The dry rot fungus, Serpula lacrymans, is often regarded as the ‘cancer’ of a building. Many myths have built up concerning what this fungal decay is capable of doing, occasionally leading to the belief that the fungus is indestructible and that the whole of the building will have to be pulled down.
However, dry rot is vulnerable to certain environmental effects and, like all wood destroying fungi, it has essential needs, and it is those needs that limit the extent of spread and damage that this organism can inflict. Unfortunately dry rot is a very secretive organism, favouring dark, damp stagnant conditions to develop. This is frequently why it is able to spread extensively before the damage is first noticed.
‘Dry Rot and its Control’ sets out to describe the fungus its biology, but what it can and can’t do, the conditions it must have, and most importantly how it can be readily controlled with the proper combination of environmental and building considerations coupled with the proper use of timber and masonry preservatives. Many people expect large volumes of chemicals to be used and that they will have to put up with the risk of any toxic effects and unpleasant odours and fumes which may be a part of the treatment.
‘Dry Rot and its Control’ describes the use of ProBor 50, ProBor 20 and ProBor 10, a new series of fungicides based on boron, a naturally occurring mineral. These new products are virtually odourless and have a mammalian toxicity generally in the order as that of common household salt! Furthermore, ‘ProBor’ formulations are ‘environmentally friendly’ and have a very significant advantage over the traditional dry rot preservatives in that they are water diffusible and therefore diffuse into those areas that are particularly susceptible to dry rot and other decays, i.e., where the wood is wet; the traditional preservative will not diffuse into wet timber therefore leaving such wood at great risk of decaying. The correct use of ‘ProBor’ products as described in ‘Dry Rot and its Control’, coupled with good building practice, will ensure that a building will be at very little risk from further dry rot activity and yet not put the occupants or the environment at risk from the problems which can arise from the use of traditional timber and masonry preservatives.
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DRY ROT AND ITS CONTROL
WOOD AS A FOOD SOURCE
FORMATION OF WOOD
Wood is a natural material being the end product of a complex chemical process, photosynthesis, which occurs in green plants. Wood basically consists of boxes and tubes made of sugars which are linked together to form cellulose, the basic building material of plants. Chains of cellulose are laid down in different orientations and bonded by another material, hemicellulose. A further material, lignin, adds rigidity and strength. It is the arrangement of cellulose with the two other materials which give wood its characteristic properties and its ‘cellular’ structure.
The wood forming the outer part of the tree is known as the sapwood and transports sap and stores food . This is the most vulnerable part of wood to fungal decay and attack by wood boring insects. The inner wood is the heartwood and forms the older wood in the centre of the tree; it does not conduct sap or store food but it does contain some excretory products and is more resistant to decay than the sapwood. It is also more resistant to the movement of water and preservatives in general. The heartwood of different timbers varies in its resistance to fungal decay and it is this heartwood resistance to decay by which timbers can be classified, i.e., non-durable, durable, etc.
THE INITIATION OF FUNGAL DECAY
The essential requirements for any fungal decay to take place are both food and water, especially the latter at a sufficient level. Fungal decay is generally initiated in several stages. First the water penetrates the wood and this allows bacteria and micro fungi to colonise. These break down part of the cell structure but do not cause weakening of the wood. Instead, the wood becomes more porous which allows it to become even wetter. Provided that the wood is now sufficiently wet and remains wet and that other conditions are suitable the wood rotting fungi such as dry rot can colonise.
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DRY ROT - SERPULA LACRYMANS
The fine filaments of fungal growth, the hyphae, develop into a larger mass, the mycelium, which grows into and across the damp wood. Under humid conditions the mycelium is white and cotton-wool like, and in a very humid and stagnant environment droplets of water will form on the mycelium rather like tear drops; hence the name ‘lacrymans’. These droplets are probably caused by the fungus removing excess water from the wood.Under less humid conditions the mycelium forms a silky grey coloured skin which is often tinged with yellow and lilac patches. This form of the mycelium can be peeled rather like the skin on the cap of a mushroom.
Strands: Within the mycelium special thick walled hyphae develop — these are known as strands. They are resistant to desiccation and assume their real importance when the fungus spreads over and into ‘inert’ materials such as mortar and brick. In these situations they conduct water and nutrients to the growing hyphal tips so allowing the fungus to continue to spread over non-nutrient substrates. It is this ability to travel away from the food source, over and through inert materials allowing the fungus to reach more timber, which makes dry rot so potentially destructive.
FRUITING BODY (SPOROPHORE)
When growth is usually advanced a fruiting body (sporophore) may develop. This can occur as the result of two different mycelia meeting, or the onset of ‘stress’ conditions such as drying out of the wood/environment. Light is also thought to be the cause of fruiting body formation in some situations. The fruiting body takes the form of a ‘fleshy pancake’ or a bracket, the surface of which is covered with wide pores or corrugations. The surface is orange/ochre coloured. The corrugations form the spore bearing surface.
The spores themselves are very small (about 0.01mm), ovoid in shape and orange in colour. They develop on a structure known as the basidium, four spores to each basidium. When fully developed a small droplet of fluid forms at the junction between the spore and the fine stalk on which it developed. The pressure exerted by the droplet of fluid trying to form a true sphere is sufficient to eject the spore some 20mm away from the fruiting body into the surrounding air currents for dispersal. Large numbers of spores frequently collect around the fruiting body under still conditions and form the red ‘dust’ often visible where there is a significant attack of dry rot.
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GROWTH AND SURVIVAL
Whilst timber needs to be wet for growth to be initiated, at moisture contents of around 22% existing mycelial growth ceases and the fungus will eventually die; decay just above 22% is likely to be very minimal. However, for practical purposes when dealing with fungal decay as a whole moisture contents of 20-22% should be taken as the threshold figure and assume moisture contents in excess of this level put the timber at risk.
The fungus flourishes under humid, stagnant conditions; hence growth tends to be secretive and hidden and is therefore often extensive before it becomes evident. Unlike other wood destroying fungi dry rot can grow significantly on and through damp masonry; under special conditions very limited growth might occur over and through dry materials. Distances in excess of 2 metres away from its food source have been recorded, and it is this ability to grow over and through inert material that can lead to significant problems of spread.
Like all wood destroying fungi dry rot flourishes in the slightly acidic conditions found in wood. But unlike the others it also flourishes under slightly alkaline conditions which explains the frequently encountered rapid growth behind and through old mortars and renders. Growth rates of up to 4 metres per annum have been recorded; in other cases the organism may only have spread a few millimetres in the same period of time. However, Building Research Establishment give a figure of about 0.8 meters per year as a general purpose maximum growth rate (BRE Digest 299) and Coggins (1980) gives a general figure of about 1 metre per annum. Because there are large variations in growth rates, the age of an outbreak cannot be positively determined. The problem is further complicated since it is not always possible to tell if an outbreak is the result of a single outbreak or the coalescing of numerous outbreaks.
Without a source of food (wood) growth will quickly cease and the fungus eventually die. But research has shown that in the laboratory the food reserves in the mycelium may allow up to 20% growth before spread ceases. This might have important implications in control measures since it could theoretically allow the infection to pass to immediately adjacent non-infected wood even though the original food source had been removed but leaving the mycelium on, say, damp brickwork.
Identification of Dry Rot
• The wood shrinks, darkens and cracks in a ‘cuboidal manner (typical ‘brown’ rot damage)
• A silky grey to mushroom coloured skin frequently tinged with patches of lilac and yellow colouration develops under less humid conditions. This ‘skin’ can be peeled like a mushroom.
• White fluffy cotton wool-like mycelium develops under humid conditions: ‘teardrops’ may develop on the growth.
• Strands develop in the mycelium; these are brittle when dry, and crack on bending.
• Fruiting bodies are a soft fleshy pancake or bracket with an orange. ochre surface: the surface has wide pores.
• Rust red coloured spore dust frequently seen around fruiting bodies.• Active decay produces a musty, damp
A number of insects cause decay of timber in our buildings. The best known are Anobium punctatum (woodworm) and Xestobium rufovillosum (Death watch beetle). They lay their eggs on the surface of the wood, the hatching larvae tunnel in the timber and create galleries. The tunnelling causes structural damage to the timber.
Often the insects’ activities are not significant, as the timber species may dictate that only the sapwood is consumed, which may only be a small cross-section of, for instance, a floor joist. The adult insects can be seen on the timbers during the flight season (April to August) and dust (frass) may be seen on the floor beneath infected timbers as the insects emerged from the wood.
Most buildings surveyed will have extinct infestations. These do not need treatment; however, inexperienced surveyors and parties with vested interests will recommend chemical treatments. This often means that extinct outbreaks have been treated many times, particularly if a house has changed hands often in the last thirty years
If an active infestation is found then the circumstances surrounding the attack need to be considered carefully. For instance, does the timber have high sapwood content, what species of timber is it, how wet is the timber, what would be the cost of replacement rather than treatment? More often than not infestations only require changes in the environmental conditions to reduce the moisture content of the wood, for instance, increasing ventilation to a roof void.
The infestation will eventually die out as the timbers dry. Rarely, some targeted chemical treatments and monitoring may be needed. Beware of someone recommending complete insecticide treatment of all timbers in a building, as this is unnecessary, does not address underlying causes, and kills natural predators of the insects.