Is it Wet Rot or Dry Rot?! Edward Hamilton starts the debate
Timber decay

Edward Hamilton is a Building Surveyor within our Bristol office and enjoys nothing more than a good defect pathology debate. Here he looks into the conundrum (and often mis-diagnosed) Wet Rot/Dry Rot differences.

Timber decay occurs when the timber becomes wet for a prolonged period of time. The type of dampness and localised conditions defines the type of fungus, both wet and dry rot can destroy timber and cause significant issues if left long term. 

Wet Rot

Wet rot is always located close to a source of water, as moisture is required for it to grow, such as a leaking pipe or roof or damaged render, it can also begin due to excessive condensation internally. Wet rot leads to the localised weakening of timber, causing structural instability and failure in the long term. Removing the source of water stops the growth of wet rot fungus. Wet rot is usually identified by the timber turning black and having a very wet, soft feel when touched. It can often be disguised by a paint finish, however below, the timber is cracked and flaking and can be easily scraped away. Long term wet rot does lead to a white fungal growth spreading across the decaying timber. A distinguishing feature of wet rot is that it is confined to the timber only and doesn’t spread on other surfaces.

Dry Rot

Like wet rot, dry rot still requires moisture to develop, however not as much, it grows in areas with only 20% moisture content. As opposed to wet rot, the water source is also not always the location of the dry rot. Another difference is that dry rot can spread over bricks and masonry as well as timber. Dry rot is usually hidden from view and its origin may be behind the surface finish, such as below floor boards or in a wall cavity. As dry rot is usually hidden, it is often not identified until it is too late and the fungal growth has spread considerably. Dry rot can be identified by the timber turning brown and being brittle and crumbling, with the timber itself being dry to the touch. It is also often characterised by cuboidal cracking of the timber itself. The dry rot fungus produces a ‘fruiting body’ close to the attack, which appears red with a yellow outer skin and white cotton wool type
features. Grey strands (known as mycelium) grow outwards from the rot which causes the spreading of the fungus onto surrounding surfaces. The main identifying feature of dry rot is the rust red coloured dust which settles to areas around the rotting timber.

Timber suffering from wet rot must be allowed time to dry out, the worst affected areas can then be removed and replaced if required. Wet rot is localised to the area of moisture ingress, the rot itself does not spread throughout the property like dry rot. Chemical treatments are not required with wet rot, only the timbers close to the source of moisture ingress which have been damaged need be replaced.

Dry rot can cause much more damage and can spread throughout the property. Dry rot can spread through timber and masonry as such the remedial actions are more substantial. The entire effected area requires chemical treatment to ensure all signs of dry rot have been removed. Depending on the seriousness of the attack all timbers and plaster should be removed from the area with all masonry treated. If not treated correctly the dry rot lay dormant within the area and could grow again if a new source of moisture arises and can go on to infect new timbers.

In both cases it is essential to identify and prevent the cause of the moisture ingress first. The type of rot can then be identified and treated correctly. Moisture within a property can lead to irreversible damage being caused, keeping your property watertight can prevent wet or dry rot occurring in the first place.

At 3SIXTY, being located in the heart of Clifton, Bristol, we routinely deal with period commercial buildings that are often the most prone to such decay. It is critical to source accurate and methodical diagnosis with a Surveyor who follows the trail rather than relying upon (as many do) a “specialist” contractor to diagnose and then administer a scope of works – itself a clear conflict for ensuring the correct level of advice from the start.

For further information or to discuss any of the above further, please do feel free to contact Edward – 0117 247 0120 or