Does your timber cladding measure up?

The Timber Decking & Cladding Association (TDCA) is an independent, not-for-profit advisory body and is recognised as a reliable and authoritative source of expert opinion in the UK.

British standard BS805-1: 2014: External Timber Cladding Part 1: Method of specification was specifically developed to cover the manufacture of external timber cladding, up to the point the product leaves the factory.

The standard is also a very useful reference document for specifiers and users of timber cladding.

Yet it appears that the profile dimension guidance it provides is being ignored by some external timber cladding suppliers.

The TDCA has been called out to inspect several instances of cladding failures due to wrongly sized profiles. The industry needs to be aware that there are specific requirements for cladding used outdoors.

Why size matters

Profile dimension guidance is designed to prevent the occurrence of movement-related issues which can occur on external cladding applications due to the effects of moisture.
Board width to thickness ratios for all profiles is set within the range of 4:1 to 6:1, unless using timber classed as very small movement, such as modified wood Accoya.
Interlocking profiles can be particularly problematic if movement issues occur, especially with denser species.

The standard advises maximum board widths of 125mm for tongue and groove boards. Tongue width and movement gaps aligned to board widths are detailed in various tables within the standard.

When boards are too wide and/or tongues to small, disengagement is highly likely as the timber contracts during drier weather. Conversely, at wetter times of year when timber boards expand, the danger is that the cladding will pull away from the wall.

Complaints are costly in more ways than one

The best installer in the world cannot always compensate for incorrect product specification. But add poor workmanship into the mix, and the likelihood of cladding failure is increased.

The TDCA is warning suppliers who sell non-compliant profiles that they could become engulfed by complaints. Not only are complaints costly and time consuming to rectify, but they also risk damaging the reputation of the entire timber cladding industry. This is a huge shame given the environmental benefits of this natural construction material.

Improving standards

When TDCA members are considering their product ranges, they have direct access to our team of cladding professionals who can advise them so that potentially problematic profiles can be avoided.

It’s worth noting that due to the nature of generic guidance, which tends to be conservative, there are always exceptions to the rules. Some profiles which are technically non-compliant may be acceptable if there is compelling proven evidence to support fit for purpose performance.

If you supply timber cladding, be diligent and check your cladding profiles – make sure they are clearly marked if they are suitable for use outdoors.