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

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.

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 www.Rightsurvey.co.uk/survey-types for advice on booking a home survey.

What is the best wet rot treatment?

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.