Dry Rot

Dry rot in timber is caused by a wood-destroying fungus with the scientific name Serpula lacrymans. Over a period of time, woodwork affected by dry rot will be totally destroyed, so any occurrence of dry rot is potentially dangerous. This is particularly true if the rot occurs in structural building timbers such as roof trusses, joists or beams.

Although the fungus only attacks wood, it can spread through or across masonry, especially when damp. An outbreak in one part of a building can spread to timber throughout the building, making dry rot one of the most damaging forms of rot in buildings. In severe, long-untreated cases, expensive works may be required to eradicate the fungus and to replace decayed timber.

Despite its name, dry rot only occurs when timber is damp. The fungus thrives in dark moist conditions, in parts of the property hidden from view or easy access, and therefore often spreads extensively before the damage is first noticed.

It is more likely to occur in older homes, as timber in modern properties is now treated against rot.

These are some of the typical signs of dry rot:

  • Wood shrinks, darkens and cracks in a characteristic rectangular block pattern.
  • In humid conditions white, fluffy ‘cotton wool’ appears and ‘teardrops’ may develop on the growth. 
  • In less humid conditions a silky grey to mushroom coloured skin often develops which can be peeled off. 
  • Active decay produces a musty, damp odour.

A RICS Level 3 Survey will indicate if there is any evidence of dry rot, and further investigation may be required. As dry rot tends to develop in parts of buildings which are not easily accessible, inspection can be difficult.

Treatment will usually be a twofold process. It will first be necessary to investigate the cause of the dampness which has enabled the fungus to grow, and remedy that. If damp has been caused by condensation within the building e.g. from a bathroom or kitchen, the provision of adequate ventilation may remedy the problem. However if rot has occurred because structural timbers have been in contact with damp masonry, it will either be necessary to prevent damp penetrating the wall or if that cannot easily be done (e.g. in old buildings with solid brick walls) then provision will have to be made so that timbers are not in direct contact with the masonry and new treated timber may have to be inserted.

Secondly it will be necessary to determine whether affected woodwork can be treated to eradicate the fungus, which will depend upon how far it has decayed. Provided that the structural integrity has not been affected, it may only be necessary to apply one of the various proprietary treatments which are now available. Otherwise it may be necessary to replace affected timber.

Post Author: Frances Traynor