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Dry rot and its control

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, 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.

The wood destroying fungus, Serpula lacrymans, is commonly known as dry rot. However, the name ‘dry rot’ might be considered rather inappropriate since like all wood destroying fungi it requires water for germination, growth and survival. Indeed, water/dampness is the fundamental need of all wood destroying fungi plus, of course, a food source (wood); without either the fungus ceases to grow and dies.


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.      {short description of image}

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.

Wood decay is basically the reverse of wood formation. Dry rot attacks the cellulose and hemicellulose of the wood to break it back down into its basic sugar components . The sugars are respired with air by the fungus to produce carbon dioxide, water and the energy for growth. However, the lignin is not metabolised and this gives rise to the darkening in colour of the wood. A number of wood destroying fungi other than dry rot also decay the wood in the same manner, leaving the lignin untouched. The characteristic darkening of the wood by these fungi together with the typical cuboidal cracking give them the title of ‘brown rots’; dry rot is one of the brown rots.     

When the wood is broken down and utilised for food, shrinkage, loss of weight, loss of strength and cracking occur. It is the shrinkage which causes the typical ‘cuboidal’ cracking (cracks to form small cubes) of dry rot and the other 'brown rots'. Indeed, it is this shrinkage and cracking which is often the first signs of a problem.


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.


A minute spore of dry rot lands on wet wood and germinates. In the laboratory this takes around 7 - 10 days, but may be longer in the wild under less than ideal conditions,

The first growth that emerges from the spore is known as the 'germ tube'. This grows and divides to produce fine filaments, hyphae, which invade the timber and secrete enzymes to break down the wood. As the wood is broken down by the enzymes secreted by the growing fine filamentous hyphae the wood becomes even more porous so allowing further water to penetrate into the timber. Furthermore, the by-product of the decay process is water which can also contribute to the moisture within the wood.

It is reported that the spores can remain viable (in a 'dry' state) for up to 3 years; however, one report identifies that such conditions in the spore stage can be tolerated for up to 30 years. thus spores may land and be present but not germinating until the timbers become suitably wet (probably greater than 28% moisture content)

It should also be considered that the spores are 'omni-present', ie, they are everywhere. They are extremely small and literally float around in air currents (along with many other organisms, pollen, spores, etc); effectively they are present in all houses - and even found at elevations of 5 miles above the earth's surface.

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% but the above figures do vary depending on the author of the researches.

(Note: moisture contents of wood in a well ventilated environment should be between 8 -16%.)

Strands: Within the mycelium special thick walled hyphae develop — these are known as 'strands' - not rhizomorphs!!!. 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 Growthdestructive. 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.

How far can dry rot grow away from its food source? Anecdotal figures suggest around 2 metres.

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 Strandscorrugations 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.


Listed below are some details regarding dry rot growth and survival.

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.

Growth: 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.

Survival: 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 18-20ºC 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 7-8ºC, 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.


The principles for the control and eradication of dry rot are outlined as follows:


The most vulnerable feature of the fungus is it requirement for water, and it is the control and elimination of this essential requirement that forms the fundamental measure for the control and elimination of dry rot.

  • Locate and rectify the source of water causing and maintaining the rot.
  • Promote and maintain rapid drying conditions.

The removal of the source of water is the first point of attack. It is therefore absolutely essential to stop further water ingress. This action alone will eventually control and eliminate the activity. Indeed, it is the fundamental measure in eradicating the organism. Included in this action is the promotion and maintaining of rapid and good drying conditions.



  • Remove infected wood the removal of the food source will relatively quickly stop the spread of growth. This does not mean 1 metre as commonly reported. It can mean considerably less - it will depend on each particular situation. Although note that in cases where dry rot occurs in historic timbers/building then a far less destructive approach may be taken; this will rely mostly on drying techniques and will need to be very carefully monitored!!
  • Reinstate using pre-treated timber (double vacuum/pressure impregnated as appropriate), or use inert materials such as concrete, steel, etc. Consideration should also be given to the use of preservatives for steeping joist ends prior to reinstatement.
  • Spatial and physical isolation: for example reinstate timbers using joist hangers, joinery wrap. These deny the fungus a potential food source and they also prevent timbers from becoming wet.
  • Contain the fungus within the masonry away from potential food sources as follows:

a. Physical containment: joinery lining around adjacent timbers.

b. Fungicidal renderings and paints: these effectively form chemical barriers. They are based on the use of zinc oxychloride (ZOC).

c. Masonry sterilisation: This involves the application of a special water based fungicide to the masonry in the following manner:-
A surface spray with a masonry biocide is usually all that is required. However, in more severe situations a ‘cordon sanitaire’ (‘toxic box’) could be used. This involves drilling perimeter of the infected area and injecting masonry sterilant into the masonry under pressure; the work is finished with a liberal surface spray or brush treatment with the sterilant.
The traditional full wall irrigation using standard water based fungicides injected under pressure where the whole of the wall is drilled and injected usually introduces too many problems and is basically unnecessary. It introduces excess water into the masonry which frequently causes more damage than the dry rot. It is also unlikely that full saturation will be achieved, and it is also unnecessary use of a biocide. 

* Insitu chemical treatment of timber:

a. Surface spray: These are likely to be relatively ineffective since they afford very little protection of the wood. Only the outer surface of the wood receives fungicide, the greater proportion of the timber remaining untreated.

b. Conventional fungicidal pastes: Conventional pastes consist of an oil/water emulsion with the consistency of a thick ‘mayonnaise’. Because of their high oil content which carries the fungicide there is the potential for deep penetration provided that sufficient is applied and that the wood is not too wet. In most situations, however, the wood is already damp and therefore at risk from decay. In such situations conventional paste preservatives are unlikely to penetrate to any great extent because of the resident moisture in the wood. Furthermore, any surface applied paste relies on diffusion to reach deep within the wood, and even with a paste preservative the levels of fungicide necessary to prevent rot are unlikely to be achieved since the highest loading remain towards the surface therefore affording little protection towards the interior. Indeed, a research paper by Holland and Orsler (1992) reporting the evaluation of traditional paste treatments was of the opinion that, “ — treatment against wood destroying fungi — may be insufficiently effective for more severe risk situations in the longer term.”

c. Fluid injection: This involves the injection of fungicides carried in organic solvents via special plastic valves driven into the wood. The fluid is injected under pressure and can potentially give good distribution of the fungicide provided the wood isn’t too wet. Unlike conventional paste preservatives the fluid is injected within the wood and doesn't depend on penetration from the surface. But it is likely that insitu timbers are damp/wet and when such a treatment is applied it can lead to very poor distribution of the preservative due to the presence of resident moisture.

d. Borate rods: This preservative is supplied as glass-like rods which consist of a special fusion of boron compounds; these are inserted into holes drilled into the wood. The rods are soluble in water and should timber become damp then the rod slowly dissolves and distributes the preservative by diffusion into the wet areas. Because the rod is embedded in the wood the preservative distributes precisely into those areas which become at risk to decay. Their use is ideal in those areas which are at risk to decay but not yet affected, e.g., embedded joist ends, window joinery, etc. However, it must be appreciated that diffusion of the borate preservative from the rod does not take place at any significant speed at moisture contents below 26-27%. Thus, it is possible to have a situation which will propagate the spread of dry rot yet the preservative is not diffusing.

e. Boron/glycol preservatives: These consist of an inorganic boron preservative dissolved in a glycol - usually sodium octoborate in monopropylene or monoethylene glycol. The 20% 'boron' material is a liquid and is usually surface applied and the 50% material is a paste which can be either surface applied or preferably injected into holes in timber or into masonry by means of a caulking gun.

Like the borate rod boron/glycol preservatives are water soluble and will readily diffuse into damp wood, even from the surface. But unlike the rods diffusion is significantly more rapid and efficient because the material is supplied as a water miscible liquid/paste. The glycol in which the borate is dissolved is hygroscopic thereby causing rapid mixing of the soluble borate with any resident moisture. The nature of the boron/glycol formulations also ensures that diffusion will occur in wood with a moisture content considerably lower than 20% because of the hygroscopic liquid base, but this can take some time. However, this adds to the initial protective value of the treatment should the wood eventually become wetted because the preservative would have already diffused to some extent and distributed.
The overall advantages of boron based preservatives are that they are designed to distribute effectively within timbers at risk thereby affording good protection, unlike the more conventional formulations which suffer from a distinct limitations where they are required to penetrate damp, susceptible wood.  

Note: Whatever the strategy employed to control fungal decays, it is essential that the primary measures are instigated immediately before deciding on the secondary and supportive chemical treatments. All risks should be thoroughly evaluated where wood is like to remain embedded or in damp conditions, even where treated, and it is essential that in such cases the centre of the wood receives full treatment. The limitations of traditional preservatives and their application in conditions of sustained dampness must be fully understood.


In considering the requirements for the growth and survival of dry rot and methods and practices for its control, the emphasis is on attacking the essential requirements for growth and survival. Where chemical control is used as a support measure or to reduce the risk of decay to damp timbers it is essential that the whole of the area of wood at risk is treated, i.e., deep within the timber. This is unlikely to be achieved with 'conventional' preservatives.

Unlike conventional preservative pastes the boron based materials are designed to work under high risk situations, i.e., when the timbers are damp and at risk to decay. The boron/glycol formulations have the added advantage in that they will distribute more rapidly than the solid borate rods thereby ensuring greater potential protection and lowering more rapidly the risk of rot. This is especially important where the moisture contents for dry rot are marginal for survival -- the solid borate rods will not distribute so effectively under these marginal conditions.

The control of dry rot should be the total responsibility of specialist treatment companies; this includes all the attendant building works as well as any chemical treatment where deemed necessary. Fundamentally, the specialist contractor will fully understand the factors involved with the outbreak of dry rot and the significance of the control measures and associated risks. Furthermore, the use of the single specialist contractor will eliminate the problem of ‘split responsibility’ where part of the required and essential work is undertaken by a third party and part by the specialist contractor. The elimination of this split responsibility certainly serves to eliminate the cause of many continued outbreaks following failure of third parties to comply fully with the instructions issued. It certainly can eliminate some potentially very expensive disputes!

Please note: where dealing with historic properties and where it is deemed necessary to keep as much of the historic timbers as practically possible then it is essential that dry techniques are used. This WILL require careful monitoring of both conditions and the state of the rot; such practices should only be conducted by specialist professionals in the conservation of historic timbers


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