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Natural durability

Natural durability refers to the natural resistance of heartwood to decay, insect or marine borer attack – sapwood is rarely regarded as anything but “non-durable”.

The natural durability of a timber species is conveniently allocated to one of four Durability Classes which are based on the “average life” (average time for a set of samples to fail from biological attack) of timber in the ground, or exposed in a marine environment.

Examples of NZ species assigned to the different Durability Classes are shown in the table below:

Natural
Durability
Class
Average time
to failure 50 x 50 mm
tests stakes
NZ Examples
1 >25 years Totara, Robinia, Silver pine
2 15-25 years Red beech, Hard beech
3 5-15 years Rimu, Matai, Kauri. Macrocarpa
4 <5 years Tawa, Silver beech, Corsican pine

Above-ground end-use applications for timber are recognised as being less severe than in-ground applications.

Therefore above-ground average time to failure natural durability classes are generally higher for any given timber species.

A useful reference is the Australian Standard AS 5604-2005, which assigns Durability Class to mainly Australian species, with some imported species relevant to NZ as well.

Durability ratings are given for both in-ground and above-ground end applications, and there is some information on performance in a marine environment.

The biological hazards include reference to termites which are not recognised as commercially important in New Zealand.