The principal standard for cladding specification is BS8605:2014 External Timber Cladding Part 1 - Method of Specifying. Other relevant standards are listed at the bottom of the page.
SPECIFICATION CHECK LIST (click on arrow to expand)
The principal considerations when choosing timber cladding are:
Price can vary significantly from material to material. Naturally durable species from which all sapwood is excluded tend to cost more than less durable softwoods protected with wood preservative by an industrial process. Cost relates to the availability of a species, the quality of material, performance and level of processing involved.
Desired service life
The prediction of service life is not precise because a range of factors can affect it but generally length of service will relate to the durability of the timber against fungal and insect attack. For extended service life (30 to 60 years) always choose an appropriate naturally durable, preservative treated or modified wood species.
Durability against decay and insects
Cladding is a BS EN 335-1 Use Class 3 application. Where sapwood is not excluded, appropriately durable timber should be specified.
Fire retardant treatment
If required by building standards, always check to ensure that the cladding species, profile size and cavity design reflects accurately the Fire Certificates of the treatment under consideration.
Careful detailing to prevent moisture retention and to minimise the effects of moisture movement (expansion and contraction) are essential in all designs. Materials with enhanced durability provide an insurance against detailing failures and poor installation practice/maintenance.
Is a natural or coated finish required
If a natural look is required then only naturally durable or species treated with an appropriate preservative or modified wood should be specified. Surface coatings do afford a degree of protection to untreated low durability softwoods but for best results they work best on cladding materials that are more dimensionally stable like modified woods or on preservative treated wood.
What maintenance regime is acceptable
If maintenance, repair and replacement is likely to be difficult and costly then the highest durability of cladding permitted by the budget should be chosen. All coated cladding will require recoating at some point.
Resistance to impact damage and vandalism
Species with a low density (physical hardness) are more susceptible to impact and surface damage and should never be used at low levels where there is the potential for abrasion, scraping or vandalism.
Source certification schemes from managed forest resources are now well established for both home grown and imported species. Look for materials from FSC and PEFC accredited sources for absolute reassurance.
For any given species, appearance, strength and durability are determined by the quality selected. Knot size, knot frequency, grain orientation, type of machining and whether sapwood is included or excluded all have a bearing on quality. Where naturally durable, preservative treated or modified woods are specified, the TDCA recommends that the cladding is processed using a Factory Production Control system (FPC), with third party accreditation such as ISO9001.
Supplier support services
Make sure that the supplier you select has the ability to provide the range of support services required prior to, during and after installation. Samples supply, supply volume and quality consistency, technical expertise on the chosen material, handling advice and snagging and quality dispute resolution policies are all critical service factors to be considered. Look for suppliers with the TDCA CladMark quality accreditation which means their capabilities and policies have been independently assessed.
- Cladding shall be produced by processors operating a Factory Production Control System with third party accreditation
- CladMark is the quality scheme operated by the TDCA for cladding materials.
On this web site, quality is related to both the material and to the factory procedures used to manufacture and/or install it. The TDCA operates CladMark, a quality assurance scheme that verifies factory production control procedures to ensure quality consistency.
For any given species, appearance, strength and durability are determined by the quality of the grade selected. Knot size, knot frequency, grain orientation, type of machining and whether sapwood is included or excluded all have a bearing on quality.
BS1186:3 1990 (Timber for and workmanship in joinery) is the current British Standard relating to cladding. The definitions of quality largely relate to visual quality and the size and frequency of knots. Four classes are defined:
- Class CSH – knot diameter limited to 6mm
- Class 1- knot diameter limited to 22.5mm
- Class 2 – knot diameter limited to 35mm
- Class 3 – knot diameter limited to 50mm or no more than 35% of board width
No limit is placed on the size or density of knots on the concealed face of boards. The standard also places limits on other features such as resin pockets, splits, grain slope and, most importantly, sapwood content. The standard requires that sapwwod is not present on exposed surfaces unless the material is to be pre-treated prior to installation.
CSH and Class 1 are used for high quality joinery applications whereas Class 2 and 3 grades have traditionally been used for cladding and external wood trim particularly where the cladding will be left uncoated to weather or a translucent finish is required. Class 1 grade may be preferred for planed cladding that is to be unfinished but it is not readily available in all species. Hardwoods for cladding tend to be Class 1 quality. CSH quality is not generally associated with cladding but is sometimes used for high value pre-fabricated cladding panels made from hardwoods.
Whilst BS1186 is the recognised standard for cladding, it was last reviewed in 1990 and modern day manufacturing practices and material approval procedures have largely superseded its use though some of the guidance such as that on moisture, faults, sapwood and factory production control are still relevant.
The sapwood of all timber species can be subject to decay and insect attack. The natural durability ratings defined in BSEN350:2 relate specifically to the heartwood of a tree. As such, where timber is to be used for cladding, all sapwood should be excluded. Obtaining cladding from which all sapwood is removed can add cost and is sometimes difficult to achieve totally. Where small amounts of sapwood are detectable these should not appear on the external face of the board – particularly where there is a distinct colour change between sapwood and heartwood.
Where a cladding board is machined from material in which sapwood is present it should always be pre-treated with an appropriate preservative or made from modified wood.
Where naturally durable, preservative treated or modified woods are specified, the TDCA recommends that the cladding is processed using a Factory Production Control system (FPC), with third party accreditation such as ISO9001.
Standards relating to cladding specification
BS 8605: External Timber Cladding Part 1: Method of specifying.
This is a relatively new British Standard which was published at the end of 2014. It covers aspects that can be controlled by the manufacturer and it provides useful information for those specifying and using timber cladding. A Part 2, which will deal with installation best practice, is currently being written. Its publication is anticipated sometime in 202.
BS EN 14915: Solid wood panelling and cladding – Characteristics, evaluation of conformity and marking.
This standard describes the data set required for CE marking and labelling to comply with EU Construction Product Regulations that came into force on 01 July 2013.
BS 1186-3: Timber for and workmanship in joinery - Part 3: Specification for wood trim and its fixing.
BS EN 350:2016 Durability of wood and wood-based products. Testing and classification of the durability to biological agents of wood and wood-based materials.
BS8417 Preservation of wood. Code of practice.
BS EN 335: Durability of wood and wood based products. Use classes: definitions, application to solid wood and wood-based products.
Classifies timber applications into five “Use Classes” depending on the risk a component faces from a biological hazard i.e. decay, disfigurement by surface moulds or insect attack during service. External timber cladding is a Use Class 3 application. This means it is considered to have a medium risk of attack from such biological organisms. However, poor detailing, installation or maintenance may result in cladding being exposed to more severe conditions so it is good practice to choose timber that has a more than adequate durability to meet the service conditions.
BS EN 1310: Round and sawn timber. Method of measurement of features.