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.
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
