The durability of wood
'Durability' means the ability of wood to resist decay and insect attack.
All mature trees have a stiff inner core known as “heartwood” surrounded by an outer layer of younger “sapwood”. The sapwood is where the tree stores the nutrients essential to growth. These food reserves remain after the tree is logged and sawn into wooden products.
The heartwood of some species contains naturally occurring chemicals that make it relatively durable.
The degree of natural durability varies from species to species. Sapwood on the other hand is a source of food for many species of fungi and insects and is always vulnerable to attack.
The risk of attack by such biological organisms increases significantly if the moisture content of wood rises above 20% for any reason – for example poor installation practice or maintenance, persistent condensation and damp.
Increasing utilisation and management of forest resources means that wooden components are more likely to contain high proportions of sapwood with low durability. However, thanks to the science of wood protection technology, wood can be made highly durable with a reliable long term performance in almost any situation.
Sawn timber showing heartwood (darker) and sapwood colour variation. Heartwood is not always distinguishable and the proportion of sapwood can vary significantly from one piece of wood to another. All sapwood is non-durable.