Info Brochure WKW10 1 Timber Cladding in Wales
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1 Information brochure WKW10-1 T imber cladding in Wales Dennis Jones Project officer, Woodknowledge Wales (WKW) Cladding buildings with timber in Wales is becoming fashionable, due to increased interest from architects and designers. The Welsh timber trade can play a major part in providing quality timber for this sector, instead of the use of imported materials. This information brochure aims to provide technical guidance on how to best use locally grown materials, and to achieve the high service lives expected by the end-user. Collated from the major information sources and using the in-house expertise of Woodknowledge Wales (WKW) partners, this information brochure will provide the first port of call for people wishing to use home-grown timbers for exterior cladding. Desired properties Ideally the timber chosen for cladding should exhibit the following characteristics: Desired properties of home grown timber for cladding Property Description Movement Low to moderate Natural durability Durable to slightly durable Coatings Moderate coating uptake, extended maintenance periods Weathering of uncoated cladding Uniform greying on exposed areas Surface and Workability Easy to process, low blunting of machinery tools, smooth finish possible on cladding surface Many or all of these properties can be readily achieved with home grown timber. More information can be found on the Woodknowledge Wales Images courtesy of: Top: WKW website: http://www.woodknowledgewales.co.uk or by contacting Dennis Jones (Tel: 0777 2228711; E-mail [email protected]) Middle: Hughes Architects Bottom: Ivor Davies 2 Material choice There are a wide range of factors to consider when choosing timber cladding. These include The main home grown species that species choice and whether the timber needs may be considered for exterior to be protected. timber cladding are: – Oak (Quercus robur) – Larch (Larix spp.) Ideally, timber cladding should have a service – Sweet chestnut (Castanea life of at least 30 years, and the best way to sativa) achieve this is through the use of a more – Scots pine (Pinus sylvestris) durable species. The order of natural durability – Sitka spruce (Picea sitchensis) of home grown timbers is: – Norway spruce (Picea abies) Oak, sweet chestnut > western red cedar, – Douglas fir (Pseudotsuga larch, Douglas fir > Scots pine, Norway spruce, menziesii) Sitka spruce. – Western red cedar (Thuja plicata) Heartwood NOTE: All sapwood (darker area on picture), regardless of species chosen, is perishable, and should not be present on cladding, as this will rot and fail far quicker than the rest of the board (between 6 months to 20+ years Sapwood depending on the moisture load). This is a more common problem with waney edge Waney edge (picture courtesy of Ivor Davies) boards. It is possible to increase the service life of cladding by undergoing wood protection. The most common form of wood protection is through the use of preservatives, though innovative methods are now available through wood modification. Acetylation, in effect a pickling process for wood using a reactive form of vinegar, causes a permanent chemical change in the wood cells, resulting in a material that is highly stable and durable. The process has been commercialised in The Netherlands and is marketed as Accoya ®. This is sold in the UK under licence. Accoya® cladding (picture c ourtesy of Titanwood) Thermal modification changes the way the internal components of wood behave, again increasing its stability and durability, as well as improving machining properties. The major commercial developments have taken place in The Netherlands and Finland. Now there are operating thermal modification facilities in Monmouthshire and Anglesey, one coming on line in 2010 in the Heads of the Valleys and another being considered in Pembrokeshire. The main focus of the Welsh facilities will be for treatment of softwood species such as Corsican pine, larch and western hemlock for Heat treated Sitka spruce products such as cladding, though other species are also cladding under test (picture being treated and tested. courtesy of BRE) 3 Style of cladding There are a variety of styles of timber cladding available. Horizontal Boards Depending on the final surface requirements, various designs are possible. These include overlapping, feather or square edging or rebated feather edging. In addition to tight fitting horizontal cladding, looser panelling is also possible. This is more suitable for green timber, because it masks minor distortions. Wider spacing can also be used to provide shading systems for buildings. The way in which the panels are attached may also alter the appearance of the cladding. Board ends can finish along the same vertical line, or in a staggered effect (similar to conventional brickwork). The method chosen will determine the number of battens required behind the cladding. Diagonal Boards Diagonal boards (laid out at 45o to the horizontal) are mainly used cosmetically; designs normally require long cladding pieces. There can be a risk of wet rot if not designed correctly, especially at the point where the different diagonals meet (i.e. the design should be such that the V points upwards to minimise moisture collection). Whilst the example shown here is at a dock, diagonal boards are a common feature on a front gable of a house. Picture courtesy Akzo Nobel Vertical Boards Vertical cladding is commonly fitted using either a tongue and groove system, or using a board on board method. Usually vertical cladding is only used if boards of sufficient length are available (though this may be overcome by finger jointing shorter panels). Additionally, there needs to be sufficient overlap to compensate for shrinkage (so that no gaps are revealed). The less-favoured open-faced vertical cladding (i.e with gaps between the cladding pieces) has increased moisture risks due to rain passing between the boards (compared to horizontal systems). Picture courtesy West Coast Energy Company Consideration needs to be given to the risk of swelling in the lower sections of the timber cladding, where water draining off the board might be absorbed by the end grain. This may be countered by covering the bottom of the boards with horizontal flashing. For all types of cladding, the lower sections need to have some protection against splash-back from the ground. The easiest way to achieve this is to ensure the cladding stops approximately 15cm from the ground. It will also be further helped by not having hard surfaces (paths, driveways) flush to the wall of the property. 4 Detailing Having selected the timber species for use as an exterior cladding, it is necessary to ensure the right quality of material. There are a range of appearance grading rules used for cladding, which deal with the number and size of knots present. Examples are given in the reference section. Ideally, timber should be relatively free from knots, especially if it is to be left uncoated. Where there are knots present, these should not be dead knots, which will fall out during service. The Larch cladding (picture presence of knots may also result in resin exudation, courtesy of BSW Timber) depending on the species used. Another key issue is the · At moisture contents < 30% timber shrinks thickness of the cladding board. as it dries and expands as it wets The use of a thicker cladding · Installation should be at approximately board will help prevent excessive 16% moisture content moisture variation, as well as · ‘Shrinkage’ describes dimensional change reducing the risk of due to initial drying warping/cupping. · ‘Movement’ describes size change due to ongoing moisture content fluctuations. It is important to remember that · Cupping can also occur when cladding is wood will continue to “move” installed whilst too wet. due to moisture uptake and release (i.e. wet in winter, dry in summer). The level of movement depends on timber Oversized drill holes (picture courtesy Ivor species and the type of Davies) treatment used. Boards must be fitted with sufficient gaps (for expansion) as well as sufficient overlap (to avoid separation during drying). · Install boards as soon as possible after board conversion o Green oak can shrink 4-7% across its width as it dries o Avoid moisture contents varying greatly from atmospheric conditions during installation · Keep it simple · Avoid profiles that are too thin o Cladding board thickness preferably ≥15mm o Thicker profiles for timbers with greater movement · Use overlapping or open joints W et installation No expansion c ausing cupping gaps causing · Ensure joints can accommodate anticipated ( picture courtesy lifting (picture shrinkage I vor Davies) courtesy Ivor · Use single fixings if possible or drill holes Davies) oversize · Use corrosion resistant fixings, e.g. stainless steel o 304 stainless steel recommended, use 316 grade stainless steel near the coast · Support battens at max. 600mm separation Blue Planet, Stoke (picture courtesy BCL Timber Projects Ltd) 5 Design specifications In order to prevent premature failure of cladding, it is important to draft detailed specifications, and follow these accordingly during installation. A range of different information sources are available, however it is always recommended that advice is sought from a company with a proven expertise in cladding prior to installation. The following show some typical examples of specifications for timber cladding. These designs may alter depending on the style of cladding being erected. Board layout and corner detail. Courtesy of Woodspec Above: Corner detail for cladding on block wall Below: Cladding detail around window cill. Courtesy of BCL Timber Products Ltd Horizontal cladding detail around a window. Courtesy of TRADA Timber Technology 6 Finishes The key issue with exterior timber cladding is whether a person wishes for it to be coated or not. Studies in Canada have shown that, whilst still invisible to the naked eye, surface damage to wood starts to occur within several hours’ exposure to UV. If erected uncoated, it is recommended the cladding should remain uncoated, and be allowed to weather (silver).