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:
to failure 50 x 50 mm
|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.