information/FAQs

Table 1 Durability classes

Durability of timber and durability classes

In general, there are two main factors that influence the durability of timber in service. The first is the natural durability of the particular species. The second is the type and degree of hazard to which the timber is exposed.

The natural durability of particular species is expressed by rating the timber as one of four Durability Class. These classes are based on field trials of untreated heartwood in the ground and indicate the resistance of the heartwood of the species to fungal and insect (termite) attack. These classes are:

Class 1: Timbers of the highest natural durability which may be expected to resist both decay and termite attack for at least 25 years and up to 50 years in the ground.

Class 2: Timbers of high natural durability which may be expected to have a life of about 15 to 25 years in the ground.

Class 3: Timbers of only moderate durability which may be expected to have a life of about 8 to 15 years in the ground.

Class 4: Timbers of low durability which may last from about 1 to 8 years in the ground. These timbers have about the same durability as untreated sapwood, which is generally regarded as Class 4, irrespective of species.

Some examples of timbers in the four classes are given in Table 1.

The serviceability of timber may also be affected by borers, termites, or marine borers.

Good design workmanship, finishing and maintenance is helpful, together with the selection of species of high natural durability where necessary. An alternative is to use timber that has been preservative treated.

References:

NAFI Timber Manual: Timber Datafile P1 – Timber Species and Properties.
Timber Promotion Council - "Technical advisory brochure no 1"