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Dry or Wet Rot in Timber

Skill Level: Intermediate



Most homes have timber in their construction or decoration, wood is organic and will decay unless it is looked after.  Care and planned maintenance ensures your wooden structure or fitting not only looks good longer, but will continue to serve its intended purpose for many years to come. Very often, timber maintenance is easy, relatively cheap, and does not take much time. The main problem associated with wood is Rot! This can be either “Dry rot”, or, “Wet rot”. Set out below are the differences between the two and the various methods of treatment.



Dry-rot is a fungus that can spread throughout a building very quickly,

Dry-rot is a fungus that can spread throughout a building very quickly, destroying all timber in its path. The fungus, which thrives in unventilated and moist conditions, will penetrate brickwork and infect anything made of wood; causing widespread destruction to structural timbers, skirting boards, door frames, and wooden flooring. Because the fungus survives in damp and unventilated environments, it can reside in those areas of a property that are unexposed and not often seen, such as under floors, and behind timber cladding.  Under such conditions damage can progress quickly and become extensive before the infection is eventually discovered.


What to look for

Dry rot often appears as an off-white powdery residue on both brickwork and timber, and as it progresses, will develop thick fungal strands as thick as your finger. Where the fungus is exposed to light, it often has a lemon-yellowish tinge. Dry rot damage is not only confined to timber.  Large flat mushroom-like growths can even grow through wall finishes such as plaster or paint. These mushroom like growths are often the first visible signs of a problem. They produce great numbers of spores which are generally a bright reddish colour. Dry-rot decayed timber can be easily crumbled to dust between the fingers. This deadly fungus carves deep cracks running across the grain of the timber, and there is usually evidence of off-white layers of the fungus on the wood surface.


The treatment of dry-rot

The treatment of dry-rot involves the removal of the affected timber (including all timber for approximately a metre beyond the visible signs of the infection), and extensive chemical fungicide treatment for all adjoining timber and the brickwork of any contaminated walls and plaster. The best approach is to use environmental controls, such as isolation and improved ventilation, this will ensure that the damp, unventilated conditions required by dry-rot, do not exist. The method used to ensure that timber in a property does not become damp enough for dry-rot to thrive and attack, is to replace the dry-rot infected joists with new timber using joist hangers, instead of fixing them back into the brickwork, or by using ventilated skirting board grills to promote ventilation of a floor space. When replacing infected door frames ensure replacement frames are protected with a suitable membrane material against damp from damp brickwork.

Wet rot is not so severe a problem as dry rot!

Wet rot is basically the timber decaying naturally in the presence of high levels of moisture. There is almost always a structural problem associated with the wet rot.  It could be that a wall adjoining the timber is suffering from damp, or water collects on the timber. Any such problem must be tackled before the timber is treated otherwise the problem will reoccur. The problem may be as simple as damaged paint on the timber which will allow the wood to absorb moisture. The things to look for are; external timber frames that are not adequately painted and therefore cannot adequately protect the timber from water ingress. The rot may be caused by a missing or damaged damp proof course membrane, Where suspected, seek professional advice as the symptom may be just a sign of a much more serious problem yet to be discovered.  It is good practice to keep timber structures clear from damp soil and debris.


Check regularly your roof space for water ingress

You may not see daylight through a hole in the roof, but the water could still be running down the under felt behind the tiles, eventually affecting timber that could be some considerable distance from the actual leakage point.

Other areas susceptible to wet rot are under the kitchen sink, behind and under baths, and showers, washing basins, toilets and behind washing machines and dishwashers. Inspect all exposed and vulnerable areas of timber, such as window and door frames.

The bottoms of frames are more susceptible to rot due to water which can collect and stand. If paintwork is damaged, this can also increase the risk of wet rot. Although painted surfaces may look sound, the timber underneath may be rotting from the back. Timber suffering from wet rot will feel spongy (even through a coat of paint) and look darker than the surrounding timber. When dry, the timber will easily crack and crumble into fine particles.



Firstly, any structural problems must be repaired before treatment

Firstly, any structural problems must be repaired before treatment there is little point in repairing rot damaged timber if it is going happen again. When wet rot occurs in structural timbers such as roof trusses and floor joists, professional advice should be sought in order to fully assess the extent of the infestation. In other cases, the section of rotten timber may be removed and replaced; if the damaged area is fairly small, it can be cut away and a new piece of timber spliced into the existing member.

If the damage is confined to a very small area, an epoxy based repair kit is generally used to fill the damaged area once it has been cut back to sound timber.  Preservative tablets are available which can be inserted into the timber adjoining the repaired area which protects the timber internally. If there is any doubt that the structural problem has been eliminated, the new and adjoining timber should be treated with a proprietary wet rot treatment before redecorating. After the repair is complete, all external timbers should be protected with either a suitable paint system, or some other suitable timber preservative.


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