Hardwood Durability Classes
Four-class system rating untreated hardwood heartwood resistance to decay, from Class 1 (highly durable) to Class 4 (least durable) under AS 5604.
Australian hardwood species are rated for natural durability of heartwood under AS 5604 (Timber - Natural durability ratings) using a four-class system. Class 1 species (e.g. ironbark, tallowwood, white cypress) have an in-ground service life over 25 years; Class 2 (e.g. blackbutt, spotted gum) typically 15 to 25 years; Class 3 (e.g. mountain ash) 5 to 15 years; Class 4 (e.g. radiata pine heartwood) under 5 years. The standard also rates above-ground decay resistance, marine borer resistance, and lyctine susceptibility separately. Sapwood of any species generally has very low durability and may need treatment. For buyers, durability class drives the choice of species for untreated decking, posts, and exterior applications, but always pair durability with the right detailing (drainage, ventilation) for full service life.
Values and references on this page are indicative and drawn from common industry sources. Always verify the current AS/NZS standard, manufacturer data sheet or NIAA policy publication before relying on figures for design, specification or compliance work.
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