Wet rot is not just one fungus
The term wet rot covers several fungi that break down damp timber. What they have in common is the need for moisture. You may find the damage beside a leaking window, beneath a shower or where wood has remained in contact with a wet wall.
The moisture source is usually close
A leaking window, shower, pipe, roof or wet wall can keep one area of timber damp enough for decay while adjacent areas remain sound. This local relationship often helps trace the cause.
Appearance varies
Affected wood may become soft, fibrous, dark, bleached or cracked depending on the fungus and timber. Because the appearance is variable, moisture and loss of strength are important parts of diagnosis.
Repair is often straightforward once the cause is known
Where the damp source is local and the structural damage is limited, stopping the water, drying the area and repairing only the weakened timber can be enough. Larger hidden outbreaks need more investigation.
Useful technical sources
- Historic England — Timber Decay in Buildings and its Treatment
- Historic England — Timber Treatments and Pest Control
These links provide background guidance. A particular building still needs its own inspection.