Technical Q&A 26 : Timber-decaying Fungi

A holistic approach to fungal decay – wet and dry rot – minimises damage, expense and the use of chemicals. Douglas Kent, the SPAB’s Technical Secretary, explains how to fight an old adversary.  

Q. What are timber-decaying fungi?  
A. Most fungi affecting timber comprise microscopic strands (hyphae) that frequently grow together to produce a visible mass (mycelium) and mature into a fruit body that releases spores. They feed off cellulose in the timber, and the extent of decay depends upon moisture availability, the timber species and its durability. Fast-grown softwoods used since the First World War have low natural resistance so are particularly vulnerable.
 
A fungus is described according to the colour of the damaged timber ‑ white (primarily found in hardwood) or brown (favouring softwood). Alternatively, it may be classified as either dry rot (a brown rot caused by one fungus, Serpula lacrymans) or wet rot (white or brown rots caused by various fungi, commonly Coniophora puteana). 
 
Q. Why do fungi attack timber?  
A. Timber-rotting fungi, like wood-boring insects, only cause significant damage where dampness exists. This includes dry rot – despite its misleading name - although it thrives at lower moisture levels than wet rot. Sometimes active dry rot is inadvertently reported in a dry building, especially where evidence of a previous, long since extinct outbreak remains and new central heating or other recent work causes timber shrinkage
Good preventative maintenance and moisture monitoring can avert dampness and, therefore, ensuing damage caused by rot. However, over-reliance should not be placed on surface readings from electrical moisture meters. Sound oak roof timbers, for instance, can have surprisingly elevated surface moisture contents at high humidities. 
 
Q. How do I recognise wet or dry rot?  
A. Dry rot is the most aggressive wood-destroying fungus and thrives in dark, unventilated voids, for example, beneath floors.  It often has a musty smell (hence is sometimes identified using dogs). The fungus can develop into grey/white cotton wool-like sheets, ultimately forming orange fruits. It is capable of crossing non-timber surfaces and penetrating masonry. Timber becomes darker and develops cube-like cracking as it dries
Wet rot, meanwhile, commonly affects exposed wood, such as external joinery. The white species of wet rot leave timber softened, with a bleached, fibrous appearance. Although easily confused with dry rot, the brown species of wet rot differ in that their hyphae remain flexible when dried, rather than becoming brittle.
 
Q. What’s the solution for wet or dry rot?  
A. Because dry timber is immune to attack, the first measure for successfully arresting its decay is to eliminate all causes of dampness and promote drying to reduce its moisture content to below 20% – for example, by rectifying faulty gutters or improving ventilation. Major damage could necessitate conservative timber repairs, but avoid automatic wholesale replacement

Secondary measures may be required where an attack is serious and dampness will be hard to eliminate quickly or effectively. These can involve isolating susceptible timbers from damp masonry with an air gap or membrane. They could also entail targeted chemical treatment - but not as a substitute for promoting drying, or general precaution merely to obtain a guarantee

Where advice is sought, this should be from an independent chartered surveyor or consultant, not a remedial treatment contractor. 
 
Q. Can destructive dry rot treatments be avoided?  
A. There is a growing perception that some of more drastic treatments used are often neither desirable nor necessary. In particular, routinely cutting timber back to a safety margin of 0.3-1.0m beyond the last signs of decay is rarely justified. Similarly, the general removal of old plasterwork to expose timbers for inspection has little to recommend it. Monitoring and specialist non-destructive investigations can prevent much damage, although partial exposure of softwood timbers at risk may be unavoidable where chemical treatment is considered justified

The use of blowtorches to kill dry rot usually presents an unacceptable fire hazard. Hot air heat treatments have recently been trialled, however, and provisional results are encouraging.

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Further reading

Is Timber Treatment Always Necessary? An Introduction for Homeowners, SPAB Information Sheet 14 (1999)
Timber Decay in Buildings: The Conservation Approach to Treatment, by Brian Ridout, Spon Press, 2000
© Douglas Kent 2006