Analysis of new timber-frame technical guidance

Timber as a building material has many benefits. But because it is a natural product it can shrink because of moisture content changes and how it has been cut from the tree.

Cross-grain timbers shrink widthways as the moisture content of the timber reduces from construction to in-service environment moisture levels. In addition, the joints in the lightweight construction tighten up under load. Consequently, this movement means that timber-frame structures reduce slightly in overall height during the first two years of use.

The external leaf of buildings also tends to move slightly - brickwork may expand following thermal changes, for example. It's that difference between the movement of the timber frame and the external cladding that is known as differential movement.

With the correct design and construction, this differential movement does not cause a problem. Many design professionals are familiar with the need to incorporate movement joints.

Education process

However, as the use of timber frame increases, there is a need to continue the education process and provide technical detailing support for the scale and issues that modern building layouts encounter.

Defects caused by insufficient movement joints, particularly in multi-storey buildings, can become a claim on the third-party approval body if not considered.

It was for this reason that National House Building Council (NHBC) and the UK Timber Frame Association (UKTFA) agreed on a joint project to produce standard guidance that can be used across the industry.

So last year work began on new guidance, looking closely at practical experience and industry feedback. Knowledge from TRADA, NHBC, Wood for Good and the UKTFA was pooled together to arrive at key interface detailing at cladding and internal finish junctions.

The resulting recommendations, published last month, are now best practice to all timber-frame manufacturers and designers. They will also be adopted by NHBC for its revised technical standards, with a new Chapter 6.2 on external timber framed walls, which housebuilders must comply with from September this year.

Guidance document

The differential movement guidance document is a joint UKTFA, Wood For Good and NHBC-funded project written for architects, contractors and checking authorities. It provides the first comprehensive overview of the principles on vertical movements of timber frame for any level of building.

It includes 44 design details and technical specifications showing the appropriate movement gaps in a building, and advice on how to ensure defects caused by lack of movement joints are avoided.

Eleven detailed areas of a build are featured in the document, presenting best practice for the interfaces of platform timber frame and connected materials and claddings.

These include around windows, balconies, roofline eaves and verge, drive-throughs, stairs and common areas, lift shafts, chimneys and fireplaces, and solutions for situations where builders may be using non-masonry cladding.

There is also advice on the movement joints required where timber-framed extensions are built onto existing masonry buildings or older timber-frame structures.

Timber-frame manufacturers can help to modify the degree of differential movement by removing cross-grain timber from the frame and by using timber with a reduced initial moisture content (standard structural timber has about 20% moisture content).

It is good practice for builders to tile the roof and load-out floors before starting external claddings to help close up normal construction gaps before the external masonry cladding is built.

But the key to avoiding that shrinking feeling is to get the design right in the first place. This new UKTFA technical guidance should help the construction industry do just that.

Fire safety and zero-carbon homes

Another new technical report on structural issues such as disproportionate collapse has also been published. This will help timber-frame designers and engineers comply with the revised BS5268, which included new clauses to help meet the requirements of Part A of the Building Regulations (revised in 2004).

These regulations now bring the UK into line with European Code proposals, and continue to require certain provisions for buildings of five storeys or more.

Further reports are scheduled this year on fire safety on sites under construction, and on zero-carbon homes. On the fire safety issue, expect to see a range of commonsense measures to ensure secure and tidy sites with appropriate access.

The guidance is also likely to recommend options for specifying containment measures on timber-frame projects whenever this is required - typically following the outcomes of the client and architect's fire risk assessment.

Vertical containment measures in timber-frame buildings are considered to be one of the most logical and practical solutions for high-risk sites once all the usual site security, fire detection and suppression measures have been exhausted.

For smaller or low-risk sites the general good housekeeping and standard approach is more appropriate and proven in practice.