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Timber roof structures

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 SIPs, the roof structure is integral to the design of the roof cassettes so trusses, beams, joists, purlins etc. are not normally required

Truss design is usually carried out by the truss manufacturer (unlike traditional design)

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).

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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 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 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.

mansard roof

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 energy, more captured carbon and less thermal bridging.

Materials

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 beams.

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

I beams

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 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).

James Jones do a set of roof joist details and span tables for their I beams

European Standards

On 1st April 2010 the new CEN Eurocode standards for structural timber came into force in place of the old BS 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

Span tables

TRADA 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 NHBC 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

Building regulations

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 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). Part C, covers site remediation 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.

Books

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