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Dry Rot

Despite its name, dry rot affects damp timber and, just like wet rot, it is caused by a fungus.

The particular organism responsible for dry rot is called ‘serpula lacrymans’, the Latin ‘lacrymans’, which means ‘tears’, referring to the characteristic glistening surface that coats infected timber.

For serpula to germinate, it needs a temperature of no less than 3°C (preferably 21/22°C) oxygen, water and a food source (the wood). The brown, ridged spore-producing part of the white-edged fruiting body begins by sending down a hollow tube (or hypha) into the wood, which then grows and branches to become hyphal threads or mycelium. The mycelium breaks down the wood, causing darkening and cracking and making timber structures unsafe.

One of the main problems with dry rot is its ability to cross brick and stone to reach more timber, even using calcium salts from plaster as a food source. The grey strands, sometimes as thick as a pencil lead, nourish the growing front of the fungus as it spreads.

Searching for signs of damp

The first step in tackling dry rot is to locate the source of the damp, sort out the problem that caused it and commence rapid drying of the area. A thorough inspection of the property is often needed, both external and internal, checking the following:

External Roof

  • Blocked or defective gutters and rainwater goods
  • Missing or broken slates/tiles
  • Damaged chimney flashing
  • Defective surface to flat roofs

External Walls

  • Faulty or missing damp-proofing
  • Blocked air bricks
  • Flashing around window sills
  • Mortar deterioration in brickwork
  • Damp-course bridging
  • Cracked waste or water pipes
  • Cistern or water tank overflows
  • Damage by ivy or tree roots
  • Signs of rotten wood


  • Damp on walls
  • Poor quality or missing impervious membrane under stone or concrete flooring
  • Condensation
  • Window joinery
  • Old toilet leaks (via a cracked pan or damaged union)
  • Flood water between hard floor and timber
  • Under vinyl or laminate laid on top of poorly ventilated wooden floors

Treating Dry Rot

Once the initial problem has been solved, the treatment of the fungus can begin. The treatment used is specific to dry rot as wet rot treatments are not powerful enough.

Although dry rot is mainly confined to damp areas, the extent of damp in buildings can be underestimated so close inspection is required.

Next, badly damaged timber and plaster should be carefully disposed of, with at least 600mm of the surrounding material, while minimising the spread of spores which appear as a fine rust-coloured dust. Fungicide needs to be applied to any fruiting bodies and the walls cleaned and fungicide applied. A surface application is usually sufficient although a deeper ‘irrigation’ method can be used where decorative plaster needs to be protected or a toxic barrier created across a party wall.


Finally, the whole area needs to be dried thoroughly and any efflorescence brushed off. Any rendering needs to be done with fungicidal plaster and joists painted with fungicidal paint.

Pre-treated replacement timber is easy to come across, but any cut ends or joints will require re-treatment.