A good start in achieving excellence in construction is to choose timber frame. With sustainability such a major issue nowadays, timber frame is even more desirable.

It has a huge commercial advantage, too, by achieving weather tightness quickly – just make sure you order your windows and doors early. The National Audit Office November 2005 report “Using modern methods of construction to build homes more quickly and efficiently” also notes that timber frame buildings are completed more quickly. More information is available at www.nao.org.uk/publications.

Design for longevity. A timber frame external wall has to be able to breathe – and you have to keep the feet dry. The lowest timber should be 150mm above the external ground level and, with a properly designed and constructed wall that has a ventilated external wall cavity, the timber wall and the external environment humidity will equalise. These two concepts will prevent fungal growth and decay. TRADA Standard Details for houses and flats recommends this construction, as do Zurich and NHBC. For more information see www.trada.co.uk/bookshop.

The next question about design is how to create a level threshold without getting the feet wet. If you have a door opening where the ground slopes off to the right and left of it, moisture is going to drain out of the adjacent open perpends and will also be well ventilated, so there’s no problem. But when care homes and student accommodation, for example, require a pavement around the entire building at ground floor level, we see a perimeter concrete upstand with an 89mm wall panel fixed on top. Then the wall panel is battened out with 50mm timbers from ceiling to floor level. Rigid insulation is installed below the sole plate and 140mm of fibre insulation is installed above. This 140mm external wall also fits well with the change in Part L.

Now that Part L has been changed, using the SAP method rather than the elemental method, there are two areas where the timber frame industry can help designers achieve compliance: U-values and air permeability. You can easily achieve a U-value of 0.18 in a 240mm wall, which may be a double studded wall, but that is perhaps a bit extreme. We recommend switching from 89mm studs to a 140mm depth wall stud to achieve a 0.28 U-value. You still see 89mm walls with enhanced breather membranes, but they only achieve a U-value of 0.35, the worst you can use with SAP 2005. We do a lot of design detail evaluations and are out on site every week carrying out assessments and our

experience is that a lot of designers are changing to 140mm studs.

It is advisable to have at hand useful reference sources such as Accredited details for Part L – these can be found at www.planningportal.gov.uk. Standard Details for houses and flats recommends a continuous vapour control layer (VCL) on walls and ceilings, tight timber connections in floor zones and a seal at external/party wall to floor deck junctions to provide an air barrier. It may all be common sense, but to have these details on site also helps to ensure that the job is done properly.

Specifying ‘sustainable material’ is not enough: the timber must have chain of custody certification     Photo: Ridgeon Group

When we evaluate a design for planning permission, one of the first things we would look for is how you are transferring the loads down to the foundations and what is supporting the cladding. Cladding systems can be fixed to the timber frame where they are installed over a roofed area. Also, we ask if you are using Approved Document E (AD E) or Robust Details Ltd details to comply with Part E. If you are using AD E, the party wall cavity will be 50mm, the recommended minimum, and Robust Details Ltd requires 240mm between the face of the studs in each dwelling, which means you can have a party wall cavity of 62mm. We will check this measurement.

When the timber frame manufacturers get the architect’s drawing, they will redraw a section and plan view of the design. Engineers will design from the top down, with point loads transferring through walls and floor zones to the foundation. Remember, as architects and designers you want to think from the top down, too – consider how each element is going to be supported.

The engineers will also be responsible for a fixing schedule. On request, they should be able to supply this schedule for your building.

With the latest change in Approved Document A, all buildings are evaluated for disproportionate collapse with effective horizontal tie being justified at and above four storeys and notional removal at five storeys and above. The timber frame manufacturer should supply the main contractor with these documents when building medium-rise construction.

Now onto the groundwork. People often give a price without knowing it’s for timber frame. It may well be acceptable for brick and block to say that 2in is close enough, but that is not the case with timber frame. Ideally we recommend:

• length/width of foundation within +/- 10mm

• diagonals of foundation within +/- 5mm up to 10m, more than 10m +/- 10mm

• level of foundation within +/- 5mm from datum

During training days we suggest to the main contractors to keep the splash course down until the sole plate is installed, just because the medium of concrete isn’t millimetre perfect. If the foundation is, maybe, 10mm too wide, or falling short by 20mm, when the splash course comes up to sole plate level, which is the lowest timber, the one that’s fixed to the foundation, the external wall cavity can start at the required 50mm.

Sixty minutes’ fire resistance is required between dwellings, so care must be taken if services are installed on the party wall. Typically we see 38mm timber noggings and plasterboard installed behind and around service points with intumescent mastic installed where the cables penetrate the timber and plasterboard to provide the required fire resistance. You can have services on party walls in England and Wales, when they are finished properly, to provide the required fire and acoustic performance.

BSRIA/TRADA publication Services in Timber Framed Construction is a useful guide for the main contractor. See www.trada.co.uk/bookshop.

Make sure too, that you have technical information on any engineered products being delivered to site for drilling and cut out tolerances. Timber joist and stud drilling and cut out tolerances are important too.

The main contractor must know how much weight can be put on the joists, too. When using timber joists, or if proprietary I-joist or metal web joists are used, ask the timber frame manufacturer for information or a technical booklet that will give you the maximum loading. Timber joist loads are typically calculated by the timber frame engineer, or they may use the proprietary I-joist maximum loading.

As for owners or architects, just putting down “sustainable material” as a requirement does not mean that the timber frame is from a well-managed forest; the timber must have chain of custody certification.

You have to be careful what wording you use in your specification, too. You can ask for the timber to be graded to a certain British Standard, but do you know what you are asking for? BS 5268-2:2002 states that, in compression, members’ bow and spring should be limited to approximately 1/300 of the length, so that means that over a 2400mm stud, the spring can be 8mm. Imagine trying to put a counter top against that – or tiling. We would ask designers or main contractors to bring that tolerance down; otherwise expensive remedial work might be needed. It is good practice always to check the timber stud spring before the insulation is installed.

Finally, I do believe that to deliver excellence, you need training. It might not be frameCHECK that you use to deliver your timber frame training, but please bring someone in to talk to you about it. We cannot overstate the value of training, so that everyone involved with a timber frame project understands the concept and their role in delivering excellence.