Natural Timber Durability   Dura wood


Natural resistance




Natural Durability is defined as the inborn 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 the average environmental conditions.


Timbers are often grouped into four categories, Perishable, Non-durable, Moderately durable, Durable & Very durable.

Perishable & Non-durable are not suited for exterior use as they would break down with the elements. As for Moderately durable can be used external but protected from the elements by coatings (Paint) and above ground.

Durable & Very durable are suited for external use but may vary from species on their in-ground durability.


Natural Durability Classes


The chart below is based on the average life expectancy (in years) for a species.


Durability Class

Protected from

the weather

Above ground &

weather exposed

In-ground contact and exposed to the weather


Very Durable



25 years +




15 – 40

15 to 25 years


Moderately  Durable


7 – 15

5 to 15 years


Perishable / Non-Durable


0 – 7

0 to 5 years


Using these classifications to rate the timber cannot be done with great precision because of the inconsistency of wood properties within species and the broad variety of ground conditions in which it may be used.


As a result the classifications are a guide with the actual life dependent on the local ground conditions and other factors that include the following:


The above classifications apply only to heartwood with all sapwood having poor resistance to decay and insect attack.

Generally the inner core of heartwood around the pith, has lower durability than the rest of the heartwood.

Durability can also be influenced by the size of the timber i.e. the larger the piece size the more durable is will be.


The classifications show no natural durability rating against these hazards:


Chemical hazards

Marine hazard

Physical and mechanical hazards

Fire hazards