Dry Rot

Despite its name, dry rot actually requires moisture to take hold. Dry rot is a fungus, called Serpula lacrymans that infects the timbers of a building when they are damp enough for its spores to germinate. The fungus then grows, using and breaking down the timber it sits on as a source of food. Most alarmingly, and where the name ‘dry’ rot comes from, once it reaches a certain size, this fungus can send out strands that can cross dry, inert materials such as concrete to infect further timbers on the other side of it.

Initially the fungus appears as white cushion of what looks like cotton wool, which may produce water droplets on the surface. Where the fungus is growing in less humid conditions or is exposed to light, it appears as a thin silky, grey skin that has yellow patches tinged with lilac on it.

These sheets produce strands that carry nutrients and water with them so they are capable of crossing adjacent inert, non-nourishing materials such as brick and concrete to reach new areas of damp timber, thereby allowing the fungus to spread over large and distinct areas, potentially causing extensive damage to the structural integrity of the property.

When the fungus is advanced and preparing to reproduce, it develops a fleshy but tough pancake-like fruiting body, often through plaster. This body holds spores on its surface which are ochre to red-brown in colour, giving it the alternative name of ’brown rot’.
Rotted timber becomes dark and dry as the fungus draws all the water from it, shrinking and splitting to form ‘cuboidal’ cracks that run with the grain.

• Locate and eliminate source of moisture.
• Promote rapid drying of the structure
• Remove all rotted wood.
• Contain the fungus within the wall using preservatives.
• Use pre-treated timbers in replacement work.
• Treat remaining sound timbers with preservative.
• Introduce support measures ( ventilation, damp-proof membranes, joist hangers)

Structural woodwork such as joists should be replaced with new timbers treated with preservatives applied by pressurisation, double vacuum impregnation or dipped in organic solvent. Cut ends should be re-treated and wrapped to physically prevent the spread of rot.

Healthy timbers near a site of infection should be treated with a preservative for deep penetration. Affected plaster should be removed and replaced with fungicidal plaster. Masonry should be cleaned with a chemical fungicide and sprayed with a masonry biocide.