Dry Rot (Serpula lacrymans)

Dry rot is the common name for decay caused by the fungus Serpula lacrymans. It affects timber that has been subject to prolonged elevated moisture levels. The fungus digests components within the timber structure, resulting in a loss of strength and stiffness. If not addressed, the structural capacity of affected elements can be significantly reduced.

Although termed dry rot, the fungus requires moisture to initiate growth, typically where timber moisture content has risen above approximately 20 per cent. Once established, it can extend through masonry and concealed voids in search of additional timber, even where adjacent surfaces appear dry.

Typical Indicators

• Timber exhibiting shrinkage, surface cracking and a cuboidal fracture pattern.

• Discolouration and a brittle texture to affected wood.

• White or grey sheet-like fungal growth (mycelium), sometimes with thicker strands resembling cords.

• Rust or orange coloured spore dust, often visible beneath fruiting bodies.

• A persistent damp or musty odour in enclosed areas.

Underlying Causes

Dry rot is not spontaneous. It arises where timber is exposed to sustained moisture due to building defects or inadequate environmental control. Common sources include:

• Defective coverings of the roof, flashings or rainwater goods leading to water ingress.

• Leaks from plumbing installations.

• Bridging of damp proof courses or raised external ground levels.

• Restricted ventilation to sub-floor voids or roof spaces.

In most cases, the presence of dry rot indicates an underlying defect that requires investigation and rectification.

Preventative Measures

• Maintaining coverings of the roof, rainwater goods and external fabric to prevent water penetration.

• Ensuring sub-floor voids and roof spaces are adequately ventilated.

• Addressing plumbing leaks without delay.

• Avoiding the enclosure of damp timber within impermeable finishes.

The control of moisture within the building fabric remains the primary method of prevention.

Remedial Approach

Where dry rot is confirmed, remedial works typically involve:

• Identification and rectification of the source of moisture.

• Removal of severely decayed timber to a defined extent beyond visible damage.

• Replacement with appropriately specified timber.

• Localised treatment of adjacent masonry and retained timber where justified by inspection findings.

Extent of intervention should be proportionate to the level of decay and informed by a thorough inspection. Ayling Associates Ltd provide independent assessment and reporting in relation to timber decay, identifying the cause of moisture ingress and advising on appropriate remedial works across London and the South East.