Wet and Dry Rot

One of the most serious causes of timber damage within buildings is fungal decay, and the two main varieties are commonly known as Dry Rot and Wet rot.

A Rot outbreak may have a disastrous impact on a building, resulting in structural failure of floor and roof beams. This has been exacerbated by the widespread use of timber within traditional and modern construction. Timber has been used in structural applications, as floor joists to Purlins and trusses in roof structures.

Dry Rot and Wet Rot are rarely seen in isolation and are usually a result of another defect, such as a misaligned gutter, broken down pipe, slipped or missing roof slates. Therefore, any discovery of a Rot outbreak should immediately look to ascertain the cause, and the localised defect, which has created the favourable conditions for fungal growth.

Both Dry and Wet Rot are caused by the combination of 3 elements, these are warm, moderate temperatures, as anything above 40 degrees will kill the fungus, a source of timber, particularly the nutrient rich sapwood and a damp source, as moisture is needed to allow the fungus spores to germinate and spread.

Comparing Wet and Dry Rot

The main difference between Dry and Wet Rot, like their namesakes, is that Wet Rot only affects timber which is wet, with a high moisture content of around 50-60%, therefore, it is usually only ever found as a result of a water leak or similar. In comparison, Dry Rot can establish in timber with much lower moisture content, at or around 20%, and once developed, this may spread and even infect relatively dry timber with moisture contents of only 14-15%.

Dry Rot or Serpula lacrymans has far greater potential to cause damage then Wet Rot, due to its ability to spread extensively, as it has been known to extend through masonry and other parts within buildings able to do so by transporting water via conducting strands from affected timber. If left untreated, Dry Rot can quickly spread through a home, and become difficult to eradicate once discovered.

Dry rot often starts in hidden, concealed spaces, and if left untreated, can easily spread throughout a building.

Wet rot or Coniophora Puteana, also known as Cellar fungus whilst generally not considered to be as intrusive as Dry rot, can still cause considerable decay to timber over time. Wet rot needs very damp and moist timber; as such its presence may indicate a water leak and or faults within the building.

Unlike Dry rot, Wet rot will not travel through masonry or plaster, as such, its spread tends to be localised to the damp timber at which it is found.

Its discovery is often indicative of a water leak, and there may be further secondary problems, including e.g. wood boring weevils

Identifying Wet and Dry Rot

Dry Rot is a type of brown fungus, termed by the colour of rot-affected timber, which turns brown following the action of dry rot, caused by the breakdown of cellulose within timber.

Indications of a Dry rot outbreak include:

  • Cuboidal cracking – caused by the removal of moisture, a primary indicator of a brown rot attack.
  • Dry timber – crumbled easily within fingers
  • Hyphae strands are grey/ white in colour, 2.8mm thick
  • Distinctive fruiting body – Pancake shape, red/brown colour, due to spore covering.
  • Mushroom like smell


Wet Rot requires high moisture contents to propagate and spread, it is usually found in areas where there may be a water leak, or building failure, such as along window cills and in roof voids.

Indications of a Wet rot outbreak include:

  • Surface cracking – cracks follow timber grain.
  • Hyphae strands are thread like brown, yellowish, becoming darker brown in colour. (Key difference distinguishing from Dry rot)
  • Distinctive fruiting body – (Although rarely found), thin sheet like shape, knobbly and olive brown surface.
  • Infected timber turns dark brown in colour

Mine fungus can cause confusion amongst surveyors, as this has white mycelium and hyphae, which is more commonly seen with a Dry Rot outbreak. 

Rot Treatment

The first step to remedying Wet and Dry Rot is to find and eliminate the source of dampness, rectifying those faults that have contributed to the problem to prevent timbers becoming damp.

Wet Rot

All infected and/or damaged timber should be removed, as these may be structurally impaired due to the decaying action of the Wet Rot. To give an example, Wet Rot that is discovered on door and window frames, may be letting water in. This may subsequently be cut out, and the remaining timber treated with a suitable preservative.


If the Wet Rot is in a confined space, such as a cupboard or loft space, it may be necessary to consider methods to improve the ventilation, as particularly if it is a non breathable roof, (does not have a Tyvek or similar breathable felt) consideration should be made to ensure adequate eaves to ridge cross ventilation.

Dry Rot

Remedial action for Dry Rot is far more intrusive. The first step is to establish the full extent of the outbreak, many parts of which may be initially concealed from view, underneath floorboards, behind furniture, panels removed etc.

Affected timber, similar to Wet Rot needs to be cut out and replaced, including those within approximately 300-450mm beyond, to allow for possible further contamination. All surrounding timbers should be treated with a chemical biocide, to ensure all strands/ spores are killed off. (Usually recommended to use a solution that works effectively on Woodworm).

It is imperative, that you ensure the timbers do not be come re wetted at this stage, as this would invalidate any insurance or guarantee for the Dry Rot treatment.

Affected plaster that has come into contact with the Dry Rot needs to be removed, and wall surfaces treated with a sterilizer or biocide.

Lastly, the area should be kept dry and heated, to ensure all timbers are fully dried out to prevent or reduce the likelihood of further outbreaks. See article Dealing with Dry Rot for more information.

Further Advice

For further advice and or information, please contact us: Alternatively, you may wish to contact the companies in the resources section.