Timber Durability

Some species of trees are more effective at resisting biological hazards such as fungi and termites than other species. This natural resistance or durability is a function of the type of extractives the tree stores in its heartwood.

Durability is not determined by the density or ‘hardness’ of the species. Designers can match the natural durability of a particular species with the hazard classification to specify an appropriate species for a specific application.

Sapwood

Sapwood of most species has little durability because there are no extractives contained in the wood, and it is laden with starches (normally intended for the growth of the tree, but also essential for the growth of fungi).

Heartwood

The resistance of a particular species to attack from fungi and termites is determined by the type of extractives stored in the heartwood.

Durability Classes

Species are rated according to their ability to resist attack from biological hazards – Class 1 for highly durable species to Class 4 for non-durable species. The durability class only relates to the heartwood of the species (not the sapwood).

Natural durability refers to the durability of the heartwood of timber species with respect primarily to resistance to decay and termite attack in an in-ground situation. Natural durability ratings or classes are assigned with respect to in-ground service in an adverse environment, ie. High moisture content, high temperatures and presence of subterranean termites.

Natural durability is measured by examining the progressive decay of small wooden stakes buried in the ground at a number of locations around Australia, “graveyard tests”. The range of test sites has been selected to cover different climates and different soils around the nation. The stakes are periodically sampled to see how much degradation has taken place over time. The data on durability are continually being updated, and the table in the Australian Standard is modified as evidence of performance in tests or of performance of timber in buildings is made available.

Natural durability cannot be measured in one country and used in another. Australia has different environmental conditions that can accelerate fungal and termite degradation when compared with North America or Northern Europe. Also our range of fungal and insect species is different to other parts of the world and may give greater or lower deterioration rates (even on the same species) compared with the species mix in North America, Europe or Asia.

FUNGI	BORERS	TERMITES
Wood Decaying Fungi Brown Rot: attacks cellulose. White Rot: attacks cellulose & lignin Soft Rot: usually in very moist areas Wood Disfiguring Fungi Wood Staining Fungi: E.g. blue stain, usually in areas of high temperature and humidity. Does not attack cellulose or lignin but may enable areas for rot fungi.	Lyctid Borers Female lyctid beetles lay their eggs under the surface of the timber and the young feed on the cellulose, not found in softwoods or the heartwood of hardwoods. Anoboiid borers Common furniture beetles, they usually attack moist timber (18-26% m.c) and commonly infest old furniture in both the hardwood and sapwood of softwoods. Marine borers Molluscs and crustaceans found in all Australian waters but most active in warm, tropical zones	Subterranean Termites Living in nests in nearby trees or rotting wood, termites will forage in tunnels to find food and eat out susceptible wood. Dry-wood termites Only found in humid and coastal areas and do not need subterranean protection deriving their moisture requirements from the wood they ingest.

For each of these Durability Classes there are three different service conditions, In ground contact, Outside above ground and Marine borer resistance.

These classes are based on field trials of untreated heartwood and indicate the resistance of the heartwood of the species to fungal, insect attack and borer attack. These classes are:

Durability classes are appropriate for heartwood only. All untreated sapwood is considered non-durable. Core wood (wood from the very centre of the tree) generally also has marginally lower natural durability than the rest of the heartwood as it was laid down when the tree was immature and production of the full range of extractives was not fully developed.

The durability class can be matched to the hazard level to estimate the long-term performance of untreated timber heartwood. However, the life expectancy for any structural material is difficult to predict, and this is particularly so for timber.
The life of the material in service is affected by the microclimate (especially by presence of moisture – in the air or direct water contact), and also by the quality and regularity of maintenance. Maintenance includes termite inspections and remedial measure, checking and replacement of surface protection (including paints), and checking for corrosion of connections.

The following table is a simplistic presentation of the expectation of lifetime performance of timber, given an environment in which wise detailing has been used, and regular termite inspection and maintenance schedules are used.

Some general principles

• Class 3 and 4 timbers should not be used for structural members in H3 environments unless stringent detailing and workmanship, treatment or sealing has been specified.
• H6 environments require the use of Class 1 timbers. Note that where maintenance is neglected, it is possible for even a Class 1 timber to degrade in service within 25 years if it is intermittently in contact with water.
• Absolute life expectancy is almost impossible to determine. None the less, the relative durability of species and treatments will assist in selecting between available materials to satisfy durability expectations.

NATURAL DURABILITY, PROBABLE LIFE EXPECTANCY

* Ratings in this Table are based on expert opinions and the performance of the following test specimens:(a) In-ground: 50 × 50 mm test specimens at four sites around Australia. (b) Above-ground: 35 × 35 mm test specimens at eleven sites around Australia.