How is natural durability quantified?
BS EN 350-1 recognises five classifications of natural durability for timbers. These relate to the resistance of the heartwood to attack by wood-decaying fungi:
- Class 1 very durable
- Class 2 durable
- Class 3 moderately durable
- Class 4 slightly durable
- Class 5 not durable
For cladding, timbers rated Class 3, moderately durable or better, can be used without preservative treatment (providing the non-durable sapwood is excluded). Timbers rated Class 4 or Class 5 should be pre-treated with preservative.
For more detailed information about timber cladding, take a look at our book External Timber Cladding, a comprehensive guide to the design and specification of timber cladding.
We also provides a range of services for building designers and product manufacturers to assist with timber cladding queries, as well as training and cpd.
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