Classifying naturally durable timber
Natural Durability Classes
(Standards Australia, 2003. Revised 2005. AS 5604 – Timber-Natural durability ratings.)
Definitions
- Natural durability is defined as the inherent resistance of a specific timber to decay and to insect attack.
- Natural Durability Classes provide the basis for rating the timber’s performance and longevity in contact with the ground when exposed to average environmental conditions.
The classification system is based on the average life expectancy (in years) for any timber species, as shown in the table below.
Natural durability – probable life expectancy.
Class |
Probable in-ground life expectancy (years) |
Probable above-ground life expectancy (years) |
1 |
> 25 |
> 40 |
2 |
15 - 25 |
15 - 40 |
3 |
5 - 15 |
7 - 15 |
4 |
0 - 7 |
0 - 7 |
Other factors
Using these classifications to rate the timber of different tree species cannot be done with great precision because of the variability of wood properties within species and the wide variety of ground conditions in which it may be used.
Therefore the classifications are a guide with the actual life dependent on the local ground conditions and other factors that include the following.
- These classifications only apply to heartwood with all sapwood having poor resistance to decay and insect attack.
- The inner core of heartwood around the pith, generally has lower durability than the rest of the heartwood.
- Durability is also influenced by the size or diameter of the post i.e. the larger the piece size the longer is will last.
- The age of the tree used to cut the timber also generally influences natural durability with mature trees producing more durable timber than semi mature trees
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