Use Class is a term taken from BS EN 335:2013 (Durability of wood and wood-based products. Use classes: definitions, application to solid wood and wood-based products). This standard groups possible end uses for timber into 5 classes referred to as Use Classes. Use Class 3 relates to timber used in above ground, outdoor situations and therefore this is the category that corresponds to timber cladding.
Timber species with insufficient natural durability to be used successfully outdoors can be upgraded by the application of a factory-applied timber preservative to improve its long-term performance. The application of preservative is not a ‘one size fits all’ process; it is tailored specifically to the intended end use of the timber product.
BS8417 BS 8417:2011+A1:2014 (Preservation of wood. Code of practice) is the relevant standard for wood preservation in the UK. It defines treatment by preservative type, penetration and retention i.e. amount in the wood relative to species and desired service life (15, 30 or 60).
Specifiers may find the following sample specification clause helpful:-
[Insert species] cladding to be treated with [insert preservative type] according to the recommendations in BS 8417 for a use class 3 application from which a desired service life of 30 years is required.
Preservative type can either be copper organic or micro-emulsion (waterbased). Copper organic types give the timber a green (unless coloured) hue whilst micro-emulsions are generally clear or slightly tinted but mirco-emulsions must be used in conjunction with a maintained paint.
Thirty years is the industry default desired service life although 60 years may be possible with certain products from certain suppliers.