Often, when reading information about timber products the word “durability” is mentioned. What does this actually mean? What is wood durability? Mature trees all contain Heartwood which is an inner core and this is surrounded by an outer layer known as Sapwood.
Sapwood is new wood. It is like a pipeline which moves water through the tree up to the leaves. Sapwood is where a tree stores its nutrient which are essential to its growth. When a tree if felled these nutrient food reserves remain in the timber after it sawn into components. Sapwood is a source of food for many species of fungi and insects and is always therefore more vulnerable to attack. The of attack increases significantly if the moisture content of wood, for any reason, rises above 20% . This can be caused for example by poor installation practice or maintenance.
Heartwood, the central, strong inner core of a tree of a tree is dead or retired sapwood, it doesn’t decay easily. As long as the tree’s outer layers are intact, heartwood remains strong. In some species the Heartwood contains naturally occurring chemicals that make it relatively durable i.e. the ability to resist decay and insect attack. The natural durability varies from species to species.
The timber we use to build with today is more likely to contain high proportions of low durability sapwood. This is due to increasing forestry management practice and production yields.
The addition of a preservation treatment can improve wood durability, offering longer term performance of less durable Sapwoods. Wood that has been tanalised pressure treated will be more resistant to rot, fungus, mould, and insects, as well as hardier in extreme weather conditions. However, what it does not protect against is the wood reacting over time to weather conditions they are in; particularly heat and humidity.