Stop the Rot

The rule that dry wood kept dry will not decay is one that designers, builders and all involved with timber buildings and their maintenance should keep in mind.

Timber kept in a favourable environment lasts indefinitely. However, it is equally important to note that wood is biodegradable. Such biodegradation is accomplished in part by insects (borers and termites), but by far the greatest breakdown is caused by wood decay. Decayed wood is rotten wood. Decay fungi, acting as scavengers, play an important role in the forest, returning nutrients to the soil and preventing the forest from becoming choked with debris. The same natural process may also occur in wood in structures when the conditions are suitable. In these cases preventative measures are necessary.

How do we avoid creating the conditions favourable for decay? Wood decaying fungi have four basic requirements for growth - air (oxygen), water, a favourable temperature and a food source. The food is, of course, the wood itself; the favourable temperature range is even wider than what humans tolerate; the amount of oxygen required is so little that it is not possible to limit it sufficiently. This leaves water, or more specifically, preventing its entry into dry wood, as the remaining factor that is usually the most economic and effective way of preventing decay.

The rule that dry wood (less than 20 percent moisture content) kept dry will not decay is one that designers, builders and all involved with timber buildings and their maintenance should keep in mind. It is for this reason that timber floors are built with a sub-floor space adequate to allow ventilation that will prevent a build up of moisture, why the end sections of timber members are protected from the weather whenever possible, and why it is important to promptly control plumbing leaks in the vicinity of timber components. Timber in contact with the soil is a condition requiring a special attention described later.

Decay starts when minute fungal spores, always present in the atmosphere and light enough to be transported by the wind, alight on timber under conditions conductive for decay. This usually means when sufficient moisture is present. Once the spores germinate they produce filament-like strands which, by branching and re-branching while feeding on the wood substance, form a network or mat which may eventually appear as a cottony mass on the wood surface. If allowed to proceed, mushroom-type growth, called fruiting bodies, is formed. It is from these fruiting bodies that further spores are produced to continue the cycle. Often it is the presence of the fruiting body that is the obvious first sign that decay has been active.

The most serious effect of decay in timber is the reduction in strength which can be quite serious even at an early stage. Decay also renders wood more permeable and it therefore absorbs water more readily, making the situation worse. Decay in its advanced stages is obvious, but in the early stages it can be difficult to detect.

Any flecks or streaks of discolouration be they paler or darker than the normal colour of the wood, call for closer inspection. The suspect area should be tested by inserting the blade of a strong pen-knife and slowly prising up the grain. In sound wood it should be possible to raise a splinter, but if there is decay the fibres will break off short with a brash, carroty fracture. If a moisture meter is available, testing of the surrounding wood is advisable. Should the moisture content be above 20 pc the extension of decay to these positions is most likely if no preventative measures are taken.

When decay is detected, the most obvious move is to look for and eliminate the source of moisture. This may have originated through contact with damp materials such as site concrete, through plumbing leakages, faulty gutters, condensation, by rain water seepage in exterior woodwork, usually at joints, points of paint failure and bolt holes. In particular, any contact of the end grain of timber and moisture is a potential trouble spot.

Experience shows that window frames, the mitred ends of fascias, the end of stair treads, the exposed ends of balcony beams, pergolas, handrails and weatherboards are the most likely positions for decay to appear in exterior joinery. Internally, floorboards with poor sub-floor ventilation and places where flooding has occurred, such as laundries and bathrooms, are potential trouble-some areas.

The soil normally contributes a major source of both moisture and fungal spores so that placing unprotected timber in or on the ground is to expose it to very severe conditions of service. House stumps, fence posts, pergola supports and veranda posts are just a few of the components that may be subject to such a hazard.

At the planning stage, awareness of the above factors can eliminate many of the potential risks. Careful design to separate timber and moisture, or by using either pressure preservative treated timber or durable species in positions of high risk, is recommended.

Such a regime, while desirable, is not always achievable. Decay may be already present, it may not be possible to completely eliminate water absorption, or access to the trouble spots may be difficult.

This is when you need to call the Rot Doctor