Type of structure
Whether living space is required in the roof is the most crucial decision from a structural point of view because it decides whether you can use prefabricated gang nail roof trusses or whether you have to build a traditional roof. The roof trusses are cheaper and much faster (two people can lift and position a truss) but they preclude the use of attic space because the roof is full of small timber members which cannot be cut or modified (at least not to any useful extent). Roof trusses can also span much greater distances without intermediate support than traditional beams, purlins and joists. Over 20m. is possible compared with about 5m. for traditional structures. Of course if there is intermediate support you can get away with a lighter and cheaper truss.
With 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, the roof structure is integral to the design of the roof cassettes so trusses, beams, joists, purlins etc. are not normally required
There are many types of truss design available. Try any of the manufacturer’s web sites. North Americans tend to have a broader range of designs. For instance Construction Components Inc. do all kinds of roof profiles (including of course the Polynesian).
There is no reason these should not be available here as the manufacturer’s computer design programs can handle this.
A special design of attic space gang nail truss is available from some manufacturers (see the ‘ATTIC’ design above) but they tend to be a much heavier item than normal trusses and will require a crane to get them into place so may not be worth it. If you do want to use the attic space and go for a traditional structure then it is very likely that you will have to design in some vertical support for the ridge, running right down to the ground (or otherwise some steelwork). This is in order to reduce the overall span. It also requires that the purlins are not too long.
Typical Georgian or Victorian roof design
With both Georgian and Victorian buildings (which constitutes a great deal of existing houses) there was a need to utilise attic space (particularly for servant accommodation) so large timber purlins were used to help support the roof joists. Purlins are large timbers (typically 275mm x 75mm which span right across the room (in terrace houses, between party walls).
To the left is a typical design of a Victorian or Georgian roof with two purlins on each side, one above the 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.) and one below (sometimes there is only one purlin, depending on the total span of the roof, and occasionally there may be three). The support for the apex of the roof (i.e. where the sloping joists meet at the highest point in the centre) is provided by a masonry supporting wall which runs up the middle of the house. It is usually possible to use the standard span tables to calculate the sizes of the various timber members with this arrangement.
King post truss
King post truss. For longer spans the King post truss was often used. With this construction the top of the king post is held at point A by the two rafters which are in compression. In turn it holds up the centre of the chord (C-D) at centre point B. It also supports the bottoms of the two diagonal struts and they support the centres of the two rafters. In this way the chord and the rafters can have a larger span without bending or breaking. The Approved Documents Approved documents are detailed publications which come under the English 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). They are based on tried and tested methods of building and if you follow them you are assured of complying with the Regs. The equivalents for Scotland are the Technical HandbookUnder the Scottish Building Regulations, the Technical Handbook gives construction principles, which, if you follow them guarantee compliance with the Regulations, for Wales: the Approved documents (Wales), and for N.I. the Technical BookletsUnder the Northern Ireland Building Regulations, the Technical Booklets give construction principles, which, if you follow them guarantee compliance with the Regulations do not cover this arrangement and the design of a new truss or the repair of an existing damaged truss will require calculations from a structural engineer.
the mansard roof. This is a roof with two slopes, one very steep and the other quite shallow. Its advantage over the normal type of attic living area is the greater space it gives. At the same time it gives the impression of being a conventional sloping roof, especially if viewed from quite close. This may be an advantage if the planners are asking for a relatively low roofline. The shallow part of the roof must be steep enough to be within the constraints of the tiles or slates used and the geographic location (achieving an angle of less than 20° can be problematic).
Stairs to attics
Using an attic for storage only requires a loft ladder which can reach down to any part of the floor below whereas if you want to use it for living or working in then it needs its own stairs built to building regulation standards. Also there are implications for fire safety which can affect lower storeys
With existing houses which do not have a proper floor in the attic (only ceiling joists) these joists are not load bearing. At best they will be able to carry a small amount of storage. If you want to carry more storage or put a living area up there then the joists will need increasing in size. There are many companies which specialise in upgrading loft areas.
Structure for large spans
Compared with the structures available to Georgian or Victorian house builders, it is now possible to build in steelwork which increases the spans of open space. Better still is the use of engineered timber such as Glulam because there is less embodied energythe total amount of energy it takes to make a material (or a building). See more on embodied energy, more captured carbon and less thermal bridgingthis is a pathway where heat can easily escape (or get in) through some part of the structure. It is usually caused by some element of structure such as a steel lintel or wooden studwork. Also known as a cold bridge. see more on thermal bridging.
From a green standpoint the main thing to remember is that timber should be woodmarked and sourced as locally as possible, preferably in the UK.
Structural timber has the advantage over most other structural materials of being renewable and also a relatively good thermal insulation material.
Structural timber is usually of the following types
- normal softwood sections such as floor and ceiling joists as specified in the building regulations.
- larger sections of softwood or hardwood forming beams, trusses etc. and usually calculated by a structural engineer.
- laminated structural timber
- engineered timber such as I beamsprefabricated timber beams with a web of OSBOriented Strand Board and softwood flanges. They are very light, efficient and regular.
The normal softwood sections are usually graded as C16 or C24 (respectively replacing SC3 and SC4) and this will be stated in the approved building regulation drawings. It is often available from UK plantations
Larger sections of softwood are also available from the UK and it is often possible to get them from local sawmills.
The larger hardwood sections may be more of a problem to source sustainably and in the case of tropical hardwoods, only the woodmark can be relied upon. (However it is seldom that tropical hardwoods are used structurally in housing). There is a considerable amount of timber produced sustainably in the UK which is not woodmarked mainly because it is produced in such small quantities that the certification procedure would not be warranted. E.g. there is a constant supply of hedgerow ash (although this might become threatened by the recent outbreak of Chalara) and to a lesser extent oak and other species which is not woodmarked but which gets replaced. There are also organisations such as Woodlots which may be of help in sourcing local timber.
Laminated timber (sometimes known as Glulam) is generally well sourced environmentally. However, due to the poor understanding of timber building culture in the UK it has been marketed mainly towards large structures such as offices, swimming pools, theaters etc rather than the housebuilding market, so it can be difficult to find merchants who are supplying off-the-peg structural members. The Glued Laminated Timber Association has a list of member companies. Also try Panel Agency Limited and Lamisell
Engineered timber such as Masonite beams represents a huge step forward in timber technology. Compared with traditional beams and joists Masonite sections are, for the same structural strength, much lighter, much more dimensionally regular and use considerably less timber. The dimensional stability with traditional beams and joists can be a major problem if they are not supplied at a moisture content of 12%, as shrinkage can cause considerable movement. It is not unusual to hear of 20mm movement over a two storey[for the purposes of part B (fire) of the Approved Documents to the Building Regulations] this includes - (a) any gallery[for the purposes of part B of the Approved Documents] - A raised area or platform around the sides or at the back of a room which provides extra space. if its area is more than half that of the space into which it projects; and (b) a roof, unless it is accessible only for maintenance and repair. timber structure in the first year . They also help with the insulation because the webs, being much thinner, cause minimal thermal bridging. They do however run at about twice the price of solid timber sections with merchants such as Arnold Laver quoting around £4.80/m for 220 x 38 I beams. (supplied in 12m lengths).
On 1st April 2010 the new CEN Eurocode standards for structural timber came into force in place of the old BSBritish Standard standard. These are –
- BS EN 1995-1-1 Eurocode 5: Design of timber structures. Part 1-1 General – Common rules and rules for buildings
- BS EN 1995-1-2 Eurocode 5: Design of timber structures Part 1.2 General – Structural fire design
TRADATimber Research and Development Association A trade association with a strong reputation for research and publication on all things timber have published span tables in a new softback book called Eurocode 5 Span Tables: For Solid Timber Members in Floors, Ceilings and Roofs for Dwellings and various companies do online calculation software. However for practical purposes the self builder will still find the span tables in the old (archived) Approved Documents from 1992 to be useful in determining sizes for floor, ceiling and roof joists, binders, rafters and purlins. There is extremely little difference between the old span tables and the new ones. The slight discrepancy is mainly in spans shorter that 2.4m. See more about the BS to Eurocode changeover on the Timber in Construction web site.
Pitched roof construction
The NHBCThe National House-Building Council describes itself as being "the leading warranty and insurance provider and standards setter for UK house-building for new and newly converted homes" do a useful pdf on the technical requirements for pitched roofs
Reciprocal frame roofs
see the ‘Living in the Future’ video showing the practical aspect of building such a roof featuring Tony Wrench
The Building Regulations part AThe Approved documents, (England) part A, deals with building structures 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 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 BThe Approved documents, (England) part B, deals with fire (Fire Safety). Part CThe Approved documents, (England) part C, deals with Site preparation and resistance to contaminants (C1) and Resistance to Moisture (C2), 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.
Approved document L1B, concerning extensions and alterations to existing dwellings has quite a lot to say about changes to roof loadings as roofs are altered and repaired. There is a useful Building Control Guidance Note on this.