“Rot” is one of the words most likely to alarm a buyer reading a survey report. It’s worth understanding properly, because the two main kinds — dry rot and wet rot — behave very differently, cost very different amounts to put right, and are both, at root, a moisture problem rather than a timber problem.
The short version
- Both are fungal decay, and both need timber to be wet for long enough to take hold.
- Wet rot stays where the water is. Dry it out and it stops.
- Dry rot can spread well beyond the original wet area, across masonry and behind plaster, which is why it’s taken more seriously.
- In both cases, the lasting fix is removing the source of moisture. Chemical treatment on its own doesn’t solve either.
What is wet rot?
Wet rot is the general name for decay caused by several fungi that need timber to be consistently damp. It’s far more common than dry rot. You’ll typically find it where water is getting at timber: window sills and frames, the bottom of door frames, skirting boards against a damp wall, floor joists near a leak, and roof timbers under a leaking gutter or flashing.
What wet rot looks like
- Soft, spongy or crumbling timber that a screwdriver sinks into.
- Darkened wood, often cracking along and across the grain, or bleached and stringy in some types.
- Flaking or bubbling paint over a soft patch on windows and joinery.
- Confined to the damp area — it doesn’t travel beyond it.
What causes wet rot
Persistent moisture from a defect: a leaking gutter or downpipe, failed pointing, a plumbing leak, a failed window seal, or penetrating or rising damp reaching timbers.
What is dry rot?
Dry rot is caused by one particular fungus, Serpula lacrymans. The name is misleading: it needs damp timber to start, but once established it can spread across drier areas by sending out strands that carry moisture and nutrients, including through brickwork and behind plaster. That’s what makes it more serious.
What dry rot looks like
- White, cotton-wool-like growth (mycelium) in damp, still, dark areas, sometimes with yellow or lilac patches.
- Grey strands, like thin roots, spreading across masonry and timber.
- Timber that shrinks and splits into cubes, dry and brittle to the touch.
- A fruiting body like a flat, pancake-shaped mushroom with a rusty-red centre, often shedding fine red-brown dust (spores) nearby.
- A damp, mushroomy smell.
The Property Care Association’s guidance on the early signs of dry rot is a good reference for homeowners.
What causes dry rot
The same thing: moisture that has been allowed to persist, usually in poorly ventilated spaces — under suspended timber floors, in cellars, behind panelling, and around long-term leaks. Blocked airbricks and raised ground levels outside are common culprits under floors.
Dry rot vs wet rot at a glance
| Wet rot | Dry rot | |
|---|---|---|
| Cause | Several fungi | Serpula lacrymans |
| How common | Common | Much less common |
| Spread | Stays in the wet area | Can spread across masonry and behind plaster |
| Timber | Soft, spongy, dark | Dry, brittle, cracks into cubes |
| Tell-tale signs | Localised softness, flaking paint | White growth, grey strands, red spore dust |
| Priority | Fix the leak, repair the timber | Act promptly, find the full extent |
How rot is fixed properly
The Property Care Association puts it plainly: controlling dry rot is less about treatment and more about eliminating moisture. The same principle applies to wet rot. The steps are:
- Find and fix the source of moisture — the leak, the defect, the blocked ventilation. Without this, everything else fails.
- Establish the full extent of the decay, which for dry rot may mean opening up to trace the strands.
- Remove and replace decayed timber, with appropriately treated replacement timber where it’s at risk.
- Dry the structure out and improve ventilation — reopening airbricks, lowering external ground levels.
- Use targeted treatment only where it’s justified. Blanket spraying of timbers and masonry is rarely necessary if the moisture is under control.
For older buildings in particular, aggressive treatment that involves stripping out large amounts of sound historic fabric is increasingly seen as unnecessary where the building can be dried out and kept dry.
Rot and buying a house
A survey that finds rot isn’t necessarily a reason to walk away, but you need to know the cause, the extent and the likely cost before you exchange. A Home Survey Level 3 will look closely at timbers and flag signs of decay. Where it’s suspected, an independent damp and timber survey confirms the type, the extent and the cause. Because we don’t sell treatment, we can tell you honestly whether the “rot” needs a specialist contractor or just a repaired gutter and some new timber.
Not sure what’s causing damp near your timbers? Try our damp checker, or get in touch.
Sources and further reading
- Early signs of dry rot — Property Care Association
- Dry rot treatment advice — Property Care Association
- Damp in historic buildings — Historic England