The Timber Decking & Cladding Association (TDCA) answers the common timber cladding questions its technical team are often asked.
External timber cladding is an arrangement of boards covering the exterior of a building. It provides extra protection against the elements and also adds an attractive design feature to the building.
In the UK, timber cladding on occupied buildings should be designed as a rainscreen – meaning it is fitted over a drained and ventilated cavity (ventilation top and bottom). The cavity is created by the battens on which the cladding boards are fixed.
Material selection, design and installation techniques are key to ensuring an effective and well-performing application. Timber is a natural material, so the cladding design must accommodate natural movement, as well as avoiding moisture traps to facilitate drying following inevitable periods of wet weather.
What material and profile options exist?
The timbers used to manufacture cladding boards are selected for their appearance, durability and suitability for use in an above-ground outdoor environment. This includes pressure-treated softwoods, naturally durable softwoods like western red cedar, naturally durable hardwoods such as Cumaru, and modified wood brands including Accoya and Abodo. Coloured and textured factory finished products are also on the market. Lifespans can range from 15 to 60 years.
Profile styles are varied and include close-jointed styles such as tongue and groove, shiplap and featheredge, rectangular for board-on-board arrangement, and parallelogram for open jointed designs. The suitability for vertical, horizontal or diagonal arrangement is dictated by the profile style. For example, board-on-board is only recommended for vertical arrangements, and parallelogram for horizontal use.
Does profile size matter?
Guidance on profile dimensions is given in British Standard 8605 – External Timber Cladding part 1: Method of specifying. For all profile styles, a board width-to-thickness ratio of between 4:1 and 6:1 is recommended. It’s possible to go higher than the 6:1 ratio, but only with timber species classed as ‘very small movement’ (e.g Accoya), and lower than 4:1 if the additional material and installation costs are acceptable. Board characteristics and dimensions, such as movement gaps and overlaps, are also detailed in the standard.
What kind of battens should be used?
The preferred batten material is softwood; usually pine, pressure-treated with a wood preservative to a Use Class 3 specification for external use above ground. Roofing battens are not suitable as they are only treated to Use Class 2 specification.
Is a breather membrane needed?
The cladding assembly should be designed and constructed as a rainscreen, meaning the external timber cladding should be separated from the exterior insulation (or backing wall) by a drained and ventilated cavity.
Some water will always penetrate the cavity, so the rear face of the cavity needs to act as a secondary drainage plane. This is usually termed a ‘breather layer’, constructed using a breather membrane or equivalent moisture-resistant, vapour-open material, protecting against wet weather penetration while allowing moisture to escape.
In some instances a breather membrane may not necessary. Where a masonry construction (stone, brick or breeze block) has been specified and constructed for external conditions, no additional weather protection is needed – presuming the surface is sound, with a rendered finish or coated with a water repellent. If, however, the surface is loose (eg, an old stone wall) then a breather membrane is an economical way of providing a drainage plane. The membrane can be held in position by the cavity battens.
Where can I find out more?
The TDCA is a not-for-profit, independent trade organisation that provides technical advice to the timber industry, working closely with TDUK to promote best practice around timber decking and cladding products. More information is available at www.tdca.org.uk
- This article was originally published in Issue #01 of Supplying Timber magazine.