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If you’re using softwood, whether spruce (whitewood) or pine (redwood) you need good quality, slow growth timber, typically from Sweden and other Nordic countries. Its moisture content should be between 16% and 19%.
Both pine and spruce need to be preservative pressure-treated to Use Class 3, even if you are going to paint them. Make sure the surface of the wood is thoroughly dry after treatment, otherwise any decorative coating won’t adhere well.
Other softwoods, such as Siberian larch, Douglas fir and Western red cedar may be used untreated, but you will need to specify heartwood timber (the sapwood excluded).
Many hardwoods, such as sweet chestnut, red louro and yellow balau can also be used untreated, as can modified timbers like Accoya™. In the latest version of EN350, European oak has been re-classified as Durability Class 2-4; to ensure oak is suitable for cladding, verify it meets Durability Class 2-3.
Some modified timbers and high tannin species, such as oak require special stainless steel fixings.
There are two main things to remember about timber cladding:
Getting the detailing and fixing right are as important to the life of the cladding as choice of species.
BS 8605:2015 is the standard covering external timber cladding.
Refer to Wood Campus specification sheets for design and fixing details for different applications.
Wood’s moisture content will change relative to its surroundings. Different species have different degrees of movement and this must be accounted for in timber cladding design.
Timber cladding should have a moisture content of 16% ± 3% at the time of installation. Wood expands across the grain, rarely along it. Oak, pine and spruce will change 1% in dimension for every 4% change in moisture.
Good design and installation practice will help minimize the effects of moisture:
There is a wide choice of standard profiles available in softwoods, modified woods and hardwoods typically up to 150mm in width. Are you laying the boards horizontally, vertically or diagonally? Check which profiles are suitable.
Styles can vary from one manufacturer to another, so always obtain samples and agree quality parameters before ordering.
If you need a bespoke profile or specific species, contact a member of the TDCA.
Only specify cladding manufactured under a recognised quality scheme such as the TDCA CladMark/ISO09001.
Thickness is largely determined by the profile required. Guidance is available in British Standard BS8605: External Timber Cladding Part 1: Method of specifying.
Timber Cladding can be applied in any one of three directions, which will have an impact on the methods of fixing as well as on the overall look of the façade.
Available with overlapped or profiled sections.
Shiplap or feather edge boards should have a minimum 15mm overlap, and 2mm gaps between the up-stands. Tongue and groove boards should have a maximum face width of 125mm, with a 2mm clearance above the tongue for expansion. Install with the tongue upwards.
Open joint boards should have an 8-15mm gap at the ‘water face’. Chamfered edges allow the boards to overlap slightly, reducing any exposure of the cavity.
Board-on-board, shiplap and tongue and groove profiles are used. Tongues need to be long enough to allow for slight movement of the timber so that open gaps do not develop with atural movement. The face width of tongue and groove vertical boards should not exceed 125mm.
The most versatile fixing method is board-on-board. Any overlap should be a minimum of 20mm.
Overlapping boards are not suitable. Shiplap boards are advisable and should be fixed to battens. Use counter-battens to provide ventilation.
Avoid sharp edges on cladding. A minimum radius of 3mm on the bottom of horizontal cladding will help shed water and prevent it tracking up the back of the cladding.
Use softwood battens that are preservative treated for a BSEN335:1 use Class 3 application and structurally graded to ensure they are able to carry the weight of the board material.
Fix vertical boards to horizontal battens, with vertical ‘counter’ battens to facilitate drainage and ventilation. Horizontal battens should have the top edge machined at an angle to help shed water into the cavity.
Specification sheets for design details are available on Wood Campus:
Create a well-ventilated cavity of at least 21mm. The more open the cladding style, the wider the cavity required to protect against moisture penetration.
On timber frame buildings, the minimum sized batten (21mm) may be used so long as its position coincides with wall studs.
Timber frame buildings: The inner wall structure should be fitted with a breather membrane to seal the building against damp and weather penetration. The membrane should be durable and tear resistant in accordance with Type 1 membranes in BS4016.
Masonry buildings: Fitting a breather membrane between cladding and battens attached to a property with cavity walls is not essential. Use a waterproof coating, membrane or wax-treated insulation board where cladding is fitted to an existing building with solid walls.
Some modified timbers, such as Accoya™, are acidic and require high quality stainless steel fixings.
Use a specialist water-based micro-porous coating, opaque or semi-translucent. Coatings adhere better to sawn than planed boards. Re-coating before failure of the system means you don’t have to spend a lot of time preparing the surface. Pre-finished boards are widely available.
Broadly speaking, Building regulations allow untreated timber on buildings less than 18m high but do not allow any combustible materials for cladding on residential buildings over 18m high.
The Confederation of Timber Industries recommends that all timber-based cladding and balcony components should be treated using a quality-assured factory-applied flame retardant to Euroclass B, unless shown not to be necessary by an appropriate risk assessment process.
Generally, this should be fire treated to Euroclass C-s3, d2.
See Cladding resources on Wood Campus.
See Cladding resources on Timber Decking and Cladding Association website www.tdca.org.uk
Timber is the most sustainable mainstream building product. It is naturally renewable. Over 90% of timber used in UK construction comes from Europe, where more trees are grown than harvested (source: TTF Statistical Review 2016).
Softwood and temperate hardwood forests in Scandinavia, Europe, Canada and North America are stable or growing. Growing forests act as carbon sinks; wood products act as carbon stores.
Ask for PEFC or FSC Chain of Custody certification.
See Wood Campus RIBA CPD module Procuring Sustainable Timber for more on timber certification and sustainability and government requirements.