Extensive red spore dust in the bathroom from dry rot spores © Dr Jagjit Singh
Dry rot or Serpula lacrymans is an environmentally sensitive fungus – though the term is a misnomer because the decay is invariably a result of damp. Presumably, the name arose from the crumbly, dry-looking state of wood that is completely decayed by the fungus, in contrast to other kinds.
Dry rot is a cord-forming basidiomycetes fungus that infects damp timbers in buildings, and grows across intervening spaces to colonise and decay successive damp timbers. Like other problems such as wet rot and beetle infestation, dry rot is primarily due to a lack of maintenance and defects that allow water penetration into the fabric, providing favourable conditions for infestation and ultimately resulting in timber decay.
Dry rot requires 20% moisture content in wood to begin growing and colonise, with an optimal temperature of at least 20C. Unlike wet rot, the dry rot fungus is able to spread through a building from one timber to another across surfaces lacking in nutrients when the environmental conditions are favourable as described above. The fungus has a serious impact on UK housing stock, as well as buildings of historic and architectural merit.
The early, destructive stages of dry rot are often hidden from view, as it thrives in dark unventilated cavities and voids under floors, behind walls and in ceilings. These can therefore be missed by the surveyor.
Dry rot mycelium and timber decay in the floor void
In fact, the first signs of dry rot – with the appearance of its fruiting body and resulting decay – may in fact be the final stages of an infestation, as major damage to structural timbers will already have occurred.
Typical damage to a floor joist by dry rot showing cuboidal cracking
Dry fruiting bodies
Dry rot can adversely affect the structural integrity of the timbers and disrupt the use of the building and is potentially hazardous to human health. Building surveyors in particular have to look out for dry rot and wet rot fungi when they are appraising properties, and consider the factors that may indicate the likely occurrence of internal decay.
The technical and legal implications of S. lacrymans should not be underestimated, and building surveyors must alert their clients to the risk of timber decay when carrying out either a structural building survey or a HomeBuyer Report and valuation.
Correct and early diagnosis of dry rot in a building is essential if a later widespread outbreak is to be avoided and proper repair is to be effected. Proper, timely diagnosis also minimises the chance of carrying out inadequate or excessive treatment where S. lacrymans is mistaken for wet rot or vice versa, and will help the surveyor avoid a negligence claim in the event of a future outbreak, particularly in the early stages of dry rot infection.
Dry rot in its early stages is difficult to distinguish from other wood rots without the help of a building mycologist and laboratory analysis, which involves growing samples of the fungi on artificial media under controlled conditions. In the late stages dry rot fruiting body develop and this is helpful to identify dry rot.
The most common method of dealing with dry rot, wet rot, beetle infestation and timber decay involves large-scale exposure and destruction of the building fabric to determine the full extent of the infection.
This is followed by the large-scale removal of infected timbers for fear of the rot spreading, and spraying non-infected timbers and drilling and irrigating walls with chemicals.
The remedial chemical timber treatments employed can affect building occupants and are themselves a cause of concern to environmental health authorities. This common practice, although often recommended by surveyors to clients, is therefore untenable. I would like to see a culture in which this treatment is viewed as potentially negligent.
Yet overkill of this kind remains a common practice for sales-based contractors who are judge and jury in their own court. In my opinion, they can be more damaging than the dry rot and timber decay itself.
Misidentification leads to misdiagnosis and mistreatment. Correct identification of the infestation is important because not all fungi are equally destructive – and may even be dead by the time the investigation is made. Therefore, surveyors should consider recommending that their clients engage the services of independent and impartial professionals for correct identification and treatment to avoid misdiagnosis and mistreatment.
Environmental control of dry rot and timber decay and preventative maintenance are in my experience always preferable to traditional remedial chemical drilling and irrigation. Environmental management offers a long-term, sustainable solution because by addressing the underlying causes of the problem the risk of future outbreaks is minimised.
A remedial timber treatment company dealt with an outbreak of dry rot in an embassy in central London and provided the client with a 30-year guarantee.
During subsequent renovation works to provide additional space for a passport office, the refurbishment contractor uncovered a substantial decayed timber bressummer infected with dry rot.
The client returned to the timber treatment company to inspect, review and remedy the situation. The remedial company found that the dry rot had developed after the completion of its original timber treatment works.
I was asked to investigate the outbreak and comment on the treatment company's findings. I calculated the age of dry rot and decay, based on the extent of the mycelium spread and decay to the bressummer, and confirmed that the decay was more than ten years old.
I have also come across cases where RICS HomeBuyer Reports carried out by chartered surveyors have missed dry rot outbreaks – often for the reasons described above. The surveyors in question have not adequately covered themselves with limitations of survey, exposing themselves to litigation, for example the report should include hidden timbers or inaccessible timbers not investigated.