Press Release

UKTFA Publishes First New Guidance Document On Differential Movement In Timber Frame Buildings

5 June 2008

The UK Timber Frame Association (UKTFA) and wood for good have published the first dedicated guidance document on how to deal with differential movement in timber frame buildings.

The timber frame industry worked closely with NHBC to produce standard guidance that can be used across the housebuilding industry. The recommendations are now best practice to all timber frame manufacturers and designers and will also be adopted by NHBC for its revised technical standards and a new Chapter 6.2 on external timber framed walls, which housebuilders must comply with from this September.

Stewart Dalgarno, Chairman of the UKTFA, says: “The differential movement guidance is the first of our suite of new technical documents to be published from the timber frame industry. We worked closely with the industry - TRADA, NHBC, wood for good and the Wood Protection Association - to ensure we had the right knowledge, feedback and practical experience to develop the guidance.

“Timber has many benefits as a building material, but it’s well understood that as a natural product it can undergo shrinkage due to moisture content changes and how it has been cut from the tree. Cross grain timbers shrink across the width as the moisture content of the timber adjusts from an outdoor ‘site environment’ to an ‘in-service environment’. In addition, the joints in lightweight construction tighten up under load.

“The consequence of this movement can be 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 due to thermal changes, for example. It’s that difference between the movement of the timber frame and the external cladding that is called differential movement.

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

“However, as timber frame construction grows in popularity and the scale and complexity of modern buildings put new demands on designers and housebuilders, we have seen many situations where the technical detailing is no longer appropriate. We saw there was a need to produce standard guidance which could be used as best practice across the industry.”

The UKTFA’s new guidance is written for architects, contractors and checking authorities to provide a 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.

To download a free copy of ‘Differential Movement in Platform Timber Frame’, go to www.timber-frame.org