and the design of the main investigation;
  4. Main investigation and reporting. This will usually include intrusive and non-intrusive sampling and testing to provide soil parameters for design and construction. The main investigation should be preceded by (b) and (c) above.

1.2    The extent and level of investigation need to be tailored to the type of development and the previous use of land. Typically the site investigation should include susceptibility to groundwater levels and flow, underlying geology, and ground and hydro-geological properties. A geotechnical site investigation should identify physical hazards for site development, determine an appropriate design and provide soil parameters for design and construction. BS EN 1997-2:2007: Eurocode 7:Geotechnical design with its UK National Annex14 supported by BS 5930:1999+A2:201036 provide comprehensive guidance on site investigation. Guidance on site investigation for low-rise buildings is given in six BRE Digests covering procurement15, desk studies16, the walk-over survey17, trial pits18, soil description19 and direct investigation20. Reference should also be made to BS 8103-1:201121.

1.3   Where the site is potentially affected by contaminants, a combined  geotechnical and geo-environmental investigation should be considered. Guidance on assessing and remediating sites affected by contaminants is given in Section 2: Resistance to contaminants.

Unsuitable material

1.4    Vegetable matter such as turf and roots should be removed from the ground to be covered by the building at least to a depth to prevent later growth. The effects of roots close to the building also need to be assessed. Consideration should be given to whether this provision need apply to a building used wholly for:

  1. storing goods, provided that any persons who are habitually employed in the building are engaged only in taking in, caring for or taking out the goods; or
  2. a purpose such that the provision would not serve to increase protection to the health or safety of any persons habitually employed in the building.

1.5    Where mature trees are present on sites with shrinkable clays (see Diagram 1 and Table 1), the potential damage arising from ground heave to services and  floor slabs and oversite concrete should be assessed. Reference should be made to BRE Digest 29822. Where soils and vegetation type would require significant quantities of soil to be removed, reference should be made to BRE Digests 24023 and 24124, and to the FBE (Foundation for the Built Environment) report25. The effects of remaining trees on services and building movements close to the building need to be assessed using guidance in NHBC (National House Building Council) Standards Chapter 4.226

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14    BS EN 1997-2:2007: Eurocode 7: Geotechnical design – Part 2: Ground investigation and testing; with UK National Annex to BS EN 1997-2:2007.

15    BRE Digest 322 Site investigation for low-rise building: procurement, 1987.

16    BRE Digest 318 Site investigation for low-rise building: desk studies, 1987.

17    BRE Digest 348 Site investigation for low-rise building: the walk-over survey, 1989.

18    BRE Digest 381 Site investigation for low-rise building: trial pits, 1993.

19    BRE Digest 383 Site investigation for low-rise building: soil description, 1993.

20   BRE Digest 411 Site investigation for low-rise building: directinvestigations, 1995.

21    BS 8103-1:2011 Structural design of low-rise buildings – Part 1: Code of practice for stability, site investigation, precast concrete floors and ground floor slabs for housing.

22    BRE Digest 298 Low-rise building foundations: the influence of trees in clay soils, 1999.

23    BRE Digest 240 Low-rise buildings on shrinkable clay soils: Part 1, 1993.

24    BRE Digest 241 Low-rise buildings on shrinkable clay soils: Part 2, 1993.

25    Subsidence damage to domestic buildings: lessons learned and questions remaining, FBE, 2000.

26    NHBC Standards Chapter 4.2 Building near trees, 2003.

36    BS 5930:1999+A2:2010. Code of practice for site investigations.

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See our section on drains and SUDS

1.6    Building services such as below ground
drainage should be sufficiently robust or flexible
to accommodate the presence of any tree roots.
Joints should be made so that roots will not
penetrate them. Where roots could pose a hazard
to building services, consideration should be
given to their removal.

1.7    On sites previously used for buildings,
consideration should be given to the presence of
existing foundations, services, buried tanks and
any other infrastructure that could endanger
persons in and about the building and any land
associated with the building.

1.8    Where the site contains fill or made
ground, consideration should be given to its
compressibility and its potential for collapse on
wetting, and to appropriate remedial measures
to prevent damaging differential settlement.
Guidance is given in BRE Digest 42727 and BRE
Report BR 42428.

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27  BRE Digest 427 Low-rise buildings on fill.

28  BRE Report BR 424 Building on fill: Geotechnical aspects, 2001.

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Section 2: Resistance to contaminants


Introduction

2.1    A wide range of solid, liquid and gaseous contaminants can arise on sites, especially those that have had a previous industrial use (see paragraph 0.12 for the definition of a contaminant). In particular, the burial of biodegradable waste in landfills can give rise to landfill gas (see paragraph 2.25). Sites with a generally rural use such as agriculture or forestry may be contaminated by pesticides, fertiliser, fuel and oils and decaying matter of biological origin.

2.2    Table 2 lists examples of sites that are likely to contain contaminants. It is derived from the ‘Industry Profile’ guides produced by the former Department of the Environment (DoE), each of which deals with a different industry with the potential to cause contamination29. Each profile identifies contaminants which may be associated with the industry, areas on the site in which they may be found and possible routes for migration.

2.3    In addition, there can be problems of natural contaminants in certain parts of the country as a result of the underlying geology. In this instance the contaminants can be naturally occurring heavy metals (e.g. cadmium and arsenic) originating in mining areas, and gases (e.g. methane and carbon dioxide) originating in coal mining areas and from organic rich soils and sediments such as peat and river silts. The Environment Agency has produced two guidance documents31,32 on this subject which discuss the geographical extent of these contaminants, the associated hazards, methods of site investigation and protective measures.

2.4    Natural contaminants also include the radioactive gas radon, although the specific approach for assessing and managing the risks it poses is different from other contaminants (see paragraphs 2.39 and 2.40).

2.5    Sulphate attack affecting concrete floor slabs and oversite concrete associated with particular strata also needs to be considered. Principal areas of sulphate bearing strata in England and Wales are shown in Diagram 1 and Table 1. BRE Special Digest SD133 provides guidance on investigation, concrete specification and design to mitigate the effects of sulphate attack

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29   Department of the Environment Industry Profiles, 1996.

31   Environment Agency R & D Technical Report P291 Information on land quality in England: Sources of information (including background contaminants).

32   Environment Agency R & D Technical Report P292 Information on land quality in Wales: Sources of information (including background contaminants).

33   BRE Special Digest SD1 Concrete in aggressive ground, 2003.

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Solid and liquid contaminants

Risk assessment

 

General concepts

2.6    To ensure safe development of land affected by contaminants the principles of risk assessment (as set out in paragraph 2.8 below) should be followed. The general approach is founded on the concept of the ‘source–pathway–receptor’ relationship, or pollutant linkage, where source refers to contaminants in or on the ground. This is illustrated by the conceptual model34 in Diagram 2.

2.7    When land affected by contaminants is developed, receptors (i.e. buildings, building materials and building services, as well as people) are introduced onto the site and so it is necessary to break the pollutant linkages or condition them so that they do not pose a significant risk. This can be achieved by:

  1. treating the contaminant (e.g. use of physical, chemical or biological processes to eliminate or reduce the contaminant’s toxicity or harmful properties);
  2. blocking or removing the pathway (e.g. isolating the contaminant beneath protective layers or installing barriers to prevent migration);
  3. protecting or removing the receptor (e.g. changing the form or layout of the development, using appropriately designed building materials, etc.);
  4. removing the contaminant (e.g. excavating contaminated material).

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34   The conceptual model is a textual or schematic hypothesis of the nature and sources of contamination, the pollution migration pathways and potential receptors, developed on the basis of the information from a preliminary assessment, and is refined during subsequent phases of investigation.

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Stages of risk assessment

2.8    In assessing the risks for land contamination a tiered approach is adopted with an increasing level of detail required in progressing through the tiers. The three tiers are: preliminary risk assessment, generic quantitative risk assessment (GQRA) and detailed quantitative risk assessment (DQRA). Once the need for a risk assessment has been identified, it will always be necessary to undertake a preliminary risk assessment but, depending on the situation and the outcome, it may not be appropriate to do a more detailed risk assessment. Alternatively, it may be necessary to do only one or both of the more detailed risk assessments. For each tier, the model procedures for the management of land contamination (CLR 1143) describes the stages of risk assessment that should be followed for identifying risks and making judgements about the consequences of land affected by contamination when developing a site. These are outlined below:

  1. Hazard identification – developing the conceptual model by establishing contaminant sources, pathways and receptors. This is the preliminary site assessment which consists of a desk study and a site walk-over in order to obtain sufficient information to obtain an initial understanding of the potential risks. An initial conceptual model for the site can be based on this information.
  2. Hazard assessment – identifying what pollutant linkages may be present and analysing the potential for unacceptable risks. Collect further information and undertake exploratory site investigation to refine understanding of risks and the likelihood of pollutant linkages. The results may be interpreted using generic criteria and assumptions.
  3. Risk estimation – establishing the scale of the possible consequences by considering the degree of harm that may result and to which receptors. Undertake detailed ground investigation to collect sufficient data to estimate the risks the contaminants may pose to defined receptors under defined conditions of exposure.
  4. Risk evaluation – deciding whether the risks are acceptable or unacceptable. Review all site data to decide whether estimated risks are unacceptable, taking into account the nature and scale of any uncertainties associated with the risk estimation process.

2.9    Guidance on the investigation of sites potentially affected by contaminants is provided in:

  1. the Association of Geotechnical and Geoenvironmental Specialists (AGS) document35;
  2. BS 5930:1999+A2:2010. Code of practice for site investigations36;
  3. BS 10175:2011 Code of practice for investigation of potentially contaminated sites37;
  4. the Environment Agency documents38,41,42,43,44,45.

They recommend a risk based approach to identify and quantify the hazards that may be present and the nature of the risk they may pose. They describe the design and execution of field investigations, including suitable sample distribution strategies, sampling and testing.

Hazard identification and assessment

2.10    A preliminary site assessment is required to provide information on the past and present uses of the site and surrounding area that may give rise to contamination (see Table 2). During the site walk-over there may be signs of possible contaminants (see Table 3). The information collated from the desk study and site walk-over can assist and will dictate the design of the exploratory and detailed ground investigation.

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35    Guidelines for combined geoenvironmental and geotechnical investigations, Association of Geotechnical and Geoenvironmental Specialists.

36    BS 5930:1999+A2:2010. Code of practice for site investigations.

37    BS 10175:2011 Code of practice for investigation of potentially contaminated sites.

38    National Groundwater & Contaminated Land Centre report NC/99/38/2 Guide to good practice for the development of conceptual models and the selection and application of mathematical models of contaminant transport processes in the subsurface.

41    Human health toxicological assessment of contaminants in soil (Science report – final SC050021/SR2), Environment Agency.

http://www.environment-gency.gov.uk/static/documents/Research/TOX_guidance_report_-_final.pdf

42    Updated technical background to the CLEA model (Science Report: SC050021/SR3), Environment Agency.

http://www.environment-agency.gov.uk/static/documents/Research/CLEA_report_-_final.pdf

43    CLR 11. Model Procedures for the Management of Land Contamination. Defra/Environment Agency, 2004.

www.environment-agency.gov.uk.

44    Environment Agency R & D Technical Report P5-065 Technical aspects of site investigation, 2000.

45    Environment Agency R & D Technical Report P5-066 Secondary model procedure for the development of appropriate soil sampling strategies for land contamination.

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2.11    The site assessment and risk evaluation should pay particular attention to the area of the site subject to building operations. Those parts of the land associated with the building that include the building itself, gardens and other places on the site that are accessible to users of the building and those in and about the building should be remediated to the requirements of the Building Regulations.

There may be a case for a lower level of remediation if part of, or the remainder of, the land associated with the building, or adjacent to such land, is accessible to a lesser extent to the user or those in and about the building than the main parts of the buildings and their respective gardens. This incremental approach may also apply when very large sites are subject to redevelopment in stages; it may be possible to limit remediation to the site that is subject to building operations.

In all cases the risk evaluation and remediation strategy documentation is likely to be appropriate for demonstrating that restricted remediation is acceptable. The onus is on the applicant to show why part of a site may be excluded from particular remediation measures.

Even if the adjacent land is not subject to Building Regulations, which are concerned with health and safety, it may still be subject to planning control legislation or to control under Part IIA of the Environmental Protection Act 1990.

2.12    The Planning Authority should be informed prior to any intrusive investigations or if any substance is found which is at variance with any preliminary statements made about the nature of the site.

 

Risk estimation and evaluation

2.13   The detailed ground investigation must provide sufficient information for the confirmation of a conceptual model for the site, the risk assessment and the design and specification of any remedial works. This is likely to involve collection and analysis of soil, soil gas, surface and groundwater samples by the use of invasive and/or non-invasive techniques. An investigation of the groundwater regime, levels and flows is essential for most sites since elevated groundwater levels could bring contaminants close to the surface both beneath the building and in any land associated with the building. Expert advice should be sought.

2.14   During the development of land affected by contaminants the health and safety of both the public and workers should be considered46,47.

Remedial measures

Introduction

2.15    If unacceptable risks to the defined receptor have been identified then these need to be managed through appropriate remedial measures. The risk management objectives are defined by the need to break the pollutant linkages using the methods outlined in paragraph 2.7 and described below. Other objectives will also need to be considered such as timescale, cost, remedial works, planning constraints and sustainability. Depending on the contaminant, three generic types of remedial measures can be considered: treatment, containment and removal. The containment or treatment of waste may require a waste management licence from the Environment Agency.

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46    HSE Report HSG 66 Protection of workers and the general public during the development of contaminated land, 1991.

47    CIRIA Report 132 A guide to safe working practices for contaminated land, 1993.

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When building work is undertaken on sites affected by contaminants where control measures are already in place, care must be taken not to compromise these measures. For example, cover systems may be breached when new building foundations are constructed, such as when extensions are added.

Treatment

2.16    A wide range of treatment processes is now available for dealing with contaminants. Biological, chemical and physical techniques carried out either in or ex situ exist which may decrease one or more of the following features of the contaminants: mass, concentration, mobility, flux or toxicity. The choice of the most appropriate technique for a particular site is a highly site-specific decision for which specialist advice should be sought.

Containment

The Ashley Vale self build development in Bristol was all constructed on a pre-existing concrete slab. It was a planning condition that it should not be disturbed due to contaminants beneath it

2.17    Containment in its widest sense usually means encapsulation of material containing contaminants but in the context of building development containment is often taken to mean cover systems. However, in-ground vertical barriers may also be required to control lateral migration of contaminants.

2.18    Cover systems involve the placement of one or more layers of materials over the site to achieve one or more of the following objectives:

  1. break the pollutant linkage between receptors and contaminants;
  2. sustain vegetation;
  3. improve geotechnical properties; and
  4. reduce exposure to an acceptable level.

2.19    Some of the building structures, e.g. foundations, sub-structure and ground floor, may, dependent on the circumstances and construction, contribute to measures to provide effective protection of health from contaminants.

2.20    Imported fill and soil for cover systems should be assessed at source to ensure that it is suitable for use48. Design and dimensioning of cover systems, particularly soil based ones typically used for gardens, should take account of their long-term performance where intermixing of the soil cover with the contaminants in the ground can take place. Maintenance and monitoring may be necessary. Gradual intermixing due to natural effects and activities such as burrowing animals, gardening, etc. needs to be considered. Excavations by householders for garden features, etc. can penetrate the cover layer and may lead to exposure to contaminants. Further guidance on the design, construction and performance of cover layers is given in the Construction Industry Research and Information Association (CIRIA) Report SP12449.

Removal

2.21    This involves the excavation and safe disposal to licensed landfill of the contaminants and contaminated material. Excavation can be targeted to contaminant ‘hot spots’, or it may be necessary to remove sufficient depth of contaminated material to accommodate a cover system within the planned site levels. Removal may not be viable depending on the extent and depth of the contaminants on the site and the availability of suitably licensed landfills. Imported fill should be assessed at source to ensure that there are no materials that will pose unacceptable risks to potential receptors.

2.22    Further detailed guidance on all three types of remedial measure is given in the Environment Agency/NHBC R & D Publication 66 referred to above and in a series of CIRIA publications50–55.

Risks to buildings, building materials and services

2.23    The hazards to buildings, building materials and services on sites affected by contaminants need to be considered since these are also receptors. The hazards to consider are:

  1. Aggressive substances. These include inorganic and organic acids, alkalis, organic solvents and inorganic chemicals such as sulphates and chlorides which may affect the long-term durability of construction materials (such as concrete, metals and plastics).
  2. Combustible fill. This includes domestic waste, colliery spoil, coal, plastics, petrol-soaked ground, etc. which, if ignited, may lead to subterranean fires and consequent damage to the structural stability of buildings, and the integrity or performance of services.
  3. Expansive slags. The two main types are blast furnace and steel making slag which may expand some time after deposition – usually when water is introduced onto the site –   causing damage to buildings and services.
  4. Floodwater affected by contaminants. Substances in the ground, waste matter or sewage may contaminate floodwater. This contaminated water may affect building elements, such as walls or ground floors, that are close to or in the ground. Guidance on flood resilient construction can be found in Improving the flood performance of new buildings – Flood resilient construction8

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48    BS 3882:1994 Specification for topsoil.

49    CIRIA Special Publication SP124 Barriers, liners and cover systems for

containment and control of land contamination, 1996.

50    CIRIA Special Publication SP102 Decommissioning, decontamination

and demolition, 1995.

51    CIRIA Special Publication SP104 Classification and selection of remedial

methods, 1995.

52    CIRIA Special Publication SP105 Excavation and disposal, 1995.

53    CIRIA Special Publication SP106 Containment and hydraulic measures, 1996.

54    CIRIA Special Publication SP107 Ex-situ remedial methods for soils,

sludges and sediments, 1995.

55    CIRIA Special Publication SP109 In-situ methods of remediation, 1995.

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2.24    Although the building and building materials are the main receptors with these hazards, ultimately there could be harm to health. A particular concern is the effect of hydrocarbons permeating potable water pipes made of polyethylene. Guidance on reducing these risks is given in a Water Research Centre report58. Further guidance on the assessment and management of risks to building materials is given in an Environment Agency document59.

Methane and other gases from the ground

Introduction

2.25    The term ‘methane and other gases’ is used to define hazardous soil gases which either originate from waste deposited in landfill sites or are generated naturally. It does not include radon which is dealt with separately in paragraphs 2.39 and 2.40. However, the term does include volatile organic compounds (VOCs). As stated in Limitations on Requirements above, measures described in this document are the minimum that are needed to comply with the Building Regulations. Further actions may be necessary to deal with the requirements of other legislation.

2.26    Landfill gas is generated by the action of micro-organisms on biodegradable waste materials in landfill sites. It generally consists of methane and carbon dioxide together with small quantities of VOCs which give the gas its characteristic odour. Methane and oxygen deficient atmospheres (sometimes referred to as stythe or black-damp) containing elevated levels of carbon dioxide and nitrogen can be generated naturally in coal mining areas. Methane and carbon dioxide can also be produced by organic rich soils and sediments such as peat and river silts. A wide range of VOCs can also be present as a result of petrol, oil and solvent spillages. Methane and other gases can migrate through the subsoil and through cracks and fissures into buildings.

2.27    Methane is an explosive and asphyxiating gas. Carbon dioxide although non-flammable is toxic. VOCs are not only flammable and toxic but can also have a strong, unpleasant odour. Should any of these gases build up to hazardous levels in buildings then they can cause harm to health or compromise safety.

Risk assessment

2.28    The risk assessment process outlined in paragraph 2.8 should also be adopted for methane and other gases. Further investigation for hazardous soil gases may be required where the ground to be covered by the building and/or any land associated with the building is:

  1. On a landfill site, within 250m of the boundary of a landfill site or where there is suspicion that it is within the sphere of influence of such a site. The Environment Agency’s policy on building development on or near to landfills should be followed.
  2. On a site subject to the wide scale deposition of biodegradable substances (including made ground or fill).
  3. On a site that has been subject to a use that could give rise to petrol, oil or solvent spillages.
  4. In an area subject to naturally occurring methane, carbon dioxide and other hazardous gases (e.g. hydrogen sulphide).

2.29    There are documents that cover hazardous soil gases in these specific contexts:

  1. Waste Management Paper No. 2760 gives guidance on the generation and movement of landfill gas as well as techniques for its investigation. Complementary guidance is given in a document61 by the Chartered Institution of Wastes Management (CIWM).
  2. The Institute of Petroleum has prepared a guidance document covering petroleum retail sites62.
  3. The BGS report on naturally occurring methane and other gases63 gives guidance on the geographical extent of these contaminants, the associated hazards and methods of site investigation. This is supported by a report sponsored by the former DoE on methane and other gases in disused coal mining areas64.
  4. In addition, CIRIA has produced three relevant guidance documents on methane and other gases which describe how such gases are generated and move within the ground65, methods of detection and monitoring66 and investigation strategies67.

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8    Improving the flood performance of new buildings – Flood resilient construction, Communities and Local Government, Defra and the Environment Agency, May 2007.

58    Foundation for Water Research Report FR0448 Laying potable water pipelines in contaminated ground: guidance notes, 1994.

59    Assessment and management of risks to buildings, building materials and services from land contamination, Environment Agency, 2001.

60    HMIP Waste Management Paper No. 27 Landfill gas, 2nd edition, 1991.

61    Monitoring of landfill gas, Chartered Institution of Wastes Management (CIWM), 2nd edition, 1998.

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2.30   During a site investigation for methane and other gases it is important to take measurements over a sufficiently long period of time in order to characterise gas emissions fully. This should also include periods when gas emissions are likely to be higher, e.g. during periods of falling atmospheric pressure. It is also important to establish not only the concentration of these gases in the ground but also the quantity of gas generating materials, their rate of gas generation, gas movement in the ground and gas emissions from the ground surface. This is an important part of the risk estimation stage. Indications about the gas regime in the ground can be obtained through surface emission rate and borehole flow rate measurements, and guidance on this is given in CIRIA Reports 15168 and 15269.

2.31    Construction activities undertaken as part of building development can alter the gas regime on the site. For example, a site strip can increase surface gas emissions as can piling and excavation for foundations, and dynamic compaction can push dry biodegradable waste into moist, gas-active zones.

2.32    When assessing gas risks in the context of traditional housing there is a need to consider two pathways for human receptors: (i) gas entering the dwelling through the sub-structure, and building up to hazardous levels, and (ii) subsequent householder exposure in garden areas which can include where outbuildings (e.g. garden sheds and greenhouses) and extensions are constructed, and where there may also be excavations for garden features (e.g. ponds).

2.33    Guidance on undertaking gas risk assessment is given in CIRIA Report 15269, and the GaSIM model is also available for assessing gas emissions from landfill sites70. There is further discussion of gas risk assessment in the Defra/Environment Agency document CLR 1143.

2.34    CIRIA Report 14972 and the Department of the Environment, Transport and the Regions (DETR) Partners in Technology (PIT) report73 describe a range of ground gas regimes (defined in terms of soil gas concentrations of methane and carbon dioxide as well as borehole flow rate measurements) which can be helpful in assessing gas risks.

2.35    Depending on the proposed use, for non-domestic development the focus might be on the building only, but the general approach is the same.

Remedial measures

2.36    If the risks posed by the gas are unacceptable then these need to be managed through appropriate building remedial measures. Site-wide gas control measures may be required if the risks on any land associated with the building are deemed unacceptable. Such control measures include removal of the gas generating material or covering together with gas extraction systems. Further guidance is contained in CIRIA Report 14972. Generally speaking, expert advice should be sought in these circumstances.

2.37    Gas control measures for dwellings consist of a gas resistant barrier across the whole footprint (i.e. walls and floor) above an extraction (or ventilation) layer from which gases can be dispersed and vented to the atmosphere. They are normally passive, i.e. gas flow is driven by stack (temperature difference) and wind effects. Consideration should be given to the design and layout of buildings to maximise the driving forces of natural ventilation. Further guidance on this and detailed practical guidance on the construction of protective measures for housing is given in the BRE/Environment Agency report BR 41473. (In order to accommodate gas resistant membrane, for example as shown in BR414, the position and type of insulation may have to be adjusted). The DETR/Arup Environmental report74 compares the performance of a range of commonly used gas control measures and can be used as a guide to the design of such measures.

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43    CLR 11. Model Procedures for the Management of Land Contamination. Defra/Environment Agency, 2004. www.environment-agency.gov.uk.

62    Institute of Petroleum TP 95 Guidelines for investigation and remediation of petroleum retail sites, 1998.

63    BGS Technical Report WP/95/1 Methane, carbon dioxide and oil seeps from natural sources and mining areas: characteristics, extent and relevance to planning and development in Great Britain, 995.

64    Methane and other gases from disused coal mines: the planning response, DoE, 1996.

65    CIRIA Report 130 Methane: its occurrence and hazards in construction, 1993.

66    CIRIA Report 131 The measurement of methane and other gases from the ground, 1993.

67    CIRIA Report 150 Methane investigation strategies, 1995.

68   CIRIA Report 151 Interpreting measurements of gas in the ground, 1995.

69    CIRIA Report 152 Risk assessment for methane and other gases from the ground, 1995.

70    Environment Agency GasSIM – Landfill gas assessment tool.

72    CIRIA Report 149 Protecting development from methane, 1995.

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2.38    Gas control measures for non-domestic buildings use the same principles as those used for housing, and the DETR/Arup Environmental report can also be used as a guide to design. Expert advice should be sought as the floor area of such buildings can be large and it is important to ensure that gas is adequately dispersed from beneath the floor. The use of mechanical (as opposed to passive) systems and monitoring and alarm systems may be necessary. There is a need for continued maintenance and calibration of these systems, so they are more appropriate with non-domestic buildings (as opposed to dwellings) since there is usually scope for this. Again, expert advice should be sought. Special sub-floor ventilation systems are carefully designed to ensure adequate performance and should not be modified unless subjected to a specialist review of the design. Such ventilation systems, particularly those using powered ventilation, are unlikely to be appropriate for owner occupied properties as there is a risk of interference by users.

Radon

2.39    Radon is a naturally occurring radioactive colourless and odourless gas which is formed in small quantities by radioactive decay wherever uranium and radium are found. It can move through the subsoil and so into buildings. Some parts of the country, notably the West Country, have higher levels than elsewhere. Exposure to high levels for long periods increases the risk of developing lung cancer. To reduce this risk all new buildings, extensions and conversions, whether residential or non-domestic, built in areas where there may be elevated radon emissions, may need to incorporate precautions against radon.

2.40    Guidance on whether an area is susceptible to radon, and appropriate protective measures, can be obtained from BRE Report BR 21175. The maps in BR 211 are based on the indicative atlas published by Public Health England (formerly the Health Protection Agency) and the British Geological Survey. Radon risk reports may be used as an alternative approach to the maps for assessing the need for protective measures. These reports are available from:

• UK Radon, www.UKradon.org, for small domestic and workplace buildings (and extensions) that have an existing postal address.

• BGS Georeports, www.shop.bgs.ac.uk/Georeports, for other development sites.

• Public Health England (formerly the Health Protection Agency), radon@phe.gov.uk, for large workplaces.

BR 211 provides guidance on basic radon protective measures appropriate in areas where 3% to 10% of homes and full radon protective measures in areas where more than 10% of homes are predicted to have radon at or above the Radon Action Level of 200Bq/m3.

Note: Use of the alternative radon risk reports approach will provide a more accurate assessment of whether radon protective measures are necessary and, if needed, the level of protection that is appropriate.

The Ionising Radiations Regulations76 and other legislation set out relevant requirements including a national reference level for radon in workplaces. See also the BRE guide Radon in the workplace77.

The Health and Safety Executive provides guidance on protection from radon in the workplace (www.hse.gov.uk/radiation/ionising/radon.htm). Additionally, techniques for installing radon resistant membranes described in BR 211 may be suitable for use in domestic sized buildings with heating and ventilation regimes similar to those used in dwellings but this should be done with caution. Information in ‘Radon in the workplace’ provides guidance for existing non-domestic buildings.

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73    BRE/Environment Agency Report BR 414 Protective measures for housing on gas-contaminated land, 2001.

74    DETR/Arup Environmental PIT Research Report: Passive venting of soil gases beneath buildings, 1997.

75    BRE Report BR 211 Radon: Guidance on protective measures for new buildings (including supplementary advice for extensions, conversions and refurbishment), 2007.

76    The Ionising Radiations Regulations 1999 (SI 1999/3232).

77    BRE Report FB 41 Radon in the workplace: A guide for building owners and managers (Second edition), 2011.

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Section 3: Subsoil drainage

3.1    The provisions which follow assume that the site of the building is not subject to general flooding (see paragraph 0.8) or, if it is, that appropriate steps are being taken.

3.2    Where the water table can rise to within 0.25m of the lowest floor of the building, or where surface water could enter or adversely affect the building, either the ground to be covered by the building should be drained by gravity, or other effective means of safeguarding the building should be taken.

3.3    If an active subsoil drain is cut during excavation and if it passes under the building it should be:

  1. re-laid in pipes with sealed joints and have access points outside the building; or
  2. re-routed around the building; or
  3. re-run to another outfall (see Diagram 3).

3.4    Where there is a risk that groundwater beneath or around the building could adversely affect the stability and properties of the ground, consideration should be given to site drainage or other protection (see Section 4: Floors).

3.5    For protecting low lying buildings or basements from localised flooding where foul water drainage also receives rainwater, refer to Approved Document H (Drainage and waste disposal). In heavy rainfall these systems surcharge and where preventative measures are not taken this could lead to increased risks of flooding within the property.

3.6    Flooding can create blockages in drains and sewers that can lead to backflow of sewage into properties through low level drain gullies, toilets, etc. Guidance on anti-flooding devices is given in a CIRIA publication79.

3.7    General excavation work for foundations and services can alter groundwater flows through the site. Where contaminants are present in the ground, consideration should be given to subsoil drainage to prevent the transportation of water-borne contaminants to the foundations or into the building or its services.

 

Section 4: Floors

See our section on floor design

4.1    This section gives guidance for five situations:

a. ground supported floors exposed to moisture from the ground (see paragraphs 4.6 to 4.12);

b. suspended timber ground floors exposed to moisture from the ground (see paragraphs 4.13 to 4.16);

c. suspended concrete ground floors exposed to moisture from the ground (see paragraphs 4.17 to 4.20);

d. the risk of interstitial condensation in ground floors and floors exposed from below (see paragraph 4.21);

e. the risk of surface condensation and mould growth on any type of floor (see paragraph 4.22).

4.2    Floors next to the ground should:

  1. resist the passage of ground moisture to the upper surface of the floor;
  2. not be damaged by moisture from the ground;
  3. not be damaged by groundwater;
  4. resist the passage of ground gases. To meet requirement C1 (2) floors in some localities may need to resist the passage of hazardous ground gases such as radon or methane. Remedial measures will include a gas resistant barrier which, with proper detailing, can also function as a damp proof membrane. For specific guidance for methane and other gases refer to paragraphs 2.25 to 2.38, and for radon refer to paragraphs 2.39 and 2.40. Guidance is provided in reports BR 41480 and BR 21175 respectively.

4.3    Consideration should be given to whether 4.2(a) need apply to a building used wholly for:

  1. storing goods, provided that any persons who are habitually employed in the building are engaged only in taking in, caring for or taking out the goods; or
  2. a purpose such that the provision would not serve to increase protection to the health or safety of any persons habitually employed in the building.

4.4    Floors next to the ground and floors exposed from below should be designed and constructed so that their structural and thermal performance are not adversely affected by interstitial condensation.

4.5    All floors should not promote surface condensation or mould growth, given reasonable occupancy conditions.

Ground supported floors (moisture from the ground)

4.6    Any ground supported floor will meet the requirement if the ground is covered with dense concrete laid on a hardcore bed and a damp-proof membrane is provided. Suitable insulation may be incorporated.

Technical solution

4.7    Unless it is subjected to water pressure, which is likely in the case of buildings on very permeable strata such as chalk, limestone or gravel (in which case see Alternative approach, paragraph 4.12), a concrete ground supported floor may be built as follows (Diagram 4):

a. well compacted hardcore bed, no greater than 600mm deep82, of clean, broken brick or similar inert material, free from materials including water-soluble sulphates in quantities which could damage the concrete (BRE Digest 27683; and

b. concrete at least 100mm thick (but thicker if the structural design requires) to mix ST2 in BS 8500 or, if there is embedded reinforcement, to mix ST4 in BS 850084; and

c. damp-proof membrane above or below the concrete, and continuous with the damp-proof courses in walls, piers and the like. If the ground could contain water soluble sulphates, or there is any risk that sulphate or other deleterious matter could contaminate the hardcore, the membrane should be placed at the base of the concrete slab85.

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75    BRE Report BR 211 Radon: Guidance on protective measures for new buildings (including supplementary advice for extensions, conversions and refurbishment), 2007.

80   BRE/Environment Agency Report BR 414 Protective measures for housing on gas-contaminated land, 2001.

82    If the hardcore bed is deeper than 600mm, there may be a risk of excessive settlement and cracking of the floor slab. In such cases, a suspended floor slab is advised.

83    BRE Digest 276 Hardcore, 1992.

84    BS 8500-1:2002 Concrete. Complementary British Standard to BS EN 206-1 Method of specifying and guidance for the specifier.

85    BRE Special Digest SD1 Concrete in aggressive ground, 2003.

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4.8    A membrane below the concrete could be formed with a sheet of polyethylene, which should be at least 300μm thick (1200 gauge) with sealed joints and laid on a bed of material that will not damage the sheet.

4.9    A membrane laid above the concrete may be either polyethylene sheet as described above (but without the bedding material) or three coats of cold applied bitumen solution or similar moisture and water vapour resisting material. In each case it should be protected by either a screed or a floor finish, unless the membrane is pitchmastic or similar material which will also serve as a floor finish.

4.10    Insulants placed beneath floor slabs should have sufficient strength to resist the weight of the slab and the anticipated floor loading as well as any possible overloading during construction. In order to resist degradation insulation that is placed below the damp proof membrane should have low water absorption. If necessary the insulant should be resistant to contaminants in the ground.

4.11    A timber floor finish laid directly on concrete may be bedded in a material which may also serve as a damp-proof membrane. Timber fillets laid in the concrete as a fixing for a floor finish should be treated with an effective preservative unless they are above the damp-proof membrane. Some preservative treatments are described in BS 1282:199986.

Alternative approach

4.12    The requirement can also be achieved by following the relevant recommendations of Clause 11 of BS CP 102:197387. BS 8102:199088 includes recommendations for floors subject to water pressure.

Suspended timber ground floors (moisture from the ground)

4.13    Any suspended timber floor next to the ground will meet the requirement if:

  1. the ground is covered so as to resist moisture and prevent plant growth; and
  2. there is a ventilated air space between the ground covering and the timber; and
  3. there are damp-proof courses between the timber and any material which can carry moisture from the ground.

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86    BS 1282:1999 Wood preservatives. Guidance on choice, use and application.

87    BS CP 102:1973 Protection of buildings against water from the ground.

88    BS 8102:1990 Code of practice for protection of structures against water from the ground.

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Technical solution

4.14    Unless it is covered with a floor finish which is highly vapour resistant (in which case see the Alternative approach in paragraph 4.16), a suspended timber floor next to the ground may be built as follows (Diagram 5):

  1. Ground covering either:

    i.
    unreinforced concrete at least 100mm thick to mix ST 1 in BS 850089. The concrete should be laid on a compacted hardcore bed of clean, broken brick or any other inert material free from materials including water-soluble sulphates in quantities which could damage the concrete;or ii. concrete, composed as described above, or inert fine aggregate, in either case at least 50mm thick laid on at least 300μm (1200 gauge) polyethylene sheet with sealed joints, and itself laid on a bed of material which will not damage the sheet.To prevent water collecting on the ground covering, either the top should be entirely above the highest level of the adjoining ground or, on sloping sites, consideration should be given to installing drainage on the outside of the up-slope side of the building (see Diagram 6).
  2. Ventilated air space measuring at least 75mm from the ground covering to the underside of any wall-plates and at least 150mm to the underside of the suspended timber floor (or insulation if provided). Two opposing external walls should have ventilation openings placed so that the ventilating air will have a free path between opposite sides and to all parts. The openings should be not less than either 1,500mm2 /m run of external wall or 500mm2 /m2 of floor area, whichever gives the greater opening area. Any pipes needed to carry ventilating air should have a diameter of at least 100mm. Ventilation openings should incorporate suitable grilles which prevent the entry of vermin to the sub-floor but do not resist the air flow unduly. If floor levels need to be nearer to the ground to provide level access sub-floor ventilation can be provided through offset (periscope) ventilators.
  3. Damp-proof courses of impervious sheet material, engineering brick or slates in cement mortar or other material which will prevent the passage of moisture. Guidance for choice of materials is given in BS 5628:Part 3:200190.
  4. In shrinkable clay soils, the depth of the air space may need to be increased to allow for heave.

4.15    In areas such as kitchens, utility rooms and bathrooms where water may be spilled, any board used as a flooring, irrespective of the storey, should be moisture resistant. In the case of chipboard it should be of one of the grades with improved moisture resistance specified in BS 7331:199091 or BS EN 312 Part 5:199792. It should be laid, fixed and jointed in the manner recommended by the manufacturer. To demonstrate compliance the identification marks should be facing upwards. Any softwood boarding should be at least 20mm thick and from a durable species93 or treated with a suitable preservative.

Alternative approach

4.16    The requirement can also be met (see paragraph 4.14 above) by following the relevant recommendations of Clause 11 of BS CP 102:197394.

Suspended concrete ground floors (moisture from the ground)

4.17    Any suspended floor of in situ or precast concrete, including beam and block floors, next to the ground will meet the requirement if it will adequately prevent the passage of moisture to the upper surface and if the reinforcement is protected against moisture.

Technical solution

4.18    One solution for a suspended concrete floor could be:

  1. in situ concrete at least 100mm thick (but thicker if the structural design requires) containing at least 300kg of cement for each m3 of concrete; or
  2. precast concrete construction with or without infilling slabs; and
  3. reinforcing steel protected by concrete cover of at least 40mm if the concrete is in situ and at least the thickness required for a moderate exposure if the concrete is precast.

4.19    A suspended concrete floor will meet the requirements if it incorporates:

  1. a damp-proof membrane (if the ground below the floor has been excavated below the lowest level of the surrounding ground and will not be effectively drained); and
  2. a ventilated air space. This should measure at least 150mm clear from the ground to the underside of the floor (or insulation if provided). Two opposing external walls should have ventilation openings placed so that the ventilating air will have a free path between opposite sides and to all parts of the floor void. The openings should be not less than either 1500mm2 /m run of external wall or 500mm2/m2 of floor area, whichever gives the greater opening area. Any pipes needed to carry ventilating air should have a diameter of at least 100mm. Ventilation openings should incorporate suitable grilles which prevent the entry of vermin to the sub-floor but do not resist the air flow unduly.

4.20    In localities where flooding is likely, consideration may be given to including means of inspecting and clearing out the sub-floor voids beneath suspended floors. For guidance, see Improving the flood performance of new buildings – Flood resilient construction8.

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8    Improving the flood performance of new buildings – Flood resilient construction, Communities and Local Government, Defra and the Environment Agency, 2007.

90    BS 5628-3:2001 Code of practice for use of masonry. Materials and components, design and workmanship.

91     BS 7331:1990 Specification for direct surfaced wood chipboard based on thermosetting resins.

92    BS EN 312-5:1997 Particleboards. Specifications. Requirements for load-bearing boards for use in humid conditions.

93    BRE Digest 429 Timbers: their natural durability and resistance to preservative treatment, 1998.

94    BS CP 102:1973 Protection of buildings against water from the ground.

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Ground floors and floors exposed from below (resistance to damage from interstitial condensation)

4.21    A ground floor or floor exposed from below, i.e. above an open parking space or passageway, as shown in Diagram 7, will meet the requirement if it is designed and constructed in accordance with Clause 8.5 and Appendix D of BS 5250:200296, BS EN ISO 13788:200297 and BR 26298.

Floors (resistance to surface condensation and mould growth)

4.22    A floor will meet the requirement if:

  1. a ground floor is designed and constructed so that the thermal transmittance (U-value) does not exceed 0.7W/m2K at any point; and
  2. in the case of all floors, the junctions between elements are designed to Accredited Construction Details99, or follow the guidance of BRE IP17/01100.

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96    BS 5250:2002 Code of practice for the control of condensation in buildings.

97    BS EN ISO 13788:2002 Hygrothermal performance of building components and building elements. Internal surface temperature to avoid critical surface humidity and interstitial condensation. Calculation methods.

98 BRE Report BR 262 Thermal insulation: avoiding risks, 2002.

99 Accredited Construction Details which can be downloaded from www.planningportal.gov.uk/buildingregulations/approveddocuments/partl/bcassociateddocuments9/acd.

100 BRE Information Paper IP17/01 Assessing the effects of thermal bridging at junctions and around openings, 2001.

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Section 5: Walls

see our section on walls

5.1    This section gives guidance for four situations:

  1. internal and external walls exposed to moisture from the ground (see paragraphs 5.4 to 5.6);
  2. external walls exposed to precipitation from the outside, covering:i. solid external walls (see paragraphs 5.8 to 5.11);
    ii. cavity external walls (see paragraphs 5.12 to 5.15);
    iii. framed external walls (see paragraph 5.17);
    iv. cracking of walls (see paragraph 5.18);
    v. impervious cladding systems (see paragraphs 5.19 to 5.28);
    vi. the joint between window and door frames and external walls and door thresholds (see paragraphs 5.29 to 5.33);
  3. the risk of interstitial condensation in any type of wall (see paragraphs 5.34 to 5.35);
  4. the risk of surface condensation or mould growth on any type of wall (see paragraph 5.36).

A wall includes piers, columns and parapets. It also includes chimneys if they are attached to the building. It does not include windows, doors and similar openings, but does include the joint between their frames and the wall. In the following, the term ‘precipitation’ includes the effects of spray blown from the sea or any other body of water adjacent to the building.

5.2    Walls should:

  1. resist the passage of moisture from the ground to the inside of the building; and
  2. not be damaged by moisture from the ground and not carry moisture from the ground to any part which would be damaged by it, and, if the wall is an external wall:
  3. resist the penetration of precipitation to components of the structure that might be damaged by moisture; and
  4. resist the penetration of precipitation to the inside of the building; and
  5. be designed and constructed so that their structural and thermal performance are not adversely affected by interstitial condensation; and
  6. not promote surface condensation or mould growth, given reasonable occupancy conditions.

5.3    Consideration should be given to whether provisions 5.2(a) and (d) need apply to a building used wholly for:

  1. storing goods, provided that any persons who are habitually employed in the building are engaged only in taking in, caring for or taking out the goods; or
  2. a purpose such that the provision would not serve to increase protection to the health or safety of any persons habitually employed in the building.

Internal and external walls (moisture from the ground)

5.4    Any internal or external wall will meet the requirement if a damp proof course is provided.

Technical solution

5.5    An internal or external wall will meet the requirement if it is built as follows (unless it is subject to groundwater pressure, in which case see the Alternative approach – paragraph 5.6):

a. damp-proof course of bituminous material, polyethylene, engineering bricks or slates in cement mortar or any other material that will prevent the passage of moisture. The damp proof course should be continuous with any damp-proof membrane in the floors; and

b. if the wall is an external wall, the damp-proof course should be at least 150mm above the level of the adjoining ground (see Diagram 8), unless the design is such that a part of the building will protect the wall; and

c. if the wall is an external cavity wall, (see Diagram 9a) the cavity should be taken down at least 225mm below the level of the lowest damp-proof course, or a damp-proof tray should be provided so as to prevent precipitation passing into the inner leaf (see Diagram 9b), with weep holes every 900mm to assist in the transfer of moisture through the external leaf. Where the damp-proof tray does not extend the full length of the exposed wall, i.e. above an opening, stop ends and at least two weep holes should be provided.

Alternative approach

5.6    The requirement can also be met by following the relevant recommendations of Clauses 4 and 5 of BS 8215:1991101. BS 8102:1990102 includes recommendations for walls subject to groundwater pressure including basement walls.

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101    BS 8215:1991 Code of practice for design and installation of damp-proof courses in masonry construction.

102   BS 8102:1990 Code of practice for protection of structures against water from the ground.

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External walls (moisture from the outside)

5.7    As well as giving protection against moisture from the ground, an external wall should give protection against precipitation. This protection can be given by a solid wall of sufficient thickness (see paragraphs 5.8 to 5.11), or by a cavity wall (see paragraphs 5.12 to 5.18), or by an impervious or weather-resisting cladding (see paragraphs 5.19 to 5.28).

Solid external walls

5.8    Any solid wall will meet the requirement if it will hold moisture arising from rain and snow until it can be released in a dry period without penetrating to the inside of the building, or causing damage to the building. The wall thickness will depend on the type of brick and block and on the severity of wind-driven rain. A method of describing the exposure to wind-driven rain is given in BS 8104:1992103; see also BS 5628-3:2001104.

Technical solution

5.9    A solid external wall in conditions of very severe exposure should be protected by external impervious cladding, but in conditions of severe exposure may be built as follows:

  1. brickwork or stonework at least 328mm thick, dense aggregate concrete blockwork at least 250mm thick, or lightweight aggregate or aerated autoclaved concrete blockwork at least 215mm thick; and
  2. rendering: the exposed face of the bricks or blocks should be rendered or be given no less protection. Rendering should be in two coats with a total thickness of at least 20mm and should have a scraped or textured finish. The strength of the mortar should be compatible with the strength of the bricks or blocks. The joints, if the wall is to be rendered, should be raked out to a depth of at least 10mm. Further guidance is given in BS EN 998:2003105. The rendering mix should be one part of cement, one part of lime and six parts of well graded sharp sand (nominal mix 1:1:6) unless the blocks are of dense concrete aggregate, in which case the mix may be 1:0.5:4. BS 5262:1991106 includes recommendations for a wider range of mixes according to the severity of exposure and the type of brick or block.
    Premixed and proprietary renders should be used in accordance with the manufacturer’s instructions;
  3. protection should be provided where the top of walls, etc. would otherwise be unprotected (see Diagram 10). Unless the protection and joints will be a complete barrier to moisture, a damp-proof course should also be provided;
  4. damp-proof courses, cavity trays and closers should be provided and designed to ensure that water drains outwards:
    i. where the downward flow will be interrupted by an obstruction, such as some types of lintel; and
    ii. under openings, unless there is a sill and the sill and its joints will form a complete barrier; and
    iii. at abutments between walls and roofs.

5.10    Insulation. A solid external wall may be insulated on the inside or on the outside. Where it is on the inside a cavity should be provided to give a break in the path for moisture and where it is on the outside it should provide some resistance to the ingress of moisture to ensure the wall remains relatively dry (see Diagram 11).

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103    BS 8104:1992 Code of practice for assessing exposure of walls to wind-driven rain.

104    BS 5628-3:2001 Code of practice for use of masonry. Materials and components, design and workmanship.

105    BS EN 998-2:2003 Specification for mortar for masonry. Masontry mortar.

106    BS 5262:1991 Code of practice for external renderings.

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Alternative approach

5.11    The requirement can also be met by following the relevant recommendations of BS 5628-3:2001107. The code describes alternative constructions to suit the severity of the exposure and the type of brick or block.

Cavity external walls

5.12    Any external cavity wall will meet the requirement if the outer leaf is separated from the inner leaf by a drained air space, or in any other way which will prevent precipitation from being carried to the inner leaf.

Technical solution

5.13    The construction of a cavity external wall could include:

  1. outer leaf masonry (bricks, blocks, stone or manufactured stone); and
  2. cavity at least 50mm wide. The cavity is to be bridged only by wall ties, cavity trays provided to prevent moisture being carried to the inner leaf (see paragraph 5.15 for cavity insulation), and cavity barriers, firestops and cavity closures, where appropriate; and
  3. inner leaf masonry or frame with lining.

Masonry units should be laid on a full bed of mortar with the cross joints substantially and continuously filled to ensure structural robustness and weather resistance.

Where a cavity is to be partially filled, the residual cavity should not be less than 50mm wide (see Diagram 11).

Alternative approach

5.14    The requirement can also be met by following the relevant recommendations of BS 5628-3:2001108. The code describes factors affecting rain penetration of cavity walls.

Cavity insulation

see our section on wall insulation

5.15    A full or partial fill insulating material may be placed in the cavity between the outer leaf and an inner leaf of masonry subject to the following conditions:

  1. The suitability of a wall for installing insulation into the cavity should be determined either by reference to the map in Diagram 12 and the associated Table 4 or following the calculation or assessment procedure in current British or CEN standards. When partial fill materials are to be used, the residual cavity should not be less than 50mm nominal; and
  2. A rigid (board or batt) thermal insulating material built into the wall should be the subject of current certification from an appropriate body or a European Technical Approval and the work should be carried out in accordance with the requirements of that document; or
  3. Other insulating materials inserted into the cavity after the wall has been constructed should have certification from an appropriate body and be installed in accordance with the appropriate installations code. The suitability of the wall for filling is to be assessed before the work is carried out and the person undertaking the work should operate under an Approved Installer Scheme that includes an assessment of capability. Alternatively the insulating material should be the subject of current certification from an appropriate body or a European Technical Approval and the work should be carried out in accordance with the requirements of that document by operatives either directly employed by the holder of the document or employed by an installer approved to operate under the document; or
  4. Urea-formaldehyde foam inserted into the cavity should be in accordance with BS 5617:1985109 and be installed in accordance with BS 5618:1985110. The suitability of the wall for foam filling is to be assessed before the work is carried out and the person undertaking the work should operate under an Approved Installer Scheme that includes an assessment of capability.
  5. When the cavity of an existing house is being filled, special attention should be given to the condition of the external leaf of the wall, e.g. its state of repair and type of pointing. Guidance is given in BS 8208-1:1985111. Some materials that are used to fill existing cavity walls may have a low risk of moisture being carried over to the internal leaf of the wall. In cases where a third party assessment of such a cavity fill material contains a method of assessing the construction of the walls and exposure risk, the procedure set out below may be replaced by that method.

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107    BS 5628-3:2001 Code of practice for use of masonry. Materials and components, design and workmanship.

108    BS 5628-3:2001 Code of practice for use of masonry. Materials and components, design and workmanship.

109    BS 5617:1985 Specification for urea-formaldehyde (UF) foam systems suitable for thermal insulation of cavity walls with masonry or concrete inner and outer leaves.

110    BS 5618:1985 Code of practice for thermal insulation of cavity walls (with masonry or concrete inner and outer leaves) by filling with urea-formaldehyde (UF) foam systems.

111    BS 8208-1:1985 Guide to assessment of suitability of external cavity walls for filling with thermal insulation.

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5.16    If the map given in Diagram 12 is used, determine the national exposure and, where appropriate, apply the following modifiers:

  1. where local conditions accentuate wind effects, such as open hillsides or valleys where the wind is funnelled onto the wall, add one to this exposure zone value;
  2. where walls do not face into the prevailing wind, subtract one from this exposure zone value.

(The national exposure zone value can be more accurately calculated from the larger scale maps and correction factors given in BS 8104:1992112.)

Determine the recommended constructions from the modified exposure zone values given in Table 4. Further guidance as to the use of this table is given in BRE Report 262113.

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112    BS 8104:1992 Code of practice for assessing exposure of walls to wind-driven rain.

113    BRE Report BR 262 Thermal insulation: avoiding risks, 2002.

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Framed external walls

5.17    Any framed external wall will meet the requirement if the cladding is separated from the insulation or sheathing by a vented and drained cavity with a membrane that is vapour open, but resists the passage of liquid water, on the inside of the cavity (see Diagram 11).

Cracking of external walls

5.18    Severe rain penetration may occur through cracks in masonry external walls caused by thermal movement in hot weather or subsidence after prolonged droughts. The possibility of this should be taken into account when designing a building. Detailed guidance is given in:

  1. BRE Building Elements: Walls, windows and doors114; and
  2. BRE Report 292115;
  3. Guidance for choice of materials is given in BS 5628-3:2001116.

Impervious cladding systems for walls

see our section on Rain Screens

5.19    Cladding systems for walls should:

  1. resist the penetration of precipitation to the inside of the building; and
  2. not be damaged by precipitation and not carry precipitation to any part of the building which would be damaged by it.

5.20    Cladding can be designed to protect a building from precipitation (often driven by the wind) either by holding it at the face of the building or by stopping it from penetrating beyond the back of the cladding.

5.21    Any cladding will meet the requirement if:

  1. it is jointless or has sealed joints, and is impervious to moisture (so that moisture will not enter the cladding); or
  2. it has overlapping dry joints, is impervious or weather resisting, and is backed by a material which will direct precipitation which enters the cladding towards the outer face.

5.22    Some materials can deteriorate rapidly without special care and they should only be used as the weather-resisting part of a cladding system if certain conditions are met (see Approved Document 7, Materials and workmanship). The weather-resisting part of a cladding system does not include paint nor does it include any coating, surfacing or rendering which will not itself provide all the weather resistance.

Technical solution

5.23    Cladding may be:

  1. impervious including metal, plastic, glass and bituminous products; or
  2. weather resisting including natural stone or slate, cement based products, fired clay and wood; or
  3. moisture resisting including bituminous and plastic products lapped at the joints, if used as a sheet material, and permeable to water vapour unless there is a ventilated space directly behind the material; or
  4. jointless materials and sealed joints, which would allow for structural and thermal movement.

5.24   Dry joints between cladding units should be designed so that precipitation will not pass through them, or the cladding should be designed so that precipitation which enters the joints will be directed towards the exposed face without it penetrating beyond the back of the cladding.

Note: Whether dry joints are suitable will depend on the design of the joint or the design of the cladding and the severity of the exposure to wind and rain.

5.25    Each sheet, tile and section of cladding should be securely fixed. Guidance as to appropriate fixing methods is given in BS 8000-6:1990117. Particular care should be taken with detailing and workmanship at the junctions between cladding and window and door openings as they are vulnerable to moisture ingress.

5.26    Insulation can be incorporated into the construction provided it is either protected from moisture or unaffected by it.

5.27    Where cladding is supported by timber components or is on the façade of a timber framed building, the space between the cladding and the building should be ventilated to ensure rapid drying of any water that penetrates the cladding.

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114 BRE Report 352 BRE Building elements: Walls, windows and doors, 2002.

115 BRE Report BR 292 Cracking in buildings, 1995.

116 BS 5628-3:2001 Code of practice for use of masonry. Materials and components, design and workmanship.

117 BS 8000-6:1990 Workmanship on building sites. Code of practice for slating and tiling of roofs and claddings.

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Alternative approach

5.28    The requirement can also be met by following the relevant recommendations of:

  1. BS CP 143118 for sheet roof and wall coverings made from the following materials:
    Part 1:1958 Corrugated and troughed aluminium
    Part 5:1964 Zinc
    Part 10:1973 Galvanised corrugated steel
    Part 12:1970 (1988) Copper
    Part 15:1973 (1986) Aluminium
    Part 16:1974 Semi-rigid asbestos bitumen sheets
    Recommendations for lead are included in BS 6915:2001119;
  2. BS 8219:2001120;
  3. BS 8200:1985121;
  4. BS 8297:2000122;
  5. BS 8298:1994123;
  6. MCRMA Technical Paper 6124;
  7. MCRMA Technical Paper 9125.

These documents describe the materials and contain design considerations including recommendations for fixing.

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118    BS CP 143 Code of practice for sheet roof and wall coverings.

119    BS 6915:2001 Design and construction of fully supported lead sheet roof and wall coverings. Code of practice.

120    BS 8219:2001 Installation of sheet roof and wall coverings. Profiled fibre cement. Code of practice.

121    BS 8200:1985 Code of practice for the design of nonloadbearing external vertical enclosures of buildings.

122    BS 8297:2000 Code of practice for design and installation of non-loadbearing precast concrete cladding.

123    BS 8298:1994 Code of practice for design and installation of naturalstone cladding and lining.

124    MCRMA Technical Paper 6 Profiled metal roofing design guide, revised edition, 1996.

125    MCRMA Technical Paper 9 Composite roof and wall cladding panel design guide, 1995.

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Joint between doors and windows

5.29    The joint between walls and door and window frames should:

  1. resist the penetration of precipitation to the inside of the building; and
  2. not be damaged by precipitation and not permit precipitation to reach any part of the building which would be damaged by it.

5.30    Damp-proof courses should be provided to direct moisture towards the outside:

  1. where the downward flow of moisture would be interrupted at an obstruction, e.g. at a lintel;
  2. where sill elements, including joints, do not form a complete barrier to the transfer of precipitation, e.g. under openings, windows and doors;
  3. where reveal elements, including joints, do not form a complete barrier to the transfer of rain and snow, e.g. at openings, windows and doors.

5.31    In some cases the width of the cavity due to thermal insulation and the 50mm clearance for drainage may be such that the window frame is not wide enough to completely cover the cavity closer. The reveal may need to be lined with plasterboard, dry lining, a support system or a thermal backing board. Direct plastering of the internal reveal should only be used with a backing of expanded metal lathing or similar.

5.32    In areas of the country in driving rain exposure zone 4 checked rebates should be used in all window and door reveals. The frame should be set back behind the outer leaf of masonry, which should overlap it as shown in Diagram 13. Alternatively an insulated finned cavity closer may be used.

Door thresholds

see our section on disabled access

5.33    Where an accessible threshold is provided to allow unimpeded access, as specified in Part M, Access to and use of buildings, it will meet the requirement if:

  1. the external landing (Diagram 14) is laid to a fall between 1 in 40 and 1 in 60 in a single direction away from the doorway;
  2. the sill leading up to the door threshold has a maximum slope of 15°.

Further advice for the development of accessible thresholds is given in BRE GBG 47126 and the TSO document127.

External walls (resistance to damage from interstitial condensation)

see our section on ‘breathing’ construction

5.34    An external wall will meet the requirement if it is designed and constructed in accordance with Clause 8.3 of BS 5250:2002129, and BS EN ISO 13788:2002130.

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99 Accredited Construction Details which can be downloaded from www.planningportal.gov.uk/buildingregulations/approveddocuments/partl/bcassociateddocuments9/acd.

126 BRE GBG 47 Level external thresholds: reducing moisture penetration and thermal bridging, 2001.

127 Accessible thresholds in new buildings: guidance for house builders and designers, TSO, 1999.

128 The drainage channel and adjacent paving and threshold are usually made up from precast concrete or other pre-formed components.

129 BS 5250:2002 Code of practice for the control of condensation in buildings.

130 BS EN ISO 13788:2002 Hygrothermal performance of building components and building elements. Internal surface temperature to avoid critical surface humidity and interstitial condensation. Calculation methods.

132 BRE Information Paper IP17/01 Assessing the effects of thermal bridging at junctions and around openings, 2001.

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5.35    Because of the high internal temperatures and humidities, there is a particular risk of interstitial condensation in the walls of swimming pools and other buildings in which high levels of moisture are generated; specialist advice should be sought when these are being designed.

External walls (resistance to surface condensation and mould growth)

5.36   An external wall will meet the requirement if:

  1. it is designed and constructed so that the thermal transmittance (U-value) does not exceed 0.7W/m2K at any point; and
  2. the junctions between elements and details of openings, such as doors and windows, are designed to Accredited Construction Details99, or follow the guidance of BRE IP17/01132.

Section 6: Roofs

6.1    This section gives guidance for three situations:

  1. roofs exposed to precipitation from the outside (see paragraphs 6.3 to 6.9);
  2. the risk of interstitial condensation in roofs (see paragraphs 6.10 to 6.13);
  3. the risk of condensation or mould growth on the internal surface of roofs (see paragraph 6.14).

6.2    Roofs should:

  1. resist the penetration of precipitation to the inside of the building; and
  2. not be damaged by precipitation and not carry precipitation to any part of the building which would be damaged by it;
  3. be designed and constructed so that their structural and thermal performance are not adversely affected by interstitial condensation.

Roofs (resistance to moisture from the outside)

see our section on living roofs

6.3    Roofing can be designed to protect a building from precipitation either by holding the precipitation at the face of the roof or by stopping it from penetrating beyond the back of the roofing system.

6.4    Any roof will meet the requirement if:

  1. it is jointless or has sealed joints, and is impervious to moisture (so that moisture will not enter the roofing system); or
  2. it has overlapping dry joints, is impervious or weather resisting, and is backed by a material which will direct precipitation which enters the roof towards the outer face (as with roofing felt).

6.5    Some materials can deteriorate rapidly without special care and they should only be used as the weather-resisting part of a roof if certain conditions are met (see Approved Document 7, Materials and workmanship133). The weather-resisting part of a roofing system does not include paint nor does it include any coating, surfacing or rendering which will not itself provide all the weather resistance.

Technical solution

6.6    Roofing systems may be:

  1. impervious including metal, plastic and bituminous products; or
  2. weather resisting including natural stone or slate, cement based products, fired clay and wood; or
  3. moisture resisting including bituminous and plastic products lapped at the joints, if used as a sheet material, and permeable to water vapour unless there is a ventilated space directly behind the material; or
  4. jointless materials and sealed joints, which would allow for structural and thermal movement.

6.7    Dry joints between roofing sheets should be designed so that precipitation will not pass through them, or the system should be designed so that precipitation which enters the joints will be drained away without penetrating beyond the back of the roofing system. Note: Whether dry joints are suitable will depend on the design of the joint or the design of the roofing system and the severity of the exposure to wind and rain.

6.8    Each sheet, tile and section of roof should be fixed in an appropriate manner. Guidance as to appropriate fixing methods is given in BS 8000-6:1990134.

Alternative approach

6.9    The requirement can also be met by following the relevant recommendations of:

  1. BS CP 143135 for sheet roof and wall coverings made from the following materials:
    Part 1:1958 Corrugated and troughed aluminium
    Part 5:1964 ZincPart 10:1973 Galvanized corrugated steel
    Part 12:1970 (1988) Copper
    Part 15:1973 (1986) Aluminium
    Part 16:1974 Semi-rigid asbestos bitumen sheets.Recommendations for lead are included in BS 6915:2001136;
  2. BS 8219:2001137;
  3. BS 8200:1985138;
  4. MCRMA Technical Paper 6139;
  5. MCRMA Technical Paper 9140.

These documents describe the materials and contain design considerations including recommendations for fixing.

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133 Approved Document 7: Materials and workmanship, DCLG, 2013 edition.

134 BS 8000-6:1990 Workmanship on building sites. Code of practice for slating and tiling of roofs and claddings.

135 BS CP 143 Code of practice for sheet roof and wall coverings.

136 BS 6915:2001 Design and construction of fully supported lead sheet roof and wall coverings. Code of practice.

137 BS 8219:2001 Installation of sheet roof and wall coverings. Profiled fibre cement. Code of practice.

138 BS 8200:1985 Code of practice for the design of nonloadbearing external vertical enclosures of buildings.

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Roofs (resistance to damage from interstitial condensation )

6.10    A roof will meet the requirement if it is designed and constructed in accordance with Clause 8.4 of BS 5250:2002141 and BS EN ISO 13788:2002142. Further guidance is given in the BRE Report BR 262143.

6.11    The requirement will be met by the ventilation of cold deck roofs, i.e. those roofs where the moisture from the building can permeate the insulation. For the purposes of health and safety it may not always be necessary to provide ventilation to small roofs such as those over porches and bay windows. Although a part of a roof which has a pitch of 70° or more is to be insulated as though it were a wall, the provisions in this document apply to roofs of any pitch.

6.12    To avoid excessive moisture transfer to roof voids gaps and penetrations for pipes and electrical wiring should be filled and sealed; this is particularly important in areas of high humidity, e.g. bathrooms and kitchens. An effective draught seal should be provided to loft hatches to reduce inflow of warm air and moisture.

6.13    Because of the high internal temperatures and humidities, there is a particular risk of interstitial condensation in the roofs of swimming pools and other buildings in which high levels of moisture are generated; specialist advice should be sought when these are being designed.

Roofs (resistance to surface condensation and mould growth)

6.14    A roof will meet the requirement if:

a. it is designed and constructed so that the thermal transmittance (U-value) does not

exceed 0.35W/m2K at any point; and

b. the junctions between elements and the details of openings, such as windows, are designed to Accredited Construction Details99, or follow the guidance of BRE IP17/01145 or MCRMA Paper 14146 for profiled metal roofing.

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99    Accredited Construction Details which can be downloaded from www.planningportal.gov.uk/buildingregulations/approveddocuments/partl/bcassociateddocuments9/acd.

139    MCRMA Technical Paper 6 Profiled metal roofing design guide, revised edition, 1996.

140    MCRMA Technical Paper 9 Composite roof and wall cladding panel design guide, 1995.

141    BS 5250:2002 Code of practice for the control of condensation in buildings.

142   BS EN ISO 13788:2002 Hygrothermal performance of building components and building elements. Internal surface temperature to avoid critical surface humidity and interstitial condensation. Calculation methods.

143   BRE Report BR 262 Thermal insulation: avoiding risks, 2002.

145   BRE Information Paper IP17/01 Assessing the effects of thermal bridging at junctions and around openings, 2001.

146   MCRMA Technical Paper 14 Guidance for the design of metal roofing and cladding to comply with approved document L2:2001, 2002.

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