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Fungus the bogeyman

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Fears of dry rot and wood borer attack are based on years of myth and misinformation. Try a more rational approach

At the beginning of the twentieth century, timber decay was not a serious concern. Much of the construction sector incorporated softwood taken from mature natural-forest trees, mostly from northern Europe. These trees were slow grown, tending to be about 200 years old when felled. Though not as durable as oak, the wood from these trees could withstand damp conditions fairly well and decay organisms struggled to cause much damage.

Repair was generally all that was required. But all this was to change.

During the First World War, vast quantities of wood were used for the war effort, causing a severe post-war timber shortage. Timber supplies came increasingly from plantations or natural forest regeneration. Economics dictated that trees should be felled when they reached a marketable size, and forestry practices were developed to speed up the process.

Today trees are frequently felled after 50 years or less of growth, and this has had a profound effect on timber durability.

The chemicals in timber which provide resistance to decay are not present in sapwood, and are generally slow to develop in the central juvenile core of the tree. A pine log might therefore have 15 to 20 years of growth with no durability, followed by 15 to 20 years of growth with poor durability. If the timber is 200 years old then this poor material does not matter very much, but if the tree is only 50 years old when felled, the loss in durability can be dramatic.

A further complication is that the volume of perishable sapwood is more or less constant throughout the life and length of the tree. If the volume stays the same then the thickness of the sapwood band will decrease as trunk diameter increases.

Conversely, the younger the tree, the greater the percentage of sapwood.

Unfortunately, all timber decay organisms love sapwood.

The inter-war years saw a boom in house building using poor quality materials, prompting the government to set up the Forest Products Research Laboratory (FPRL) which, because of its lack of resources, farmed out research projects to established laboratories to review and study decay organisms.

Dry rot became the province of Professor Percy Groom at Imperial College in London, who seems to have been chosen, in part at least, because most of the published earlier work was in German and he could read that language. Therefore its first report, Bulletin No 1 (Dry Rot in Wood), was based on a trawl through foreign literature, and not on research. Bulletin No 1 was the definitive account on dry rot, but it contained many of the inaccuracies and confusions that still plague peoples' understanding today. His definition of dry rot was broad and included a range of brown rot fungi.

A more accurate account of the organism, its abilities and limitations, had to await the writings of John Savory. He restricted the term dry rot to one fungus (Serpula lacrymans) in 1961 (BS 565:1963), and produced what he considered to be his definitive paper on the fungus in 1971 (Timberlab Paper 44). Savory stated that dry rot could sometimes be killed by drying the building, without recourse to fungicides. Most of his experience, however, was gleaned from an analysis of timber damage in poor inter-war, and postwar domestic construction, rather than in historic buildings.

Nevertheless, Savory's publications were a significant attempt to rationalise dry rot treatments. In fact, quick-fix chemical treatments, (which might be justified in twentieth century domestic buildings because of poor quality timbers, uncertain maintenance and the pursuit of a guarantee), are rarely, if ever, appropriate in historical buildings.

Two men, a kango hammer and a van load of chemicals usually cause far more damage than the fungus ever could. What you believe about dry rot unfortunately depends on when you learnt it, and a few up-to-date facts are now appropriate.

The facts 'Dry rot' is an eighteenth century term for a brown rot (but brown rot fungus is not necessarily dry rot).

The term was used because the damage was thought to be caused by internal 'fermentations' rather than water. In fact, dry rot fungus needs a minimum of 22-25 per cent moisture content in timber to flourish.

Dry rot strands do not transport water in order to moisten dry timber.

We have studied dry rot in buildings for 17 years and found that, in a normal situation, the fungus is restricted to the zone of moisture penetration.

Dry rot will die if the source of moisture is removed and the structure dries out. Dry rot which appears after remedial works have been completed is usually the desiccation of previously unnoticed dry rot damage. The breakdown of wood by brown rot produces water, but there is not enough to allow the fungus to flourish.

It is wrong to assume that there is too much, or too little moisture for dry rot, while a building is drying out.

Moisture requirements are no different to most other brown rot fungi, and it can thrive in wet timbers provided that there is a source of calcium (eg mortar or plaster) in the vicinity.

Wet rot does not turn into dry rot.

This myth came from Percy Groom, who considered that wet rot was dry rot that was too wet to be active.

Dry rot will not lurk in a dry wall for years to attack any timber placed in contact. The Princess Risborough laboratory showed that the fungus would not flourish at humidities below 95 per cent. Importantly, the fungus could not be revived after a year or so in a dry environment, and the limited nutrient content in a fungus strand severely limited further growth once the food source had been removed.

Dry rot, though undoubtedly a nuisance when growing vigorously in a congenial environment, is not necessarily difficult to control. For every situation that requires extensive and damaging treatments there will be dozens of small problems where the fungus is already dead, or which may be dealt with easily, perhaps by purchasing a new length of downpipe and a few floorboards. If a client demands rapid and guaranteed eradication then destruction is inevitable, but a careful evaluation can usually allow a far more sympathetic approach.

Wall irrigation makes walls even wetter when the aim should be to dry them down. It may also mobilise salts that will cause damage to finishes. It rarely, if ever, has any advantage over spray treatments, which simply inhibit surface growth.

Surface treatments will not kill fungus within timber.

Cutting back timbers to a metre past the last sign of decay is destructive and totally unnecessary. The fungus needs water and, provided that there is no moisture conduction between a wall and the fungus, then the fungus must die.

Treat a fungus according to the damage it is causing, not according to its name. Many wet rots can produce strands that penetrate walls, and cause as much damage as dry rot.

These facts and figures about dry rot are not new, but fear, inculcated ignorance and vested interest have made them difficult to accept. The next decay organism, deathwatch beetle (Xestobium rufovillosum), provides a different story. This insect was investigated to a limited extent by Professor Maxwell, at Imperial College, London, during the first two decades of the twentieth century, and in more detail by Ronald Fisher at the FPRL during the late 1930s. The main conclusions to be drawn from these periods of research are that the deathwatch beetle is a poor and supremely uncooperative research creature, and that uncritical laboratory observations alone can lead to significant misconceptions.

The beetle only emerges for a few weeks of the year and the larval growth period may take more than a decade to complete in a thick oak beam. Fisher's assistant and authority on wood boring beetles, Ernie Harris, who died recently, used to tell anguished tales of purchasing and transporting tree trunks to the laboratory at great expense, only to find that they eventually produced swarms of the wrong type of beetle.

Eventually, Fisher produced four scientific papers on the deathwatch beetle, and these formed the basis of subsequent treatment policy. He showed that the beetles laid eggs on the surface of oak blocks, and that the larvae wandered extensively before burrowing into the wood.

These observations were enthusiastically received because they implied that surface applied chemicals would kill the beetle when it bit its way out of the timber, destroy the egg or poison the newly emerged larvae.

Unfortunately, Fisher's observations on beetles in glass jars did not accurately reflect what happened in church roofs - and the resulting treatments killed few beetles.

The continuous failure of beetle treatment led the treatment companies to argue that the emerging beetles were alive, but had picked up chemicals and would soon die before they laid eggs. Years later, when beetles continued to plod across the altar every spring, a new excuse was borne. Yes the beetles were still alive, but they had been sterilised.

In 1996, English Heritage decided to investigate the problem and called together a team of international experts under the European Union DGX11 environment programme.

Three years later, some of the answers began to appear.

Beetles do not necessarily bite their own exit holes, they may use old holes, shakes or open joints to emerge from the timber. Female beetles reenter the timber to lay their eggs.

These observations are important because they mean that no stage of the insect's development has it had much contact with the surfaceapplied insecticides.

In 1924, Maxwell Leffroy posed a conundrum, the solution of which he considered would be fundamental to an understanding of deathwatch beetles.Why, if the beetles ate wood, and a roof was made of wood, did a beetle population frequently die out before a roof had been destroyed? There are two main solutions to this riddle.

The beetles can easily attack sapwood, but can only damage the heartwood of oak if the wood chemistry has been modified by fungus.

Most of the timber in a roof is probably unsuitable for the beetles to attack.

Low moisture contents in wood prolong the growth of the larvae, producing smaller adults that lay few eggs. Small male beetles are rejected by the females.

These two factors mean that active beetle infestations are likely to be localised and in timbers which have become wet. Good repairs and prolonged maintenance cause wood moisture contents and beetle populations to decline. Natural predators such as spiders, and lack of recruitment from other localised beetle populations then leads to extinction.

The corollary to this explanation is that beetle damage does not necessarily imply current beetle infestation.

Many churches and other buildings are treated at considerable expense when there are few, if any, beetles, actually present. The following procedure is recommended if beetle damage is located.

Search the surrounding surfaces and window sills for beetles. This is best undertaken during the emergence season (April to June) when beetles are alive. Old accumulations of beetles may be misleading, and should be removed.

Recommend that the surfaces are inspected frequently and that beetle numbers and positions are marked on a floor plan.

If many beetles are found in a location then check for a source of moisture and rectify the problem.

Consider localised timber treatment with a paste or gel type insecticide formulation that will penetrate deeply into the timbers.

Use light traps to kill or to monitor the population. This can be very effective in dark roof spaces. (But take care to ensure that the devices used cannot trap bats. ) Even though this type of treatment is undertaken every day throughout the country, a more realistic attitude to timber problems should be adopted, particularly in historical buildings.

The precautionary spray treatment of a Victorian roof, for example, is unjustified if 98 per cent of the timber is pine heartwood. Furniture beetle cannot attack it and fungus will ignore the treated surface.With a little thought, we can lessen the damage caused by over- enthusiastic treatments to our building heritage.

Dr Brian Ridout is the author of Timber Decay in Buildings: The Conservation Approach to Treatment, and director of Ridout Associates, Environmental Monitors and Timber Decay Analysts. Tel 01562 885135 or visit www. ridoutassociates. co. uk

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