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Dry rot

Image of dry rot fruiting body.

'Dry rot' is the decay of wood caused by the fungus Serpula lacrymans. It can be extremely destructive and is known as the cancer of buildings. Much of the mythology surrounding dry rot is founded on the ability of its strands to penetrate through non-wood building materials, to transport water to otherwise dry areas and for the fungus to 'manufacture' its own water. In reality, the delicate hyphae are the primary colonisers and the ability to conduct water is limited and can be negated by good ventilation. The process of wood decay itself produces water but in this respect dry rot is no different from any other wood-rotting fungus and, likewise, its ability to produce moisture in this manner can be negated by ventilation. Decay will cease if the moisture content of the wood is reduced to below about 20 per cent, and many extinct outbreaks of dry rot are discovered in buildings where the fungus has died out as a result of this happening, probably following maintenance which has eliminated a water source.

The fine filaments of fungal growth, the hyphae, develop into a larger mass, the mycelium, which grows into and across the damp wood; it also grows into and across nutritionally inert materials such as plaster, mortars bricks, etc. 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. It may also be anything from fawn to 'mushroom' colour. This form of the mycelium can be peeled rather like the skin on the cap of a mushroom.

As the wood is attacked it eventually breaks down in a cube-like manner (cuboidal cracking) typical of brown rots. As far as survival is concerned it appears that growth will remain viable down to moisture contents of around 20-22% although at these moisture contents it basically just survives. Optimum moisture contents are recorded as around 35 - 40%.

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 damp 'inert' materials such as mortar and brick. In these situations they conduct nutrients to the growing hyphal tips so allowing the fungus to continue to spread over non-nutrient substrates. The 'strands' basic function is to carry nutrients; it is not to wet up further substrates/wood. Indeed, dry rot is not very good at the process of wetting.

It is this ability to travel away from the food source, over and through damp inert materials allowing the fungus to reach more timber, which makes dry rot so potentially destructive. However, it must be fully appreciated that the growth cannot penetrate dry materials to any real extent. Thus the organism is only going to invade damp substrates.

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.

It is essential to understand that water is absolutely fundamental to the growth and survival of not only dry rot but all wood destroying fungi; wood decay cannot occur, exist or survive without it!

Spore germination: To initiate growth from a spore the wood must be physically wet; in other words it must be subject to a source of water ingress, e.g., leaking gutters, wood in contact with damp masonry, etc. In practical terms the wood must have a moisture content in excess of 28-30%. Spores will not germinate on dry surfaces or surfaces which are not suitably wet. In other words, unless the wood is wet dry rot cannot become initiated.

The origin of a dry rot attack is most likely to be associated with a severe source of water ingress, the most common being defective rainwater goods. Rising damp does not appear to be common as the originator of an infection although it will certainly support growth where infected wood is in contact with such dampness.

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 damp 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 millimeters 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 meter 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.

The spores are reported to remain viable for up to 3 years. They could therefore lay dormant until such times when conditions become suitable for their germination, that is, when any exposed wood surface on which they have landed becomes wet.

The mycelium can remain viable in damp masonry at around 18-20 degrees C without a food source for up to 10-12 months. But under the damp, humid conditions such as found in a cellar with temperatures of 7-8 degrees C, the mycelium may remain viable for up to 9-10 years! If untreated wood is put in contact with damp infected masonry there is always the potential for the new wood to become infected.

Because of the total dependency of dry rot on moisture, the primary control strategy must be based on environmental considerations aiming to restore and maintain dry conditions. However, in many cases drying will take a long time, often measured in years, especially where some types of historic buildings are affected. Therefore, secondary measures will often be required to prevent further damage by the fungus before it is effectively arrested by the drying. A detailed survey should be carried out to identify and locate sources of moisture ingress. Particular attention should be paid to roofs and rainwater systems with emphasis on gutters and downpipes, parapet roofs and roof coverings. Rain penetration can also be through renderings and flashings or around windows and doors. Rising dampness through missing, bridged or otherwise defective damp-proof courses must be rectified. Any plumbing should also be inspected for leaks.

Rapid drying should be encouraged through the provision of heating and ventilation which may also require specific building work to prevent moisture ingress and transfer, and to encourage aeration. Dehumidifiers can remove moisture from the air but their effectiveness in aiding drying of walls depends on the rate of evaporation from the wall surfaces.

Assessing the outbreak It is necessary to determine how far the dry rot has spread. All woodwork in the vicinity of any outbreaks should be inspected carefully to assess the extent of decay and the current moisture content of the timber. Extensive removal of plaster is necessary only if it is suspected that timber is embedded in the walls and is at risk. Removal of all timber affected by dry rot is destructive but necessary in principle. Timber already below 20 per cent moisture content presents little risk of further decay but, at higher moisture contents, the level of risk depends upon the speed with which drying can be induced and the ease of monitoring the reducing moisture content. Higher risks may be acceptable where timbers are of historic value or where their removal cannot be achieved without damage to important historic fabric - for example, where they support a fine plaster ceiling. In such cases the retention of some timbers may be essential or at least highly desirable. If the wood can be removed, it can be sterilised in a kiln. The temperature throughout the wood must be maintained at just over 40°C for 15 minutes. Care is needed to prevent splitting and distortion and this method provides no protection to the wood after reinstatement.

Special building measures are necessary if timber is to be retained, including isolation from damp masonry.

Liquid preservatives can be applied to the surface of sound timbers left in situ to help prevent new infections developing during the drying process. However, they should not be used or regarded as an alternative to physical methods of protection. If timber infected with dry rot has to be retained for special reasons and decay cannot be arrested in the short term by drying, preservative treatments that penetrate throughout the affected part of the timber can be used. For example: application of a preservative paste repeated addition of liquid preservative to sloping holes drilled into the wood or by pressure injection insertion of borate rods or tablets (these are only effective if the wood is wet). Treatment of hardwoods must include an insecticide if there is a risk of infestation by death watch beetle. All new timber used in repairs should be pre-treated with a wood preservative.

Although strands can grow through and across masonry, the dry rot fungus derives no nourishment from it. The concept of killing the fungus within masonry by wide-spread irrigation with a fungicide traditionally has provided a 'comfort factor', but it has to be questioned in each case whether this procedure can be justified. First, it is usually difficult to achieve a thorough treatment and, secondly, the treatments introduce large quantities of water which then need to be removed, increasing the risk of salt efflorescence and damage to the masonry, as well as prolonging the time it takes to dry the structure. The most important role of chemical treatments of the masonry is to prevent the fungus from obtaining access to a fresh food supply in the form of timber in adjacent areas, or replacement timbers being introduced into the area. For this purpose, localised chemical treatments of the masonry can create a useful barrier between the fungus in the wall and the wood. Examples of such treatments are: surface application of fungicidal fluid (which also helps prevent fruit-body formation during the drying phase) use of fungicidal renderings insertion of preservative plugs or pastes localised irrigation treatments. Whilst these localised treatments play a role in the overall control strategy, they must not be regarded as a substitute for getting the building dry. Heat sterilisation of masonry walls and timber in situ In the past, the use of heat to sterilise walls was condemned because it was too difficult to apply effectively and provided no residual protection.

In the search for chemical-free control measures, sterilisation with hot air is now increasingly being used, particularly in Denmark. However, the process needs to be carefully controlled so as to prevent damage to the building as well as to ensure that the necessary temperature has been achieved deep in the affected area.

Monitoring the conditions in buildings cannot be over-emphasised. Dry rot develops very slowly, so early detection and curing of moisture ingress will prevent decay occurring in the longer term. Routine monitoring can be as simple as regular visual inspection to check the integrity of the building fabric against ingress of moisture, and taking measurements of moisture content of vulnerable timbers with a hand-held probe. However, sophisticated permanent monitoring systems are now increasingly used involving computer-based equipment linked to probes permanently installed in timbers or other parts of the building fabric. Specific sensors can also be installed in rainwater goods to indicate overflows. Dry rot is potentially a cause of seriously damaging decay for timber in historic buildings, but it does not have to be devastating or outrageously expensive to cure if caught in time. A careful diagnostic approach is required to identify and cure dampness, to treat in a very selective and targeted way and to re-instate with appropriately pre-treated or naturally durable replacement timber. Installation of monitoring systems to facilitate routine maintenance checks can enable massive economies compared with destructive re-build approaches and will provide greater assurance for the future.