Durability Explained

T I M B E R     D U R A B I L I T Y

What is Durability?
Durability, or more specifically the measure of a timber species durability, is an estimation on how long that timber will perform for under different external conditions, either with no ground contact (above ground) or the tougher partially buried or in contact with the ground.  Generally, timbers are grouped into one of four groups; Perishable/Non-durable, Moderately Durable, Durable and Very Durable.
Perishable/Non-durable timbers are only suited to internal usage, where they will always be fully protected from the weather.
Moderately Durable timbers are only somewhat durable, and should be avoided for use externally.
Durable and Very Durable timbers are well suited for external use, but still have a limited In-ground lifespan.

The Sapwood of virtually all timber species is not durable, for this reason durability specifies the durability of the heartwood.  The only exception to this is Maple, which does not have a definable heartwood, but as this is a non-durable timber anyway it is never an issue.  Also of note, is if a timber is fully protected at all times, (usually this can only be guaranteed by using it internally) then there is no reason why it would rot at all, as it is the exposure to moisture that causes timber to rot.  Decay can form because of insect damage too, which can be harder to control.

Determination of a Timber's Durability
Generally speaking, most timber durabilities are learnt by placing 50x50mm heartwood stakes outside, either in the ground or above the ground. These are then checked periodically until it has been deemed that there is sufficient rot formed or insect damage to declare it decayed.

Life Expectancy of Timber Durability Classes
Due to the variable nature of tree and the variation in timber that can occur within the same tree, these life expectancies should be taken as a guide only.  Also the location of use should also be considered, eg durable timbers may last for a lot longer than 15 years in the hot dry Australian Outback, but would rot and decay very quickly in the cool damp Fiordland.

Durability Class	Fully Protected  from the weather	Above Ground  but exposed to the weather	In-ground contact and exposed to the weather
Class 1: Very Durable	50+	40+	25+
Class 2: Durable	50+	15 to 40	15 to 25
Class 3: Moderately Durable	50+	7 to 15	5 to 15
Class 4: Perishable/Non-durable	50+	0 to 7	0 to 5