Landscape and exteriors: cladding Avoiding common clangers

Avoiding common clangers

Janet Sycamore identifies how to rectify issues that can affect a cladding project.

Most cladding issues can be avoided through careful design and installation; they can be awkward and costly to rectify once a project is complete.

Fixing faux-pas

If a cladding board becomes detached from the wall or substructure, something is wrong – and it can be a mistake made with the fixing. Cladding fixings must be correctly specified and placed at specific intervals to properly secure the timber – typically dependent on timber species, board profile and arrangement.

Only high-performance coated steel or stainless steel specifically designed for use on external timber cladding should be used. Unsightly black stains (iron staining) are often a telltale sign that incorrect fixings have been used. Such staining is not easy to eradicate and ruins the aesthetic – and in the longer term there is potential for the fixing to consequentially fail.

Figure 2: An example of board-on-board fixing issues. Photo: TDCA

Fixings placed too close to the board ends is also a common problem as the boards are more prone to splitting and becoming detached from the wall. As a general recommendation, fix at quarter points of board width and a minimum of 20mm in from the board end. Use two fixings at each fixing point unless the board is less than 100mm wide when one fixing can be used.

Figure 2 illustrates two issues – the fixing type and the fixing placement. The black streaks are caused by using nails unsuitable for outdoor use and the top board is fitted through the bottom board, restricting movement and going against good practice recommendations.

Timber is natural and it moves

Wood is hygroscopic – its moisture content is affected by changes in temperature and relative humidity of the surrounding environment. This causes movement across the grain of the timber. Timber cladding should be installed with a moisture content of 16% +/- 4%. This is the mid-range for timber conditions outdoors, which can fluctuate from around 10% to 22% depending on the season. Different species have different degrees of movement and are assigned to movement classes. Ideally, cladding timber should be classed as ‘small’ or ‘medium’ movement. It is also imperative that the design and installation will accommodate the natural seasonal movement of the timber by incorporating the correct movement gaps and/ or overlaps for the profile and species used. >>

This is particularly relevant to interlocking cladding profiles such as tongued and grooved (T&G). There is a temptation to fit these boards tightly together, but problems are inevitable if you do, such as bowing or pulling away from the wall. For medium movement species, the recommended maximum width for a T&G board is 125mm, with an associated minimum tongue width of 15mm, to limit the potential for movementrelated issues.

Figure 3: Minimum movement gaps for T&G and shiplap profiles and minimum tongue dimensions for T&G profiles. Source: TDCA

Figure 4: The correct arrangement for timber battens with vertical timber cladding. Source: TDCA

What’s going on behind the cladding is just as important

Battens

A well-ventilated, free-draining cavity should always be included when installing cladding – to channel any moisture (wind-deflected rain) back to the building’s exterior. By using a series of timber battens, a cavity between the cladding and the backing wall structure can be created. Where cladding is used in a vertical arrangement with horizontal support battens, the use of a vertical counter batten is necessary to create a drainage plain and allow free passage of water through it. Counter battens should be a least 50mm wide and 25mm thick. For horizontal timber cladding, only vertical battens are required. These battens should be 50mm wide and 38mm thick, but can be 50mm x 25mm if no side jointing is to occur. These dimensions are to help ensure the battens will accept fixings without splitting. The battens should be capable of delivering a similar or better service life than the cladding boards. Pressure-treated softwood such as spruce or pine is the go-to material for cladding battens. Pressure treatment should be to a use class 3 specification with the option of a desired service life of 15, 30 or 60 years (30 years being most common). This means that roofing battens are unsuitable as they are generally treated to use class 2 (an interior grade). You should obtain written evidence of treatment detailing appropriate use class and desired service life when you purchase your battens. It would be useful if this was mentioned in any specification notes to plans. The same goes if pressure-treated cladding boards are selected; they too should be treated to use class 3.

Breather membrane

Another common question is ‘where should the breather membrane be positioned?’ It should be placed behind the cladding battens to divide the wet and dry zones of the external wall – not between the batten and the cladding board. n

About the author

Janet Sycamore Timber Decking and Cladding Association (TDCA)

Further information

All the topics in this article and more are covered in The Timber Cladding Handbook, published by the Timber Decking and Cladding Association (TDCA). The TDCA is a not-for-profit company and the money generated from publication sales and inspection services goes towards its work to raise awareness of quality materials and installation good practice. www.tdca.org.uk

Further reading

• BS 8605-1:2014 External timber cladding – Method of specifying, BSI • External Timber Cladding, 4th edition, BM TRADA, to be published in 2022 • The Timber Cladding Handbook, TDCA and TTF, 2021. Available in digital and hard copy at www.tdca.org.uk/publications • WIS 1-49 Cladding for timber frame buildings, BM TRADA, 2018 • WIS 1-50 Timber cladding for building refurbishment,

BM TRADA, 2019