How to spot wet rot — plus how to treat it according to a chartered surveyor

Rising damp or wet root seeping up a white wall
(Image credit: Getty Images)

Before we answer the question 'what is wet rot', it's important to understand how severe the issue can be if it's left unchecked. It’s hard to overstate the importance of wood as a construction material in our homes. As well as being used for load-bearing beams, lintels, joists and rafters, timber is the predominant material for interior and exterior joinery - everything from skirting boards and kitchen units to windows, doors, facias and claddings. 

Anything that threatens to weaken these key components – such as wet rot – clearly needs to be rooted out before it has potentially catastrophic consequences. 

In this article, I'll be examining what what causes damp and wet rot, as well as offering some sage advice for tackling the issue in your own home. 

What is wet rot?

Fungal decay, broadly known as ‘wood rot’ is one of the most common causes of damage to UK homes. Most people are familiar with the terms ‘wet rot‘ and ‘dry rot’ - both of which are types of wood-destroying fungi that attack timber to extract nutrients and maintain growth. 

The process by which timber decays is, of course, entirely natural on the forest floor, where old dead wood is broken down over time in damp conditions. But where rot has taken hold in dwellings, outbreaks of the wet variety are more common than its more sinister fungal cousin, dry rot, with its fabled powers to spread through solid masonry. 

Types of wet rot: What you need to know

There are several species of wet rot which attack timber, the most common of which is ‘cellar fungus’ (aka Coniophora Puteana). The ‘decay threshold’ of timber is commonly defined as a moisture content of around 20%. But for decay to really get going, wet rot fungus usually needs at least 23% for fungal spores to germinate and for the resulting rot to thrive. 

When the wet rot fungus attacks certain species of wood, it digests the lignin (an organic polymer that binds the cell walls together) stripping out the darker colours and giving the timber a bleached appearance. But in many species of timber the wet rot fungus can’t digest the lignin, so instead it just feeds on cellulose and sugars leaving the affected wood with a darker brown appearance.

A dark blue bathroom with exposed wooden beams and a freestanding bath

Any wooden components of your home can be vulnerable to wet rot, so it's best to keep a watchful eye. (Image credit: Dan Duchars)

Identifying the signs of wet rot 

The most common areas to discover wet rot in your home include cellars, ground floor skirting boards, joist ends and in timbers around windows. Although, in reality any wooden components in your home are potentially vulnerable. The quality of timber used in many Victorian and older properties was superior to modern kiln dried softwoods with significantly better resistance to fungal decay.

One common sign of wet rot is where you see cracked or flaking paint with strands of moist timber peaking through the cracks. Affected areas commonly develop a darker appearance than the surrounding surfaces, depending on the type of timber and the fungal species attacking it, and is left with cuboidal or longitudinal cracks that may crumble when touched.

You may also notice a damp, earthy smell in the immediate area. Decaying timber tends to shrink, becoming soft and spongy. So a simple test is to stick a screwdriver into the wood; if it goes in easily you will very likely have pinpointed an outbreak of wet rot. 

However, identifying wet rot is not always easy because in its early stages some of the symptoms are similar to those of dry rot.  But as it takes hold, the dry variety develops distinctive white cobweb-like strands of mycelium and a white tinged ‘fruiting body’ which left untreated can spread throughout the building. Wet rot is less destructive and remains confined to damp areas only.

What are the causes of wet rot?

Wet rot is caused by the exposure of wood to a persistent source of moisture. Where timber building elements are concealed, such as floor joists, outbreaks can be harder to detect. Common causes include leaking shower trays and defective bath seals, plumbing leaks, blocked or defective gutters, and roof leaks (particularly from flat roofs and flashings). Other causes include water penetrating through lower main walls and from condensation linked to excessively humid indoor air.

What professionals can identify wet rot?

To accurately judge how serious these sorts of issues are requires professional experience. RICS Home Surveys routinely flag up concerns about rot, noting existing outbreaks as well as drawing attention to any risk factors likely to lead to future problems. What appears to be a superficial outbreak to the untrained eye may actually be a symptom of more serious hidden structural decay. 

Chartered surveyors should be familiar with likely danger signs in different ages and types of property, and will be aware of contributory factors that can affect hidden timbers, such as combinations of damp and poor ventilation to suspended timber floors. See for advice on booking a home survey.

What is the best wet rot treatment?

The landing of a period property with exposed timber frame

Opt for specialist wood hardeners in older properties, where it’s desirable to preserve as much of the original fabric as possible. (Image credit: Future)

The good news is wet rot fungus cannot survive in a dry building because dry wood doesn’t rot. Where affected timbers have retained their structural strength, they should recover naturally once isolated from the source of damp and given time to naturally dry out. 

The first step in treating wet rot is always to eliminate the underlying source of moisture behind the outbreak. Any areas of decayed wood can then be cut out and sufficient ventilation put in place to facilitate the drying out process. Only in extreme cases would you need to completely replace the affected timbers, or where structural components such as floor joists and roof rafters have significantly deteriorated. 

Some specialist contractors advise treating timbers by wholesale spraying of fungicide during the drying out period. But this is rarely necessary once the cause of the problem has been resolved. Toxic chemical fungicides are best avoided and conservationists tend to prefer boron-based alternatives - such as this one available on Amazon - applied by brush to small, targeted areas. 

In older properties where it’s desirable to preserve as much of the original fabric as possible, such as historic doors and window frames, special wood hardeners can be applied that soak into the timber and give it strength.

Do I need a professional to treat wet rot?

To address the source of the damp that’s at the root of the problem you may need to appoint a roofer, plumber or general builder. But fixing leaks and stemming ingress of water can often be undertaken on a DIY basis. 

To facilitate the drying out process and to deter future outbreaks it’s important that timbers are sufficiently ventilated. A surveyor can advise on how best to improve ventilation in the target areas without causing undue heat loss. Where non-structural timbers like skirting boards are decayed the affected areas can simply be replaced. 

Where an outbreak of rot has been noted by mortgage valuers, banks sometimes make it a condition of the loan that a ‘timber & damp specialist’ is appointed to treat the outbreak. However it’s not unknown for contractors to have a vested interest providing extensive biocide ‘preservative’ treatments, backed up by loosely worded ‘guarantees’ riddled with get-out clauses. 

The cost of remedial work will depend on how extensive the wet rot outbreak is and how much work is involved in opening up and treating affected timbers. To fix a leaking pipe and replace a short length of skirting might take half a day’s work, whereas repairing roof leaks and replacing decayed rafters could take over a week and require scaffolding, adding significantly to the cost.

More more general advice, read our how to treat damp guide.

how to build an oak frame home

In renovation projects any signs of timber decay, no matter how small, must be addressed. (Image credit: Border Oak)

Can wet rot effect my renovation project?

If left untreated, affected timbers will start to lose their strength, potentially causing key components to deteriorate, weaken and ultimately collapse. How quickly the decay takes hold will depend on the extent of dampness and the quality of the timber. Inherently durable hardwood species such as oak, and some manufactured treated timbers have excellent resistance. 

How serious an issue this is will clearly depend on how far the decay has progressed and whether the affected components are structural, such as in lintels or rafters. Either way, in renovation projects any signs of timber decay, no matter how small, must be addressed otherwise you’ll be storing up problems for the future..  

How do I prevent wet rot from coming back?

Moisture and a warm atmosphere, combined with a lack of ventilation, are the ideal environment for fungal attack. Moisture levels in timbers fluctuate accordingly, hence the importance of ventilation to disperse any dampness before fungal decay has any chance of developing. 

Above all, keeping your home free from damp, for example routinely checking gutters and pipework and bath and shower seals for leaks, will go a long way to ensuring freedom from future outbreaks. 

Ian Rock

Chartered surveyor Ian Rock MRICS is a director is and the author of eight popular Haynes House Manuals, including the Home Extension Manual, the Self Build Manual and Period Property Manual.

Ian is also the founder of Zennor Consultants. In addition to providing house surveys, Zennor Consultants provide professional guidance on property refurbishment and maintenance as well as advising on the design and construction of home extensions and loft conversions, including planning and Building Regulations compliance.

Ian has recently added a 100m2 extension to his home; he designed and project managed the build and completed much of the interior fit-out on a DIY basis.

With contributions from