Growth capabilities of dry rot fungus


Property Remedial Articles - Olympic Construction

Serpula lacrymans requires wood (hardwood or softwood) or other cellulosic materials as a food source. The fungus grows most rapidly on wood that has a moisture content above 30% though not saturated. It is unable to colonise wood which has a moisture content below about 20% but the fungus has an ability to cause localised wetting of previously dry timber.

In well designed and maintained buildings, internal timbers will always remain below 20% moisture content and there is no risk of dry rot (or indeed wet rot) developing but risks arise where poor design or building faults lead to wetting of timber. Where wetting has occurred, damp, still air will encourage establishment and spread of the fungus, partic­ularly if these conditions are maintained for long periods. Therefore, dry rot most typically occurs in locations where timber is in contact with damp brickwork and where ventilation and heating are inadequate, for example in damp cellars, poorly ventilated sub-floor spaces and behind panelling.

Even when established, dry rot remains sensitive to the effects of low moisture content in timber. If infected wood is dried to a moisture content of about 20% or lower, the fungus will stop growing and will eventually die. Experimental observations suggest that the fungus cannot survive warm (20° C), dry conditions for more than a year, but there is evidence for longer-term survival (five to eight years) under cooler conditions.

As with other types of rot. the risk of dry rot development at any particular moisture content value will be less with timbers which have higher natural durability ratings.


Ability to spread


Besides an ability to spread on wood and other cellulosic materials, the dry rot fungus can also grow on the surface and within other substrates such as brick and plaster. It is important to stress that the fungus can derive no nourishment from non-organic materials. Fungal growth over these materials is sustained only because the dry rot strands are able to transport dissolved nutrients to the advancing hyphal edge from the wood at the source of the outbreak. In suitable damp conditions, dry rot growth can extend a distance of several metres from its food source through plaster and masonry. If the original food source becomes exhausted before the fungus reaches more timber, it will die. But if further timber is encountered this may act as a new food source, allowing further spread of the outbreak. On the basis of laboratory studies it appears that the dry rot fungus has only a limited capability for further growth once its source of food is removed.

Spread of dry rot is known to be restricted by actively drying conditions and this suggests that the fungus is not efficient at conducting water from a wet to a dry location. However, if the fungus spreads into situations which are currently dry but where introduced moisture cannot readily escape due to poor ventilation or the presence of vapour barriers, it can raise the moisture content locally to a level at which timbers can decay. Typically, this happens behind internal panelling, under badly ventilated ground floors or inside boxing to services or sash window mechanisms. If wood and the surrounding conditions are too moist for active growth, the fungus can secrete moisture to the air from its aerial hyphae. This ‘weeping’ effect, which has given rise to the specific name lacrymans, may reduce the moisture content to a level more suitable for growth.

The control of dry rot


Because the dry rot fungus cannot grow on wood which has a moisture content below about 20%, the primary aim, which all too often receives inadequate attention, should be to identify and eliminate sources of dampness and then dry the structure as rapidly as possible. In many circumstances it will take a long time, perhaps a number of years, to re-establish dry conditions and secondary measures, which are temporary but quick-acting, will often be required to prevent further damage before drying is completed — see Table 2.

Sometimes it may be particularly difficult or impracticable to dry a structure adequately and greater reliance will need to be placed on the use of secondary measures. However, in these cases or where damage is extensive, the cost of remedial treatment and maintenance will be high and, listed buildings apart, a point may be reached where demolition and rebuilding is a cheaper alternative to rehabilitation.


Table 2     Measures for controlling an outbreak of dry rot


Primary Measures Locate and eliminate sources of moisture 

Promote rapid drying of the structure.

Secondary Measures Determine the full extent of the fungal outbreak 

Remove all infested wood.


Contain the fungus within the walls


Treat remaining sound timbers with preservatives


Use preservative –treated replacement timbers.


Introduce adequate support means ‘ dpc membranes and ventilation’





Locate and eliminate sources of moisture


A detailed survey should locate such faults as defective plumbing or guttering, bridged damp-proof courses, missing or defective damp-proof courses or damp-proof membranes, missing roof tiles or damaged rendering. Some faults can be easily rectified and drying quickly achieved; in other cases drying may take years; for example, a highly porous solid wall without a DPC, which has become progressively wetter over several years. Sources of dampness can be difficult to detect and some of them may not be identified in the initial survey. Guidance is given in IP19/88.

Promote rapid drying


During remedial work, adequate heating and ventilation is necessary to ensure rapid drying of the building. The use of dehumidifiers may be advantageous in some cases though their efficient function demands that ventilation is limited


Where severe wetting has occurred, floorboards adjacent to wet walls should be removed to increase air flow to areas of dampness. If properly carried out, these measures alone would eventually control an outbreak and. a few years after dry conditions have been restored, lead to the death of the fungus. Cases of outbreaks controlled by drying alone are commonly encountered during surveys of old properties.

When dry rot has occurred in a suspended ground floor, the sub-floor ventilation should be examined to ensure there is a proper through-flow of air to all parts of the floor. If necessary, ventilation should be increased by inserting extra air vents, at least 225 x 150 mm in size and of a type that has plenty of open area. A damp-proof membrane of heavy gauge potyethylene sheeting can be laid on the oversite as an additional safeguard, especially where air may have to traverse a very wide sub-floor.



Determine the full extent of the outbreak


A thorough examination of the building is necessary to determine how far the dry rot has spread. This enables rotten wood to be identified and removed and allows nearby sound timber at risk to be treated. All woodwork in the vicinity of any outbreaks should be inspected carefully. A sharp tool (a bradawl or screwdriver) can be used to locate softened timbers. Shrinkage or distortion may be another sign that rot has occurred. Where necessary, skirting’s and floorboards should be removed to allow inspection of joists and walls below and behind them.


Woodwork in rooms adjacent to the outbreak should also be examined. Extensive removal of plaster is necessary only if timber at risk is suspected to be embedded in walls. Where no timber is embedded in walls, sound plaster can be left in position, provided a small amount around nearby woodwork (approximately 300 mm zone) is removed to ensure that the infection has not spread.


Only if the mortar has perished should it be considered essential to rebuild brickwork into which dry rot strands have permeated. Moisture meter readings of plaster may give an indication of the extent of dampness and therefore an indirect measure of the possible extent and direction of fungal spread. Alternatively, measurements of the moisture content can be made on wooden dowels inserted into holes drilled in damp masonry. In both cases the potential for inaccuracy of moisture meter readings should be borne in mind.

Remove rotten wood


Remove all wood which shows signs of softening or on which the dry rot fungus has been identified, cutting away timber 300 – 450 mm beyond the last indications of rot. This margin of safety need not be interpreted as a hard and fast rule. In the case of floorboards, for example, it is not necessary to remove fungus-free boards even though these are adjacent to one that is infected. On the other hand there may be cases where the safety margin is considered inadequate.


All timber, paper or cardboard debris must be removed from the oversite beneath suspended timber ground floors. This is good practice in any building but is vital in the case of dry rot outbreaks as the material can provide food for spread of the fungus over and through a damp oversite. Spraying timber-contaminated oversite with a surface biocide will not contain spread of dry rot from timber debris and is therefore to be avoided.


If dry rot has occurred in structural timber any remedial work must take account of Building Regulations requirements and it is recommended that a structural engineer is consulted. All decayed timber should be burnt, preferably on site. However, there are cases when, for economic or aesthetic reasons, some decayed woodwork must be retained, this must not be used unless it has been sterilised


Contain the fungus within the wall


Because the dry rot fungus derives nourishment only from wood and other cellulosic materials, it is unimportant if living fungus remains on and within brick and plaster walls, provided it cannot spread further. If measures taken to prevent further wetting and to promote drying of the building can be fully effective at an early stage, this alone may be sufficient to bring the outbreak under control.


However, there are cases, for example if there is difficulty in drying the structure, when infected walls need treating with a chemical to prevent further spread and damage. It is important then that chemical treatments remain effective until the growth of the fungus is checked by drying or exhaustion of food materials within the strands. Several fungicide treatments are available for this purpose, surface application should suffice in the majority of cases.


Surface application of fungicidal fluid

Surface application of water-based surface biocide by brush or coarse spray is the simplest and most convenient method of treating walls; atomiser sprays should not be used as they introduce unacceptable health hazards.

The treatments are usually applied to walls after removal of plaster and serve to prevent spread of the fungus under new plasterwork and to prevent the appearance of fruit-bodies on the surface of drying walls.

After treatment the walls should be allowed to dry out and any efflorescence of salts brushed off prior to replastering. Suitable surface biocides can be obtained under various proprietary names and are listed annually in Pesticides


Fungicidal renderings.

These are potentially effective even on walls fully saturated with water. They eliminate the risks of efflorescence associated with liquid treatments and provide a fungicidal barrier to a greater depth than can be guaranteed by surface application of fungicides. The only fungicidal rendering currently in use is the zinc oxychloride cement mixture available from the Preservation Centre for Wood.

The efficacy of this material has been established in tests and so far there have been no reports of failures in practice. Its distinctive colouring ensures that no surface requiring protection is missed during application, a factor which may well have contributed to its success. The major limitation to this treatment is that existing woodwork must be removed before the treatment can be applied to wall surfaces, recesses or reveals to give full protection.

Preservative plugs or pastes

These can be inserted in localised areas into holes drilled in moist walls to allow diffusion of the fungicide. They may be a better method than irrigation for providing a protective fungicidal barrier in a wall already saturated with water. If the walls are drier, but still damp enough to allow fungal growth, there is no proof that there will be enough moisture to allow the fungicide to spread within the wall.


The term irrigation is usually taken to mean the introduction of large quantities of water-based fungicidal solution into dry rot infected walls, by gravity or pressure injection, in an attempt to kill the fungus. Such treatments are unlikely to be effective owing to the difficulties in achieving thorough and even saturation of often poorly-bonded masonry or brickwork.

Moreover, the treatments involve introduction of large volumes of water They therefore delay the achievement of the primary control objective of restoring dry conditions and may, in some situations, temporarily encourage the spread of the fungus. Irrigation also considerably enhances the risk of efflorescence of salts during drying which can cause damage to new or existing plasterwork. More importantly the salts are likely to contain significant quantities of the biocide used in the treatment and will therefore need to be disposed of as toxic waste.


These disadvantages mitigate against use of large-scale irrigation. Limited localised irrigation may be justified to create a protective barrier between an outbreak and sound. but damp or poorly ventilated, adjacent timbers. Such “cordon sanitaire’ treatments cannot be guaranteed to provide control and can therefore be recommended only under exceptional circumstances where removal or preservative treatment of the timbers at risk is impracticable, for example:


  • to control the fungus and protect remaining timber in situations where stripping of the wall and/or surface treatment are undesirable, for example where an outbreak is adjacent to a valuable plastered ceiling;


  • where separating walls between properties are infected and there is no access to the other side;


  • where risks must be accepted because it is not practicable to remove woodwork embedded in a damp wall (for example bond timbers) and when there is no better alternative method of protecting the woodwork.


Treatment of  sound, damp timbers with preservative


Extensive surface treatment with a liquid preservative of apparently sound timbers left in-situ is of little value. It cannot eradicate the fungus within the timber; it may even disguise further internal decay by causing formation of a surface skin of protected wood. Surface treatments will prevent new infections developing whilst timbers dry, but such infections are in any case unlikely to develop from exposed surfaces, which will dry rapidly.


Surfaces which are unlikely to dry rapidly, and from which infection might develop, are likely to be inaccessible for treatment. In view of these limitations surface application of preservatives can be justified only to those localised areas of sound timbers which are likely to remain temporarily damp. Wherever possible, even temporarily damp timbers should be treated using a method which will achieve deeper penetration of the preservative than can be achieved by brush or spray, such as:


  • liquid preservative, applied by repeated addition of a preservative to sloping holes drilled into the wood or pressure injection.


  • application of a proprietary paste treatment to the wood surface.


  • insertion of borate rods or tablets (only if the wood is wet).


Use preservative-treated replacement timbers


New timber used in repairs should always be treated with a wood preservative. If it is likely that damp conditions will persist, as in a damp cellar, the timber should be pressure impregnated with a copper/ chromium/arsenic type of preservative to BS 4072.


Similar treatment with creosote to BS 913 is also effective, but this preservative has a strong odour and is liable to stain materials in contact with the treated wood. Consequently it should not be used in dwellings or where these characteristics would be objection­able.


If dampness is not expected to persist, an organic solvent type of preservative can be applied by immersion or the double-vacuum process. Detailed recommendations as to the level of treatment required for various timber components are described in BS 5268: Part 5, which deals specifically with structural timber, and BS 5589 which covers a wider range of uses.


These two standards also refer to naturally durable timbers which may be used without preservative treatment provided their durability rating as defined in Digest 296, is in accord with that recommended in the Standard, and that all sapwood is excluded.


Introduce support measures


These are additional procedures in common use which are really an extension of good building practice to the remedial treatment field. Any further moistening of timber can be avoided by providing a ventilation pathway between the timber and a damp wall, except at the bearing surfaces where appropriate damp proofing membranes can be used to break contact.


Alternatively, the ends of joists can be supported clear of the walls on RSJs or metal hangers if this is acceptable on structural grounds and complies with Building Regulations. Ground floor joists can be shortened and carried on new sleeper walls with an appropriate DPM. Suspended ground floors must be adequately ventilated.




Wood preservatives and wall treatment fluids are regulated under the Control of Pesticides Regulations 1986. Under these Regulations it is an offence to supply, store, sell or use these chemicals unless they have the appropriate approval.


Products approved under the Regulations are listed in Pesticides published annually by HMSO. Approved products are given an HSE number which must appear on the label.


Essential information for the safe use of products is also given on the label. Specification for supply or application of these chemicals should require that only products and processes approved under the Regulations shall be used.


Replastering specification for dry rot affected walls


Replaster with sand (Grade M, BS EN 12620-2002-PD 66821-Table Di N4MP) and cement mix 3:1 ratio with a salt inhibitor added to the gauging water to the first backing coat, to a depth of 12mm. A floating coat of Tilcon Lime- renovating plaster may be used if further coats are required. A gap of 25mm minimum is to be formed between the floor and finished line of plaster. All cement used should be sulphate-resisting cement.