Natural Durability Ratings Print E-mail
The natural durability rating of a timber species is a rating of the timber's natural resistance to attack by wood destroying fungi and wood destroying insects. The sapwood of all timber species has poor resistance and so the natural durability rating applies only to the heartwood of a timber species.

The rating is based on the testing of stakes and poles imbedded in the ground and on expert opinion of historical performance.

The rating is not intended to predict a precise life expectancy for a species - because of the variability within a species and because of the differences in conditions between sites and applications where the timber species might be used.

However, the natural durability ratings of heartwood for above ground use and for in-ground contact use, do provide a broad comparison between species.

In a given site and application, a class 1 species will last longer than a class 2 species and so on. It should be noted that a class 2 species in a less hazardous situation may last longer than a class 1 species in a different but more hazardous situation.

For each of the four classes there is an expected service life range. The above ground ranges are different from the in-ground contact ranges.

The degree of resistance to attack by wood destroying fungi and wood destroying insects is determined largely by the extractives formed when sapwood changes into heartwood as the tree grows. Termites are less easily deterred by these extractives (than are fungi) and will attack most species of timber, though slowly in the case of the very durable species. Termites tend to avoid species that have a relatively high silica content (e.g., brush box, turpentine), because of their abrasive nature. Marine organisms, too, are deterred to some extent by a high silica content.

Since the fungi responsible for decay favour wood that is neither very dry nor very wet, the danger zone for timber members embedded in the ground is between 200 mm to 300 mm above and below the ground line. This is where practically all posts and poles fail, so even with preservative-treated timber it is desirable to examine the timber in the danger zone at regular intervals for signs of decay.

The sapwood of all species is non-durable because of its life-supporting starches and sugars, therefore, for timber exposed to the weather or in ground contact, the sapwood is normally either removed or preservative treated.

The sapwood of the majority of species is comparatively easily penetrated by preservatives, whereas the heartwood of many species is very difficult to treat because of the changes that occur to the wood cells at the time of heartwood formation. The general rule of thumb is that only the sapwood of hardwoods can be penetrated where as the sapwood and some of the heartwood of most softwood species can be penetrated.

Most treatment methods for round timbers (posts, poles and piles) aim to provide a considerable loading of preservative in the ring of sapwood. Removal of bumps to improve the aesthetic appearance of the round timber should be avoided if the underlying heartwood is not of high durability.

Because it is only the sapwood that is usually commercially treatable, only durable timbers should be used as sawn timbers that are going to be placed in a high-hazard situation.

The relevant Australian Standard AS 5604 provides natural durability ratings for a large number of species in several categories including lyctid susceptibility, termite resistance, in ground-contact durability, outside above ground durability and marine borer resistance. Class 1 rated species are the most durable and Class 4 rated species the least durable.

Timber Users Guide 1 - Timber, Durability & External Applications
(PDF, 0.1 MB)
Australian Timber Importers Federation