A tree trunk has three main zones; the bark, the sapwood and the heartwood. Trees are basically triangular in shape and they gain height by growing outwards and upwards. At the start of each growing season, a new layer is added to the outside of the wood so that every part of the tree gets bigger – roots, trunk, branches and twigs.
An important consideration when building with timber is predicting how long the structure will last. Whilst insects, decay or rotting organisms and marine borers can all attack wood, some timber species have the ability to resist attack better than others. The natural durability of a species to resist attack by wood destroying organisms is an indication of how long that timber will last when it is exposed to a defined set of exposure conditions without any additional preservative protection. The natural durability of timber species varies, even so within the wood from the same species of tree, depending on whether its sapwood or heartwood.