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<B><font size="+1">Softwood</font></b>

• <B>Factors affecting durability</b>
• Cladding design Timber is susceptible to decay from fungal attack only if the moisture content is greater than 20-22%. There are various design measures that can minimise the risk of high moisture presence, including:Provision of a cavity behind the cladding for ventilation to remove any moisture that has been driven behind it
• Protection of the cladding from rain with projecting surfaces such as roofs, canopies or balconies.

Insects To prevent any insect infestation either in or behind the cavity, apply an insect mesh at the top and bottom of close-jointed boarding. For open-jointed boarding, complete protection can be provided by applying a mesh layer between the battens and boards.
<B>UV light degradation UV</b> light affects all woods, bleaching them to a silvery-grey colour. This may be a desired effect, especially for naturally durable timbers such as Western red cedar. The great advantage is that there will be minimal maintenance requirements. However, the timber will be affected more by variations in humidity, resulting in small cracks – surface-checking. These will pick up pollution, causing the timber to darken over time.
It is important to apply any coatings to softwood boarding before the timber has been exposed to sunlight for too long and surface deterioration has begun. Surface coatings should be flexible and vapour-permeable.

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• Modes of failure</b>
• <B>Decay</b> Using preservative-treated or naturally durable timber will minimise the risk of decay. Failure to use preservative to treat cut ends on site can lead to decay at joints.
• Regular recoating of boarding treated with organic solvents is essential to protect the timber. Most repainting programmes are based on three- to five-year cycles.
• Annual inspection should be carried out, certainly in the first five years, to assess the effects of the local microclimate. South-facing or particularly exposed surfaces may need regular recoating or touch–up.
• Prompt replacement of damaged timbers is essential to prevent further decay to cladding and substructure.
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• Durability tips</b>
• <B>Standards</b> Specify softwood to BS 1186-3 for wood trim and its fixing. This standard provides guidance on the suitability of the timber species for various uses, including external wall cladding. It defines the limits to the number and extent of natural defects such as knots and splits. Typically class 3 or class 2 timbers are specified for cladding boards – class 2 permits fewer natural defects.
• <B>Maintenance</b> Costs can be minimised by specifying timbers that do not require paint protection. For example:Those treated by CCA preservative; the preservative becomes fixed to the timber and does not Leach out on wetting
• Naturally durable timbers, such as Western red cedar.

Fixings Use stainless steel or non-corrosive lost-head nails. Galvanized or plated steel nails will have their protective coating damaged by hammering them into position, and rust staining will result. It is unlikely that any surface coating of the boarding will protect the steel.
Western red cedar has a high tannin content, which will corrode iron; so mild steel nails are not an option. If using Western red cedar – or indeed any timber that has been treated with CCA preservative – aluminium nails are not an option either.

<B><font size="+1">PVCu</b></font>
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• Factors affecting durability
• Exposure</b> The combined effect of wind-blown grit, grime, pollutants and UV light can degrade the PVCu surface finish to produce a matt surface, which retains dirt. Cladding should be washed at least every year with a non–alkaline detergent.
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• Modes of failure</b>
• UV degradation Prolonged exposure to UV light can lead to yellowing, pinking, embrittlement and loss of gloss finish in PVCu cladding. Self-coloured PVCu will fade when exposed to sunlight.
• Wind loading The cladding may fail due to nail withdrawal where the suction effects of winds are not allowed for by the frequency of nails or their depth of penetration into the substructure.
• Embrittlement Painting of plastics boarding may cause premature embrittlement, and use of dark colours may cause excessive distortion. Painting of PVCu claddings is not recommended.
• Substructure Plastic boarding will not exclude moisture so allowance has to be incorporated in the design to remove water that penetrates the boarding. A fully drained cavity combined with treated battens achieves a durable substructure.
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• Durability tips</b>Use austenitic stainless steel fixings. The most durable option is stainless steel of grade 1.4401 to BS EN 10088–2 (UK type 316 or A4).
• Avoid installing plastic cladding at low levels where impacts may occur. For zones above risk of impact, specify categories E and F to BS 8200, table 2.
• Make adequate allowance for thermal expansion – typically a gap of 1-2 mm – to prevent distortion and cracking of boarding.

<B><font size="+1">Glass–fibre reinforced plastics</b></font>
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• Factors affecting durability</b>
• <B>Colour</b> Through-coloured GRP boarding gives better durability of colourfastness. The structural integrity of GRP is much longer than the ability of the material to retain its appearance.
• Where flame-retardant additives have been applied to a lightly pigmented surface, yellowing may occur within a matter of years.
• <B>UV light</b> Degradation of GRP is the result of a combination of UV light, surface wetness and temperature changes. The integrity and smoothness of the surface gel coat deteriorates. Dirt pick-up is increased and glass fibres may become exposed.
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• Modes of failure</b>Fixings not allowing the GRP to expand and contract
• Damage due to vandalism or abuse
• Exposure to UV light, moisture and temperature change

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• Durability tips</b>Strong colours tend to lose their intensity more quickly. Where durability of the colour is important, specify a colour with as high a light-fastness index as possible to BS 1006.
• The material qualities and properties of GRP can vary extensively. The quality of the finished product can be specified to BS 4549.
• Low-level GRP boarding is not recommended for areas where there is a risk of vandalism or abuse.
• Degraded surfaces may be made good by rubbing down and applying a new surface treatment or, more practically, applying an appropriate paint system, such as a two-pack polyurethane type.

<B><font size="+1">High-pressure laminates</b></font>

• Factors affecting durability</b>
• The weathering qualities of the surface finish are of prime importance to the durability of high-pressure laminates – in particular, resistance to UV light in terms of colour stability, loss of gloss and surface cracking.
• Heavily-textured boarding in imitation of wood is prone to algal growth where the microclimate is damp with little ventilation or direct sunlight.
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• Modes of failure</b>
• <B>UV radiation</b> Surfaces are affected by sunlight, temperature and moisture changes. The colour of the surface dulls, although this may not be considered a problem as the colour changes should be uniform. Modern formulations include acrylic films with a protective layer against UV radiation.
• <B>Surface erosion</b> Surface layers are susceptible to scratches, erosion by wind-driven particles or use of inappropriate abrasive or aggressive cleaning agents. These effects roughen the surface of the laminate, and dirt and grime from pollutants can adhere. Where dirt is lodged in the surface layer, it is more difficult to remove.
• <B>Water absorption</b> Laminates are not impermeable to water. Although water absorption rates are very low – less than 2% – if water does penetrate the coating, it may cause delamination of the surface layers from the body of the laminate.
• Solvents High-pressure laminate sheets coated with acrylic are not resistant to solvents.
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• Durability tips</b>
• Regular cleaning with non–abrasive and non–aggressive agents should be carried out after installation rather than allowing grime to build up to an unacceptable level.