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Excellent natural ground-durability is a key feature of the NZDFI Top 5 species selected for genetic improvement. The NZDFI Top 5 selected species are all in Class 1 or 2 based on the Australian Durability Standard.
Natural Durability Classes
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 below.
|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|
(Standards Australia, 2003. Revised 2005. AS 5604 – Timber-Natural durability ratings.)
Wood properties vary within and between species. Durability also depends on the ground conditions in which the timber is used. The classifications are therefore only a guide. Actual timber life will depend on the local ground conditions and other factors: