The vast majority of the timber frame buildings constructed in the UK are softwood platform framed, where the load-bearing walls are built onto the floor decks at each storey level. Most of these will use an open panel system, where the timber frame is delivered to site covered externally with a waterproof breather membrane and internal faces are ready to receive services, insulation, vapour control layer and lining.
The speed of construction that can be achieved on a timber frame site can be surprising, but the key is organisation. One important issue to sort out in advance is how the building is to meet thermal performance requirements – with timber frame there are many choices.
One of the most common and cost-effective methods is to use a 140mm stud filled with mineral wool. This gives a
U-value of 0.30W/m2K, which meets Scotland’s Building Regulations, exceeds Northern Ireland Regulations and complies with England and Wales requirements. To maximise internal space, another solution gaining popularity is to use novel insulation materials, including reflective breather membranes. These can allow stud dimensions to be reduced to 89mm, the size regularly used before the 2002 Part L changes.
Insulation that does not fully fill the gaps between studs, perhaps because of slumping or poor installation, reduces a building’s thermal performance. To avoid cold spots, leading to thermal bridging and potential condensation damage, it is important to ensure good workmanship on site and that the insulation chosen is suitable for use with timber framed buildings.
The Part E 2003 acoustics performance requirements in the England and Wales Building Regulations have forced changes throughout the construction industry, but timber frame has adapted easily. The Robust Details Part E scheme provides timber frame solutions that comply, without the need for testing party walls and floors. Additions to the options in September 2005 now allow solid timber joists to be used. Those who don’t wish to build to Robust Details and have confidence in their standards of workmanship may choose acoustic testing on site.
Because the building is essentially complete when acoustic testing is carried out, failed buildings can be difficult and expensive to improve and retest. Without obtaining specialist knowledge of building acoustics and best practice timber frame, many builders currently seem reluctant to choose this method.
The need to provide level threshold access to buildings has led to a trend of continuing the same ground level around the whole perimeter of the building. TRADA recommends that all timber should be at least 150mm above finished ground. We have found that this can be achieved using local ramping of ground at level threshold entrances, or raising the timber frame on a masonry kerb. However, far too many designers are unaware that they risk potential problems in the future – such as moisture damage at the base of timber framed external walls – if they ignore this recommendation.
It is important to understand that differential movement of timber frame in relation to independently supported cladding (masonry, for example) will occur over the first 18 months or so of a building’s life. To accommodate the vertical downward movement of the timber frame, gaps must be allowed between the independent cladding and anything fixed to the timber frame (including cantilevered timber frame), which passes over or through the cladding. Care must be taken in the design and on site to ensure gaps are left and the timber frame can shrink, settle and compress, independently of the cladding. Necessary gaps can vary from only a couple of millimetres, up to around 30mm, depending on the height of the building and the way it has been designed.
Leaving a gap is relatively simple and cost free, but if
adequate gaps are not allowed, damage can occur which may affect the structural, thermal, acoustic and fire
performance of the building. Repairs to this damage after the building is occupied can be embarrassing, time-
consuming and very expensive. Differential movement, particularly in multi-storey timber frame, really is something you ignore at your peril.
Timber frame is a time proven and very effective method of construction. In our view, close attention to detail at the design and planning stage, as well as on site, will lead to trouble-free timber frame. And don’t try to cut corners – it will only cost more in the long term.
