Premier Guarantee Technical Standards Manager, John Gilbert, provides technical guidance on the treatment of Cross Laminated Timber. The recent interest in ‘modular buildings’ as a solution to the housing crisis and current skills shortage, has brought forward the potential use of ‘CLT’ (Cross Laminated Timber) as a structural panel to produce wall panels or indeed modular pods.
A number of CLT products have third party product approval for use as a structural plank for construction uses. However, it is also important that manufacturers have a quality management process to ensure consistent quality. Usually these approvals and manufacturing processes are for the solid plank and therefore full designs of the construction, including its external cladding, are required on a site by site basis. CLT as a structural timber product isn’t preservative treated. It is also difficult to ‘post treat’ the panels due to the compact layers of softwood timber making penetration of the preservative across the full cross section difficult to achieve. So, it is important that the design keeps the CLT panel completely dry, particularly at ground level and around critical junctions.
Where structural timber is to be used in an external wall construction, such as these wall panels, consideration should be given to whether timber treatment is necessary if the species of the timber isn’t sufficiently naturally durable.
The vulnerability of timber in external walls is particularly critical where the timber is positioned in certain areas, including at the horizontal damp proof course without the inclusion of a treated sole plate. Whilst the use of CLT panels in external walls is a relatively new occurrence in the UK, wall panels incorporating CLT have been successfully used in Europe.
Premier Guarantee are actively involved with the Structural Timber Association and have recently supported and endorsed technical guidance produced by the STA. The recently reviewed CLT guidance is available via The Structural Timber Association and BM Trada.
For our warranty purposes
Where projects are proposed that incorporate CLT wall panels; they must not be used with a render or other cladding system that is directly bonded to the wall panel. A drained and vented cavity must be provided. The CLT panel must be suitably protected as follows:
At DPC level
The CLT wall panels can be positioned directly onto the horizontal dpc (over the substructure walls) without a treated timber sole plate providing that:
Above horizontal DPC level