Wet Rot vs Dry Rot, Key Differences and How Surveyors Assess Them

The Fundamental Difference

Wet rot and dry rot are both forms of fungal timber decay, but they are caused by different fungi, spread differently, and require very different responses. The single most important practical distinction for buyers and owners is this: wet rot is contained to areas of high moisture, whilst dry rot can spread through masonry into areas that are comparatively dry.

The Property Care Association (PCA) and RICS are the primary authorities for surveying and treating both conditions in the UK.

Wet Rot, Causes and Characteristics

Wet rot is caused by several species of fungi, most commonly Coniophora puteana (cellar fungus). It requires timber moisture content above approximately 50% and will not spread beyond the zone of high moisture. Common locations include:

• External joinery, window frames, sills, and door frames where paint or sealant has failed
• Ground floor timbers subject to persistently high moisture from sub-floor ventilation failure
• Roof timbers where there is active water ingress

Dry Rot, Causes and Characteristics

Dry rot is caused solely by Serpula lacrymans. It requires a lower timber moisture content to become established (above ~20%) but, crucially, can spread through masonry, plaster, and other inert materials once active.

How Surveyors Assess Timber Decay

A RICS surveyor carrying out a Level 2 (HomeBuyer Report) or Level 3 (Building Survey) will probe and assess accessible timbers for signs of decay. However, many timbers are concealed, beneath floorboards, within wall cavities, or above ceilings, and a standard survey will not expose these.

Where there is suspicion of dry rot (from spore staining, fruiting bodies, or a musty smell), a PCA-member specialist surveyor should be instructed to carry out an intrusive inspection. PCA surveyors are qualified to the Certificate in Remedial Treatment (CertRT) or higher.

Treatment Cost Comparison

Wet rot treatment is generally less expensive than dry rot treatment because:

• Only the decayed timber needs to be replaced (plus a safety margin)
• There is no requirement to treat surrounding masonry
• The moisture source is typically a discrete defect (failed paint, leaking gutter) that is straightforward to repair

Dry rot treatment involves removing all affected and suspect timber, treating a zone of surrounding masonry with fungicidal solution, and reinstating with pre-treated timber. The extent of masonry treatment required makes dry rot remediation significantly more costly.

Buyer Checklist

• Ask the seller to disclose any previous timber decay treatment and request the guarantee document
• Check whether any guarantee is from a PCA-member contractor and whether it is assignable
• If a survey notes suspect timbers, instruct a PCA specialist before exchanging contracts
• Factor treatment costs into any renegotiation, obtain at least two quotes from PCA-member contractors

Property Passport UK can provide context on a property's age and construction, which informs the likelihood of timber decay issues in older stock.