Timber cut from the same tree and used in the same location in a building can perform very differently depending upon the amount of heartwood an individual item contains. Consider a roof pitch with twenty common rafters: not all the rafters are cut from the same position on the tree. Those rafters cut from the outside of the trunk will have more sapwood and have a much higher susceptibility to insect attack than those cut from the middle of the trunk that are predominantly heartwood.

The photograph below of a joint in two sections of an oak rafter plate illustrates the point:


The section of plate on the left hand side is cut from oak heartwood and has a small amount of deathwatch beetle damage in a knot hole (which are less durable than the surrounding trunk timber). In contrast, the section of plate on the right was cut from the outer part of the trunk: it contained a large amount of sapwood. The sapwood has been reduced to dust by deathwatch beetle attack.

In this case, the environment in which this timber was used (beneath the roof of a church tower) resulted in its moisture content being sufficiently high to support deathwatch beetle development in the sapwood. The heartwood remains durable.

This emphasises the need to specify and use good quality building timbers where possible and to focus repairs on those sections of timber that are susceptible to attack rather than employ blanket treatments or repair specifications.