Fungus in timber

Fungi are plants and therefore do not really belong in this book. Nevertheless they are often associated with animal pests and are therefore worth considering quite briefly.

There are several different species of fungus which attack timber in houses, but they only do so when the timber is damp. This may happen when timber is used before it has had time to dry out, but more often it is due to water coming in through the roof or from an overflowing drain, or through lack of ventilation.

Such fungi are most active when the timber has a water content of 30-50 per cent. Timber that has been attacked by a fungus is usually dark brown to black when wet. One of the common species is the yellow Coniophora cerebella.

Dry Rot is caused by the fungus Merulius lacrymans which can thrive in timber with a water content as low as 20 per cent, and once an attack has started the fungus can spread to dry timber, because it brings with it the water derived from the break- down of the original damp timber.

Dry rot can spread several metres through cracks and over masonry and cement, which it cannot feed on, in order to reach timber. The fungus is also distributed by its spores, produced in millions, which cover the floor and furniture as a brown dust. The spores are also spread by the wind. Dry rot mainly attacks softwoods, but sometimes also beech and oak.

Many other fungi, known colloquially as moulds or mildew, attack timber through which they spread relatively slowly, without producing any fructification comparable to the familiar mush- rooms and toadstools. It is interesting that quite a number of animal pests, mainly insects will only attack timber that has already been attacked by fungus. Certain dark mildew-like fungi produce a bluish discoloration of the timber. This occurs particularly in new timber with a water content of 50-80 per cent. The timber is not weakened mechanically.

Some moulds only grow as surface films, as for instance on wet timber and sometimes also on the inside of cold external walls where water condenses.

Prevention of fungal attacks

In order to prevent the attacks of fungi it is essential that all timber should be properly dried before it is used. Once it has been installed the next step is to ensure that there is adequate ventilation, particularly of roof spaces and of the space below the floor boards on the ground floor. Ventilation louvres should be installed and these should not be closed in winter.

Damp may also arise from leaking drains and gutters, and from condensation arising on the inside of external walls.

If such constructional measures are not sufficient to prevent fungal growth it may be necessary to resort to chemical treatment, using one of the numerous substances which render the timber unsuitable as a substrate for fungi.

The surface of the timber can be painted or sprayed with the chemical, or the wood can be immersed in it, if this is still possible.

However, this will only protect the surface as the penetration of the chemical will be very limited, depending upon the type of timber, the nature of its surface and the degree to which it has been dried. The most efficient method of protecting timber is to impregnate it with the chemical, preferably under pressure, so that this reaches all parts. The heartwood of some trees cannot be impregnated but generally speaking this is protected naturally, e.g. in oak and pine.

Once the source of the damp has been traced and suitable countermeasures have been taken, it would be advisable to identify the fungus involved. This will usually involve consultation with an expert, al- though in some cases an experienced building craftsman may be able to help.

When dry rot has been identified it is essential that all the infected timber is removed and burnt in order to prevent the dispersal of the spores. In the case of panelling and floor boards it is advisable to remove about half a metre of timber beyond the infected area.

Adjacent brickwork should be carefully cleaned and then scorched with a blowlamp. Newly installed timber must, of course, be completely dry and it should have been impregnated under pressure.

The measures recommended above would apply in cases of serious attacks by other fungi, and here again the advice of professional experts can be sought.

Animals in timber

Timber is in many ways a remarkable material, and an understanding of its nature is essential for those who wish to use it correctly. Under certain conditions timber is one of the most durable materials known to man. In Norway, for example, timber churches built about 1000 years ago are still in use. On the other hand, when conditions are different timber may disappear without trace in a few years, destroyed by a variety of animals or by fungus

It is a perfectly natural process that dead trees not only accumulate, but are broken down by other organisms, and it is only when we want to preserve the timber that we regard the process as injurious.
Timber is, in fact, very poor in nutrient, and there are only a few organisms which digest its principal constituents, which are lignin and cellulose. The bacteria and fungi which live on wood produce digestive enzymes which are able to break down the lignin and cellulose to substances, such as various sugars, which can be absorbed as food.

A few insects which feed on timber secrete the same kinds of digestive enzymes in their gut (this applies, for example, to the larvae of the house longhorn beetle), but the majority have a different method, for they have established an association with micro-organisms which help to break down the wood. Finally, there are some species, such as the powder post beetles, which are dependent upon the presence in the wood of starch or sugar.

The structure of timber

When a tree trunk is sawn through one can see first the outer layer or bark and then a thin growth layer. It is this layer that is responsible for the growth in thickness of the trunk or branch of a tree, for it forms bark outside and wood inside. A new growth ring is added every year. During the spring, large thin-walled cells are formed which transport water, while later in the summer the cells formed have walls that are thicker, and often darker. While the tree is growing the cells in the outermost annual rings are living and it is here that the transport of liquids from roots to leaves takes place. This part of the wood is called the sapwood, while the inner part, the heartwood, consists of dead cells. Right in the centre is the pith.

There are certain structural differences between coniferous trees, e.g. pine or spruce, and deciduous trees, such as oak or beech, and it is often difficult to distinguish these two types on external features. The surest method of distinguishing them is on the presence or absence of true vessels. These are present in deciduous trees, but absent in conifers. These vessels are specially adapted for the transport of water. In some species these are visible to the naked eye in a cross section. Oak and ash, for example, have large vessels.

In some deciduous trees, such as birch and beech, the vessels are more uniform in size throughout the season, and are generally quite small. Normally the darker colour of the heartwood is due to waste products, such as tannins, and to some extent these serve as a protection against fungal infections and the attacks of various animals. On the other hand, the heartwood is not receptive to impregnation with timber preservatives.

Animals that gnaw timber in buildings

In the forest, trees are attacked by many different kinds of animal. There are some which eat the leaves, but in the context of the present book we are more interested in those insects which attack the timber itself. Some of these attack only healthy, living trees, whereas others specialize exclusively in weakened or dead trees. Certain species live only in the growth layer immediately beneath the bark, whereas others penetrate right into the sapwood and the heartwood. Among those which attack dead trees, some species live in dry timber, and in the wild these will be found in dry branches or dead trunks which remain standing, whereas others will only thrive in damp, rotting timber.

When man began to use timber from the forest for equipment and houses certain of these insects must have entered his home. Some of them are merely transferred from the forest as eggs or larvae, and cannot live in timber that has been worked.

The species which had the most chance of survival, and which have become serious pests of timber, were those which could thrive in dry wood, for this is what is used in houses. In addition, there are several species which occur in places where timber structures have been damaged by damp and by fungus.

Identification of pests in timber

With a few exceptions it is insect larvae that attack timber, but normally these are difficult to find, and even if one does manage to extract one, it is difficult to identify with any degree of certainty.

The adult insects only appear for a short period, usually in summer, and so in most cases the culprit has to be identified from the tracks and signs it leaves in the timber. The type of tree, whether deciduous or coniferous, and its age and condition, may also provide clues.

Natural enemies of timber pests

It might be thought that animals living the greater part of their lives in timber would be well protected against enemies but this is not always the case. Wood-boring beetle larvae fall prey to many species of parasite and predator, and when an attack by such beetles is thought to have died out on its own, this is very often the work of these natural enemies.

The occurrence of such animals should, in fact, provide a warning that there are timber pests in the house.