Enquiries received by the Timber Decking and Cladding Association (TDCA) highlight an ongoing issue: cladding profiles that don’t conform to established best practice. Getting the specification right at the design stage is critical to the performance and longevity of any external timber cladding, with board dimensions playing a major role.
One recent enquiry was seeking a 22x219mm European larch shiplap cladding – a board with a width-to-thickness ratio of 10:1. This is outside the limits set by BS8605-1:2014, which recommends ratios between 4:1 and 6:1, and limits board width to 150mm (except timbers such as Accoya, which can extend to 195mm due to its ‘very small movement’ classification). Unless designers, specifiers, and suppliers all understand and apply the correct standard, problems are likely to surface later down the line. A width-to-thickness ratio of less than 4:1 can also be used providing the increased material and installation costs (relative to wider boards) are acceptable. For standard species, exceeding those limits increases the risk of performance issues.
UNDERSTANDING BS8605-1:2014
BS8605-1:2014 sets out detailed guidance for the manufacture and specification of external timber cladding, aiming to minimise issues caused by moisture-related movement. It describes the key profile characteristics needed to help cladding perform as intended.
The standard’s guidance is based on extensive research and was developed by Dr Ivor Davies, a leading UK timber cladding specialist, with input from an expert subcommittee. Although a Part 2 (covering installation) was never published, other trusted documents now fill that gap, including the Timber Cladding Handbook (produced by TDCA and Timber Development UK).
NATURAL MOVEMENT
Timber is a natural material that expands and contracts predictably across the grain as its moisture content changes with the seasons – referred to as ‘movement’. Correct profile design and installation will ensure that natural movement is accommodated but deviations from best practice could result in distortion issues.
Profiles such as tongue and groove (T&G) are a particular consideration because the boards are interlocked. The standard limits T&G board widths to 125mm unless a very low movement timber is used. If boards are too wide or the tongue too narrow, expansion and contraction can cause tongue disengagement or the cladding to pull away from the wall.
ROOM FOR EXCEPTION
There are occasions where profiles that fall outside BS8605’s dimensional guidance may still be suitable but only where robust, independent test evidence supports their performance. The TDCA advises that any deviation from standards should always be supported by technical documentation or certification.
DETAILING MATTERS
While proportion and profile are critical, other detailing factors have a major influence on the lifespan of cladding systems.
Timber cladding in the UK is designed as a rainscreen system, which means it must be installed over a drained and ventilated cavity to allow moisture to escape. Adequate ventilation openings should be provided at the top and bottom of the cladding, and support battens (or counter battens for vertical cladding orientation) should maintain a continuous drainage path over a waterproof, breathable membrane.
The National House Building Council (NHBC) has previously reported that many cladding systems underperform due to a lack of drainage and ventilation detailing, inappropriate film-forming coatings that trap moisture, and poor fixing practices. These are all issues that can be prevented with correct design and adherence to best practice.
Moisture-related problems and movement failures are entirely avoidable. By following the dimensional and detailing guidance set out in BS8605-1:2014 and TDCA’s Timber Cladding Handbook, specifiers can ensure their projects deliver long-term performance and visual appeal.
Alongside its technical resources, TDCA’s CladMark quality assurance scheme identifies products and installers that meet rigorous performance and quality standards verified through yearly audits. Choosing CladMark approved materials and installers provides confidence that products are manufactured under strict quality management systems, and that installers are experienced and trained to fit cladding correctly.
www.tdca.org.uk