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.

Oak, above, is a ring porous wood with visible vessels or pores that are formed at the start of every growth season. The vessels get progressively smaller as the growth season progresses. Another diagnostic feature of oak are the distinctive medullary rays that run across the trunk.

Elm, above, is also a ring porous wood. In this case the latewood vessels are uniform in size with an arrangement of latewood vessels that run parallel with the growth rings and have a wavy appearance. Elm, particularly, seems to suffer from furniture beetle damage.

Scots pine has a more consistent structure. The characteristic dark growth rings are high-density smaller cells laid down at the end of the growth season. The early wood cells are larger and the contents appear lighter.

When wood-boring insect flight holes are discovered in a structural timber there is a fear the remainder of the timber will succumb to attack. This is not the case. Without chemical modification of the timber by fungal decay, only the sapwood in susceptible to insect attack at moisture contents over approximately 18%. The heartwood will remain impervious to attack due to the various chemicals it contains that give it its durability. With this knowledge, it is possible to explain the patterns of insect damage frequently found in structural timbers in buildings.

The mechanical drying of buildings with fans can be an effective process in building refurbishment in that quadrupling the amount of air movement across a wall doubles the rate of drying. This is particularly important in the control of dry rot as can be seen in the above photograph where the sterile mycelium is growing on the wall in the sheltered position behind the door but not on the more exposed sections of the corridor walls.