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Information brochure WKW10-1

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.

Timber cladding in Wales

Images courtesy of: Top: WKW Middle: Hughes Architects Bottom: Ivor Davies

More information can be found on the Woodknowledge Wales website: http://www.woodknowledgewales.co.uk or by contacting Dennis Jones (Tel: 0777 2228711; E-mail jonesd@bre.co.uk)

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Material choice There are a wide range of factors to consider when choosing timber cladding. These include species choice and whether the timber needs to be protected. Ideally, timber cladding should have a service life of at least 30 years, and the best way to achieve this is through the use of a more durable species. The order of natural durability of home grown timbers is: Oak, sweet chestnut > western red cedar, larch, Douglas fir > Scots pine, Norway spruce, Sitka spruce.

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 depending on the moisture load). This is a more common problem with waney edge 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.

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 products such as cladding, though other species are also being treated and tested.

Accoya® cladding (picture courtesy of Titanwood)

Heat treated Sitka spruce cladding under test (picture courtesy of BRE)

The main home grown species that may be considered for exterior timber cladding are:

– Oak (Quercus robur) – Larch (Larix spp.) – Sweet chestnut (Castanea

sativa) – Scots pine (Pinus sylvestris) – Sitka spruce (Picea sitchensis) – Norway spruce (Picea abies) – Douglas fir (Pseudotsuga

menziesii) – Western red cedar (Thuja

plicata)

Waney edge (picture courtesy of Ivor Davies)

Heartwood

Sapwood

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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. 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). 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.

Picture courtesy Akzo Nobel

Picture courtesy West Coast Energy Company

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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 presence of knots may also result in resin exudation, depending on the species used. Another key issue is the thickness of the cladding board. The use of a thicker cladding board will help prevent excessive moisture variation, as well as reducing the risk of warping/cupping. It is important to remember that wood will continue to “move” due to moisture uptake and release (i.e. wet in winter, dry in summer). The level of movement depends on timber species and the type of treatment used. Boards must be fitted with sufficient gaps (for expansion) as well as sufficient overlap (to avoid separation during drying).

Wet installation causing cupping (picture courtesy Ivor Davies)

No expansion gaps causing lifting (picture courtesy Ivor Davies)

• At moisture contents < 30% timber shrinks as it dries and expands as it wets

• Installation should be at approximately 16% moisture content

• ‘Shrinkage’ describes dimensional change due to initial drying

• ‘Movement’ describes size change due to ongoing moisture content fluctuations.

• Cupping can also occur when cladding is installed whilst too wet.

Larch cladding (picture courtesy of BSW Timber)

• 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 • Ensure joints can accommodate anticipated

shrinkage • Use single fixings if possible or drill holes

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)

Oversized drill holes (picture courtesy Ivor Davies)

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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

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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.

For coated cladding it is important to ensure there is sufficient coating build-up to provide a barrier against moisture uptake and UV resistance. This is best achieved with one coating of primer and two coatings of the top-coat to be used.

Factory finishing is one way of ensuring cladding coating is correctly applied. Some companies in the UK and abroad now offer this as part of a total package. The coated product, however, requires careful installation, with remedial coating applied if the cladding lengths have been cut.

If erected uncoated, it is recommended the cladding should remain uncoated, and be allowed to weather (silver).

For site applied coatings: total thickness 40 – 75 μm For factory finished coatings: total thickness 90 – 120 μm

• Decide prior to installation if cladding is to be coated or uncoated o Once in use, it is not recommended to change from uncoated to coated

• Ensure the cladding is coated on all surfaces/edges • Apply a minimum of one basecoat (preferably biocidal) and two

topcoats, to thickness levels specified by manufacturers • Beware of thin coatings on sharp edges / arrises • For factory finished coatings ensure cut / damaged coatings are treated

on site

Range of coatings (picture courtesy of Akzo Nobel)

Greying of timber due to weathering. Picture courtesy of Akzo Nobel

Picture courtesy of Akzo Nobel

Flame treatment finishing (picture courtesy of Welsh School of Architecture)

Coated Douglas fir on a barn conversion (picture courtesy of Eddie Byrnes)

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Maintenance Having installed the timber cladding, it is important to ensure that its appearance and performance meets its specifications. For this, it is necessary to follow the recommendations of the cladding manufacturer and/or the coating manufacturer. It is essential to remember not to leave any maintenance until the last minute, as excessive damage may have already been incurred.

The maintenance period will also depend on the species of timber used. Timber that has low levels of movement will place less stress on applied coatings, such that they last longer. The best example of this is with Accoya®, where the increased stability resulting from the acetylation treatment process carried out on the timber has been shown to dramatically increase the life expectancy of the coating. This has resulted in Accoya® cladding, when factory finished, being issued with an expected service life of 30 years and predicted maintenance intervals of 10 years for factory finished transluscent coatings and 12 years for factory finished opaque coatings.

The design of the cladding will also play a role in the expected life of applied coatings. Hence the prevention of splashback from the floor onto timber cladding, avoiding water traps in the cladding design, ensuring ventilation behind the cladding to aid drying, and reducing the number of sharp edges present are all significant factors in achieving high service lives for timber cladding.

Periods between maintenance will be shorter for clear coatings compared to opaque coatings due to increased UV damage.

Properly detailed and maintained factory finished Accoya ® cladding should have 12 year maintenance cycles (picture courtesy of Titanwood).

Typical maintenance cycles for exterior timber claddings are:

• Softwood timbers 3-5 years • Hardwood timbers 5-7 years • Thermally treated woods 5-10 years • Chemically treated woods 7-12 years

Manufacturers’ guidelines should be followed.

Picture courtesy of Akzo Nobel

Failure of clear coating. Picture courtesy of Ivor Davies

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1.5

1.4

5.2

19.3

11.1

Glulam

Timber

Steel (scrap-based)

Steel (ore-based)

Concrete

Benefits versus other materials The selection of timber cladding is a personal choice, whether from the architect, specifier or the client. Among the common reasons for using timber are its looks and its environmental friendliness. A survey in Austria several years ago assessed the public perception of timber (including cladding) with other materials, which showed people were prepared to pay a premium for a high quality product with strong environmental credentials. However, there were still concerns over maintenance and lifespan for timber cladding compared to other materials. Improvements in technologies, design and maintenance have lessened these concerns, though they cannot be totally eliminated. The benefits of timber though far outweigh maintenance factors alone. Timber has the lowest carbon dioxide emission rate per square metre of building area than any other material (Source the Swedish Forest Industries Federation, 2003).

Welsh larch cladding on a construction project. Picture courtesy of Fforest timber engineering

A typical 3-bedroomed detached house has an approximate wall surface area (excluding doors and windows) of 70 m2. If clad with 19mm thick timber, this represents 1.33 m3 of timber. Each m3 of timber equates to 2 tonnes of CO2 entrapped (0.9 tonnes due to the carbon in the wood itself and 1.1 tonnes from the reduced carbon demand from the replaced cladding materials). Thus the timber cladding represents 2.66 tonnes of CO2 (over a 30 year life, this is approximately 0.09 tonnes of CO2 saved per year). A typical house uses 5.8 tonnes of CO2 per year, so the entrapped carbon in the cladding can be seen as an offset of nearly 1.5% per year throughout its 30 year service life.

All timber cladding products according to the Green Guide are rated as A+ (best environmental credentials).

Timber cladding is becoming a common feature on new developments. Picture courtesy of Ivor Davies

CO2 emissions from the manufacture of different materials

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Strengths, Weaknesses, Opportunities and Threats (SWOT) In order to better understand the potential of home grown timber in use for exterior cladding, it is necessary to consider its advantages and disadvantages against competing materials. The best way of doing this is through a SWOT analysis. Strengths Weaknesses • Home grown material

– Excellent environmental profile, low energy material, CO2 neutral, existing management and stewardship schemes

– Resource is close to market place

– Potentially large resource • Good paint performance, stable

product • Economies of scale • Proven throughout Europe

• Durability – difficult to treat/ impregnate some species with chemicals/ preservatives

• Faster growth rate than imported softwoods – perceived problems due to lower density and hardness

• Maintenance a perceived problem compared to uPVC

• UK Architect and Designer preference for Western Red Cedar

• Industry preference for slow grown timber

Opportunities Threats • Large market previously untapped • Set-up partnerships with cladding

companies • Moderate to High value adding

potential • Market a home grown product • Market environmental advantages

of home grown, sustainably produced wood

• Improved properties through defect cutting (removal of knots/cracks) and re-engineering (jointing pieces back together)

• Use of grading to improve quality of material reaching the market

• Market resistance to use of timber for cladding

• Quality must be right, any rejects would severely damage future markets

• Any set-up costs for Welsh / UK sawmills

• Price must be right to access the market

• Limited existing market or partnerships restricting growth of the timber cladding market

The SWOT analysis shows that home grown timber cladding has a lot to offer, provided the correct choices are made in the selection and design of the cladding. Any bad practice could have a severe detrimental effect on the market (in much the same way as poorer quality wooden windows led to a decline in customer satisfaction in the 1970s and 1980s, culminating in the expansion of the uPVC percentage share of the market). Picture courtesy of Hughes Architects

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Heat treated cladding at Princess Royal Sports Centre, Lincolnshire (picture courtesy of Thermowood Association)

Cladding suppliers in Wales There are many companies specialising in supplying timber cladding across Wales. The following represent a wide selection of these, though there may be other companies close to your location. Should you require further information, it is recommended you contact Woodsource Wales. North Wales Suppliers Company name

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fir

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Corwen Forestry Llandrillo, Corwen, Denbighshire LL21 0SY Tel: 01490 440316

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Gwasanaeth Coed Blaencwm Cynllwyd, Llanuchllyn, Bala, Gwynedd LL23 7DF Tel: 01678 540621

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Glasfryn Fencing & Sawmill Llanaelhaern Road, Pwllheli, Gwynedd LL53 6RN Tel: 01758 750623

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Ellesmere Timber Co. The Sawmill, Elston, Ellesmere, Shropshire SY12 9JW Tel: 01691 622441

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Llyn Wood Product Tyn Pwll, Pwllheli, Gwynedd, LL53 8TY Tel: 01758 720154

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Woodsource Wales details Address: Unit 7, Dyfi Eco Park, Machynlleth, Powys SY20 8AX Telephone: 0845 347 9372 Fax: 01654 700 050 Email:info@woodsourcewales.co.uk

Cladding test panels (picture courtesy of BRE)

Timber cladding has strong aesthetical appeal (picture courtesy of Hughes Architects)

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Mid Wales Suppliers Company name

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Oak

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(Hom

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Usk Valley Sawmills Timbercraft Park, Llangattock, Crickhowell, Powys NP8 1HW Tel: 07873 386748

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BSW Timber Ltd Newbridge on Wye, Builth Wells, Powys LD2 3RU Tel: 0800 587 8887

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ICH Joinery & Timber Ltd Berriew Road, Welshpool, Powys SY21 8RP Tel: 01938 553985

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G & T Evans Dulas Mill, Mochdre Lane, Newtown, Powys SY16 4JD Tel: 01686 622106

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Cefn Llwyn Timber Ystrad Meurig, Ceredigion SY25 6AP Tel: 01974 831560

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Heartwood 35 Dolforgan View, Kerry, Newtown, Powys SY16 4DZ Tel: 01686 670713

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Edward Thomas & Sons Usk Sawmills, Sennybridge, Powys LD3 8RS Tel: 01874 636321

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Pontrilas Timber Pontrilas, nr. Hereford, Herefordshire HR2 0BE Tel: 01981 240444

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Charles Ransford & Sons Ltd Station Rd, Bishops Castle, Shropshire SY9 5AQ Tel: 01588 638331

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South Wales Suppliers Company name

Agr

icul

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Arc

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ladd

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Oak

Sw

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Dou

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Wes

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(Hom

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Japa

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larc

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Rassau Sawmill Mountain Rd, Rassau, Ebbw Vale, Blaenau Gwent NP23 5BG Tel: 01495 302069

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AOM & Associates Garn Bwll, Mynyddcerrig, Pontyberem, Llanelli, Dyfed SA15 5BN Tel: 01269 870031

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Teifi Timber Products Ltd Llanllwni, Pencader, Carmarthenshire SA39 9DY Tel: 01559 395325

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Wentwood Timber Centre Wentwood, Caldicot, Monmouthshire NP26 3AZ Tel: 01633 400720

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Pembrokeshire Timber Store Withybush Ind. Estate, Withybush, Haverfordwest, Pembrokeshire SA62 4BW Tel: 01437 769771

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Coed Dinefwr Cwmcrib Ganol, Ffairfach, Llandeilo, Carmarthenshire SA19 6TE Tel : 01558 822833

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Celfeigan Sawmills Llanbadoc, nr. Usk, Monmouthshire NP15 1PS Tel: 01291 672805

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Caerphilly Timber Products The Old Dolomites, Wernddu Rd, Wernddu, Caerphilly CF83 3DB Tel: 02920 865111

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AW Hardwoods Castell Ddu, Waungron, Pontardulais, Swansea SA4 8DH Tel : 01792 884455

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Cwmdu Timber Blaen Cwmdu Farm, Maesteg, Bridgend Tel : 01656 738388

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Essential reading BCL (2009). External Cladding. Technical Information download. Downloadable from www.bcltimberprojects.co.uk British Standards (2000). Sawn timber. Appearance grading of softwoods. European spruces, firs, pines and Douglas fir. BS EN 1611-1:2000 British Standards (2005). Solid softwood panelling and cladding. Softwood machined profiles with tongue and groove. BS EN 14519:2005 British Standards (2006). Solid hardwood panelling and cladding. Machined profiles elements. BS EN 14951:2006 British Standards (2006). Solid softwood panelling and cladding. Machined profiles without tongue and groove. BS EN 15146: 2006 British Standards (2007). Visual grading of hardwood. Specification. BS 5756. Davies I (2010). Designing the timber façade. Arcamedia, Edinburgh. ISBN 978-1-904320-04-3 Davies I, Pendlebury J and Walker B (2001). Timber cladding in Scotland. ARCA Publications. ISBN 1 904320 00 7 Davies I and Watt G (2005). ‘Making the grade’ – A guide to appearance grading of UK grown hardwood timbers. ARCA publications ISBN 1-904320-03-1 Hislop PJ (2007) – External timber cladding. TRADA Technology. ISBN 978 1 900510 554 Holland C (2005) – Using UK-grown Douglas fir and larch timber for external cladding. BRE Digest 494. ISBN 1 86081 840 4 Jones D and Suttie E (2006) – Using UK-grown Sitka spruce for external cladding. BRE Digest 500. ISBN 1 8608 917 6 Swedish Finnish Timber Council (1982) – Exterior cladding of redwood and whitewood. Suttie E (2007) – Modified Wood. An introduction to products in UK construction. BRE Digest 504. ISBN 978 1 84806 015 9 Thermowood Association (2003). A handbook on thermal modification in Finland may be downloaded from http://www.thermowood.fi Titanwood (2009). A brochure on Accoya (commercial acetylated timber) may be downloaded from http://www.accoya.com/web_pdf_brochure.pdf TRADA Wood Information Sheet 2/3 – 1 Finishes for exterior timber TRADA Wood Information Sheet 2/3 – 6 Wood decorative and practical TRADA Wood Information Sheet 2/3 – 10 Timbers – their properties and uses TRADA Wood Information Sheet 2/3 – 60 Specifying timber exposed to weathering TRADA Wood Information Sheets 4 – 14 Moisture in timber TRADA Wood Information Sheets 4 – 28 Durability by design Acknowledgements This document would not have been possible without the help of several companies and individuals. The author particularly wishes to thank Coford, BCL, Thermowood and Titanwood as well as the following individuals: Ivor Davies (Napier University), John Alexander (BSW), Arwel Owen (Hughes Architects), Robin Aldridge (Fforest Timber Engineering), Andrew Pitman (TRADA), Andrew Bronwin (chair WFBP), Nick Tune (BRE Wales) and Phil Roberts (Warm Wales, chair WKW).

Woodknowledge Wales (WKW) is part of the Wales Forest Business Partnership (WFBP), whose funding has made this publication possible and is hereby acknowledged.

More information can be found on the Woodknowledge Wales website: http://www.woodknowledgewales.co.uk or by contacting Dennis Jones (Tel: 0777 2228711; E-mail jonesd@bre.co.uk)