Dry Rot (Serpula Lacrymans)

Dry rot is a particularly virulent form of fungal decay. The fungal attack thrives in damp, dark and
unventilated conditions. Dry rot requires relatively low levels of moisture content in order for it to
thrive which makes it more dangerous than wet rot, so is difficult and costly to eradicate from a
building.

All types of wood rot can be difficult to detect in their early stages. This is particularly true of dry rot
because it nearly always develops out of sight, often spreading behind panelling and plaster or
beneath floorboards. Some indications of the possible presence of dry rot are softening of the
timber in some areas, shrinkage, and distortion and a distinctive ‘mushroom odour’. Timber
thoroughly decayed by Serpula Lacrymans is light in weight, crumbles under the fingers and has a
dull brown colour. Often, the timber shrinks and splits into brick-shaped pieces (cuboidal cracking)
formed by deep longitudinal and cross cracks. 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 timbers.

In a well-designed and maintained building, internal timbers will always remain below 20% moisture
content and there is no risk of dry rot developing. However, risks arise when poor design or building
faults lead to wetting of timber. Where wetting has occurred, damp, still air will encourage
establishment and spreading 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 subfloor spaces and behind panelling.

Due to the conditions that dry rot needs in order to grow and thrive, it is often identified only after it
has caused severe decay to structural timbers. Therefore, it is important to ensure that trained and
competent professionals are instructed to inspect buildings to either identify dry rot, or to look for
signs that could indicate the presence of dry rot such as water ingress and inadequate subfloor
ventilation.

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