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 as one of four Durability Classes. For each of three different service conditions:
* in-ground contact
* outside above ground
* 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:
Class 1: Timbers of the highest natural durability.
Class 2: Timbers of high natural durability.
Class 3: Timbers of only moderate durability .
Class 4: Timbers of low durability. These timbers have about the same durability as untreated sapwood, which is generally regarded as Class 4, irrespective of species.
Timber is also classed as being either resistant or not resistant to attack by termite.
The probable life expectancy of each Durability Class for the three exposure conditions given in Australian Standard 5604-2005 is given below
The durability class of major species is given in the Species database.
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.
- AS 5604 2005
NATURAL DURABILITY—PROBABLE LIFE EXPECTANCY*
|
Class |
Probable in-ground life expectancy (years) |
Probable above-ground life expectancy (years) |
|
1 |
Greater than 25 |
Greater than 40 |
|
2 |
15 to 25 |
15 to 40 |
|
3 |
5 to 15 |
7 to 15 |
|
4 |
0 to 5 |
0 to 7 |
* The 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.
MARINE BORER RESISTANCE—PROBABLE LIFE EXPECTANCY
|
Class |
Probable marine-borer-resistance life expectancy
in southern waters (years) |
|
1 |
Greater than 60 |
|
2 |
41 to 60 |
|
3 |
21 to 40 |
|
4 |
0 to 20, usually less than 5 |
NOTE: Marine borer resistance is based on natural round piles containing 350 mm diameter of heartwood in southern seas reaching from Perth in the west to Batemans Bay in the east.
Only class 1 timbers can be expected to give reasonable service life (12 to 30 years) in northern waters.
