Green roof opportunities
There are several possible ways of improving the ecological design of roofs:
- Having generous roof overhangs can significantly reduce the amount of maintenance to woodwork such as doors, windows and cladding
- Careful sourcing of roofing materials is important as a way of reducing the embodied energythe total amount of energy it takes to make a material (or a building). See more on embodied energy and minimising manufacturing pollution
- The roof can have vastly increased insulation levels
- The roof can be designed to harvest rainwater
- A living roof can reduce flash flooding
- It may be able to recycle grey waterThis is the waste water that comes from the baths, basins, showers and washing machines. Kitchen sink water is known as black water. see Recycling grey water
- As a living roof it might provide a habitat for wild life or a garden
- The question of extensions and changes to roofs has a bearing on Lifetime Homes"Lifetime Homes make life as easy as possible for as long as possible because they are thoughtfully designed. They provide accessible and adaptable accommodation for everyone, from young families to older people and individuals with a temporary or permanent physical impairment". See the Lifetime Homes page and the Lifetime Homes Foundation web site
- generous overhangs on the roof of the Denby Dale PassivhausSee more on the Passivhaus standard. The PassivHaus Institute has pioneered a standard for low energy buildings. It includes very low energy usage and ways of achieving this. The word is derived from the idea of buildings which are fundamentally low energy and passive solar heated rather than using extra gadgets to heat them. See Passivhaus for the UK branch of the organisation.
Simply because most rain in the UK falls within 5º of vertical, a generous overhang will prevent most rain reaching the walls. This will prevent most rain reaching window frames, especially if they are set back slightly into the wall. In the case of timber frames, this can extend their life very considerably. While this costs slightly more initially for the extra roof area, it does save money in the long run and usually adds to the style of the house, in turn making it more valuable.
Roofing materials should be checked for embodied energy and for how polluting they are in manufacture. For instance most of the natural stone slates coming into the UK at present are coming from India and China. Lead for flashings is often manufactured in a very polluting way. This subject is dealt with well in the Green Building Handbook.
There is a potential pollution problem from zinc roofs.
With only the minimum of filtering, collected rainwater can be used for watering the garden, washing the car, the cold feed to the washing machine and flushing toilets. Particularly in parts of the country where water is becoming increasingly scarce, the low investment required for the first two of these makes a lot of sense.
Bear in mind that a living roof, (see below) is not an ideal candidate for collecting rain as it tends to absorb quite a lot itself, particularly during dry periods when you need it most.
Sustainable urban drainage systems (SUDS)
The increasing areas of hard surfaces in towns such as paths, driveways, roads, hard standing, etc. put a very high strain on the sewerage system in most towns. This is because when it rains there is sudden runoff into the drains rather that gradual seepage into the earth. SUDSSustainable urban drainage systems guidelines on how to prevent this are on the CIRIAConstruction industry research and information association Susdrain web site (registration is free). One of the ways is to have a living roof (see below).
Rooftop recycling of gray water
Research has been going on at Cranfield University into the possibilities of rooftop cleaning up of grey water for use in flushing toilets. The result is GROW by Waterworks UK Ltd.
The main design aspects of roofs concerns their two, possibly four main functions:
- to keep out the weather (rain, wind, snow, heat, cold etc)
- to provide living space or storage – this has a major influence on the type of structure needed
- to express the character of a house
- to provide an outdoor living area
weather and sound protection
Rain and snow
The Building RegulationsThese are the mass of regulations that cover safety, health, welfare, convenience, energy efficiency etc. in the way buildings are constructed. Not to be confused with Planning consent (which is more to do with whether you can put up the building in the first place) have two categories of roof covering: jointless (or with sealed joints) and overlapping dry joints. The former (typically a plastic membrane) can act as a ‘stand alone’ covering whereas the latter requires a secondary layer of protection behind the top layer. This is typically where you have tiles or slates with a layer of vapour permeable sarkinga waterproof layer in a roof which acts as backup protection in case the main layer fails. Usually a membrane or board and usually situated between roof laths and joists. behind.
The sarking (usually felt or non-woven plastic membrane) catches any water which might get past the slates and runs it out to the eaves.
In England the sarking is usually stretched between roof joists whereas in Scotland it is required to be laid on a layer of sarking boarding because of the increased exposure levels to wind and rain.
With a pitched roof which is not forming part of a habitable roomA room used, or intended to be used, for dwellinghouse[for the purposes of part B (fire prevention) of the Approved Documents] -
A unit of residential accommodation occupied (whether or not as a sole or main residence):
a. by a single person or by people living together as a family
b. by not more than six residents living together as a single household, including a household where care is provided for residents. (See also paragraphs 0.22 and 0.23.)
Dwellinghouse does not include a flat or a building containing a flat. purposes (including; for the purposes of Part B of the Approved Documents in the Building Regulations, a kitchen, but not a bathroom). then the insulation can simply be at ceiling[for the purposes of part B of the Approved Documents] - A part of a building which encloses and is exposed overhead in a room, protected shaft or circulation space. (The soffit of a rooflight is included as part of the surface of the ceiling, but not the frame. An upstand below a rooflight would be considered as a wall.) level whereas if it is habitable then insulation is required within the thickness of the roof covering. See ‘How much Insulation?
If it is a flat roof then the insulation will have to be within the roof thickness and there is a choice about whether to have a ‘warm’ roof or a ‘cold’ one.
Whichever the case, the amount and type of insulation will be determined by the SAPStandard Assessment Procedure - the method used in the building regulations for calculating the energy use of a house. see Part L and SAP calculations for the whole building. Bear in mind that the roof is normally the easiest and cheapest place to increase insulation levels. It may be worth considering the effects of decrement delayThis relates to the lag time that insulation itself takes to heat up or cool down. It introduces a delay into the effect of the insulation. This can help level out peaks and troughs of temperature. See the section on Decrement Delay on roof mass and insulation.
If a roof is to be part of a habitable area then it may be important to provide some degree of sound insulation against such such things as street noise[for the purposes of part E of the Approved Documents] - Noise is unwanted sound., aeroplanes etc.
Providing living space
Apart from the normal attic roof space there is also the slightly less usual mansard roof. See Timber roof structures
Expressing the character of a house
A bit like a hat, a roof can say a lot about what is underneath. The outstanding book A Pattern Language includes several sets of patterns concerning roofs:
- Cascade of roofs
- Sheltering roof
- Roof Garden
- Roof layout
Flat roofs first came into vogue in the UK as a result of the Modern Movement around the 1930s. Purity of shape, form, materials etc. was everything and a flat roof was seen as a pure, minimal expression of shape. The Modernist style never took a grip of the British imagination in the same way it did in much of the rest of Europe. People preferred to hark back to a romantic past, preferably Arts and Crafts, which was itself a harking back to some previous golden age.
flat roofs are an interesting subject because they can express a certain sort of character to a house, which is not always to the liking of the Great British public. They can also be the basis for some kind of a roof garden. They can occasionally be useful on extensions if a sloping roof would block off a window or obscure a view. And they can leak!
Although they have never been particularly popular on houses they may offer the opportunity to incorporate a roof garden which can be particularly attractive in an upstairs flat or studio area because of the private and immediate access it gives to a garden area or terrace. Depending on how load bearing the structure is, this can be intensively or extensively planted. (Extensive ones, which may be similar to living roofsA roof with a covering of soil or growing medium and plants. They tend to be divided into turf roofs with a 150mm layer of soil and sedum roofs with a thinner layer (about 40mm). see Living Roofs have quite shallow growing medium, say 50 to 150 mm while intensive ones can have quite deep areas of soil which are properly drained and can accommodate small trees. They need a specially designed structure for support.)
These kind of rooftop gardens need not be confined to new buildings. It is a relatively simple job to incorporate such outdoor areas into traditional pitched roofs as in the simplified diagram on the right. In this case the flat outdoor area is cut back into the roof and has little visual impact from ground level and therefore the planners give less grief. Of course there are several important considerations from the Building Regulations point of view. The whole area has to be treated like a roof. The floor has to be properly drained and have a wearing surface which will not be damaged by the odd bit of over enthusiastic gardening. It must also be structurally up to the job and properly thermally insulated from the room below (and possibly acoustically). The flanking walls have to be treated as external walls and guarding is necessary to prevent falling etc.
But what a nice spot for a bit of sun bathing.
The changing roof
Extensions and modifications are constantly being made to houses and this often involves changes to roofs. (see Lifetime homes ). Especially with major extensions the junction between an existing roof and a new one can be quite problematic unless some thought has been given to it at the original design stage. For instance, possibly one of the worst outcomes is the creation of a valley between two adjoining roofs in the proximity of trees.
The valley gutter is quite a challenge to construct well anyway (especially where condensation on the underside might be a problem) and is prone to blockage due to accumulations of leaves etc. This creates leakages down the inner wall rather than simply an overflowing gutter which is the normal case at the eaves. It is quite common to see farm house extensions with this configuration.
The Building Regulations part A covers the structure of a building. This Approved Document goes into a lot of detail for traditional masonry buildings but almost none for timber frame, steel frame, earth building SIPsStructural Insulated Panels - prefabricated (usually in a factory) timber panels often forming part of an integrated building system and aimed at fast site erection. see more on SIPs etc. For these you will need to consult a structural engineer (while SIPs structures are usually handled by the manufacturer)
With most forms of construction there will be implications concerning fire safety. These are covered in the Building Regulations and you can see examples of how to conform with these in Part B (Fire Safety)
Site preparation and resistance to contaminants
This section, Part C, covers site remediationthe term applied to the method of dealing with pollutants and contaminants in the ground. The Building Regulations cover this in detail. See info about Approved Document C – Site preparation and resistance to contaminants and moisture along with protection from nasties which might affect the construction and occupants such as damp, rain, radon etc. There is an abridged version of the Approved Document specially for houses.
Conservation of Fuel and Power (Part L of the Regulations) concerns energy saving and it distinguishes between new dwellings (part L1A) and existing ones (part L1B).
L1A has two key sections dealing with energy saving:
- Target CO2Carbon dioxide is a gas which is given off when carbon based materials such as fossil fuels (coal, oil, and natural gas) are burned. It is called a greenhouse gas because it works like the glazing of a greenhouse and causes global warming Emission Rate (TERTarget emission rate. The minimum energy performance for a new dwelling as defined in the Building Regulations. see Target CO2 Emission Rate) )
- Buildings containing multiple dwellings
- Calculating the CO2 emissions from the actual dwelling
- CO2 emission rate calculations
- Secondary heating
- Internal lighting
- Buildings containing multiple dwellings
- Achieving the target
- Fabric standards
- System efficiencies
- Party walls and other thermal bypasses
- Thermal bridges
- Air permeability and pressure testing
- Alternative to pressure testing on small developments
The Single Ply Roofing Association produces a design guide for single ply roofing with lots of useful information.
Recycled roofing materials
EBC UK Ltd do a range of recycled slates and sheets
see also Recycled and Reused Materials