Wet Rot, Dry Rot and What to Do About Them

by | Nov 16, 2009 | Building Surveys and Defects

For owners of period houses, landlords and managing agents of older blocks, the words “dry rot” can cause real alarm. Sometimes that is justified. Often the problem turns out to be wet rot, which is more common and usually less serious. Either way, the right response depends on a correct diagnosis.

Both need moisture

Timber decay is caused by fungi, and fungi need water. Sound, dry timber does not rot. Decay is therefore a sign that something else is wrong: a leaking gutter or downpipe, a defective roof, a blocked airbrick, bridged damp proofing, or a plumbing leak inside the building.

Wet rot

  • Is found where timber stays wet, such as window sills, door frames, joist ends in damp walls and timbers under leaking roofs.
  • Usually stays close to the source of moisture.
  • Leaves timber soft, darkened and cracked, often with the surface still intact.
  • Stops once the timber dries out. Weakened timber is then repaired or replaced.

Dry rot

  • Is caused by a particular fungus, Serpula lacrymans, which favours damp, poorly ventilated and undisturbed spaces.
  • Can spread across masonry and behind plaster to reach timber some distance away.
  • Shows as cuboidal cracking of timber, white or grey strands, cotton wool like growth, and fruiting bodies that shed rust coloured spores.
  • Gives off a distinctive musty smell.

What to do

  1. Find the moisture. Inspect the building as a whole, not just the affected timber. The cause is often outside.
  2. Fix the cause and allow the structure to dry, with good ventilation.
  3. Open up enough to find the extent of the decay, particularly with dry rot.
  4. Remove and replace decayed timber, using treated timber and isolating new timber from damp masonry.
  5. Be cautious about extensive chemical treatment. In many cases, drying the building is the main cure and treatment can be targeted.

Older and listed buildings

Traditional buildings need to breathe. Impermeable renders, modern plasters and blocked ventilation often cause the damp that leads to rot. Repairs should respect the way the building was built, and in listed buildings consent may be needed before historic timber is removed.

See our Building Surveys and Defect Diagnosis service, and how we carried out structural repairs to an occupied period house.