Durability

Durability refers to the natural durability of the heartwood of the timber. It has been found useful and convenient to classify durability into four groups, as shown in the table below.

Natural Durability Grouping

	Group				Number of years

	Very durable			exceeding 10 years
	Durable				5 - 10 years
	Moderately durable		2 - 5 years
	Non-durable			0 - 2 year

The placement of any timber in any of these four groups is based on its performance in 'graveyard' testing. This method of testing entails the monitoring of test-sticks measuring 50 x 50 x 600 mm buried in test grounds. The number of years that the test-sticks can last is the basis for the grouping.

Great care should be exercised when considering durability. It must be emphasised that the number of years, shown in the Table above, is relevant only to the test-sticks, which measure only 50 x 50 mm in cross-section, and that the tests have been conducted under extremely drastic environmental conditions where the test-sticks are subjected not only to weather extremities but also to attack by a very wide range of woodattacking organisms.

The number of years shown above is therefore only a yardstick for the grouping exercise and the actual service life of the timber can be expected to be much better under the more docile conditions for common usage, especially in temperate countries where climatic and other conditions are less conducive to the activity of biodeteriorating organisms.