Wet rot and dry rot are both fungal decay in timber, and both are caused by the same thing, which is wood that has been wet for long enough for fungus to take hold. The practical difference between them is how far they travel. Wet rot stays where the water is, so it rots the damp timber and stops at the edge of it. Dry rot can spread beyond the wet area, growing through masonry and through small cracks in search of fresh timber to attack, which is what makes it the more serious of the two and the reason it is worth identifying properly.
The names mislead almost everyone. Dry rot is not caused by dryness. It needs wet wood exactly as wet rot does, and it earns its name from what it leaves behind, which is timber that has gone dry, brown and crumbly. So if you have found decayed wood, the useful questions are whether the decay is confined to the damp patch or reaching past it, and above all where the water is coming from. Nothing you do to the timber will hold until that is answered.
What wet rot looks like
Wet rot is much the more common of the two in ordinary homes, and the Property Care Association's guidance on wet rot describes where it typically appears, which is joist and beam ends and timber posts that rot where they meet a wet wall or the ground.
The signs are localised and fairly plain once you know them. Timber darkens noticeably compared with the sound wood around it. It goes soft, and a screwdriver or a bradawl pushes in where it should meet resistance. Depending on the species of fungus it either turns spongy and stringy or breaks into brown crumbly blocks. Paint on the surface often looks intact while the wood behind it has gone, which is why external joinery can appear sound until it is pressed.
The usual causes are leaking pipes and overflows, penetrating damp through failed pointing or a blocked gutter, condensation and poor ventilation, and any external timber left without a maintained coating. In a home, wet rot is generally what you are dealing with.
What dry rot looks like
Dry rot is less common and more consequential. The Property Care Association's guidance on dry rot sets out the identifying features, and the ones that matter for telling it apart are worth knowing.
A mass of cotton wool like growth on the timber surface, sometimes carrying visible water droplets
Affected timber that darkens, shrinks and develops square cracking across and along the grain, rather than simply going soft
A mature fruiting body, which releases very large numbers of spores into the air
Growth that runs through masonry and through very small cracks to reach timber that was never wet in the first place
A persistent musty smell in a room where nothing looks obviously wrong
That fourth point is the one that changes what has to be done. Because dry rot can carry moisture with it and travel to reach new timber, its extent is frequently larger than the visible damage, and it favours humid still spaces such as under suspended floors, behind wall linings and behind window and door linings, where nobody looks.
Why the water comes first and the timber comes second
The single most expensive mistake with either rot is treating the wood while leaving the water. Replace a rotted joist end without fixing the leaking gutter above it and the new joist end rots too, slightly more slowly because it starts dry, and the whole job comes back within a few years.
So the correct order is fixed. Find and stop the source of the water, whether that is a leak, a defective gutter, failed pointing, a bridged damp proof course or a blocked sub floor vent. Let the structure dry properly. Then establish the true extent of the decay in dried out timber, and only then carry out the repair. Work commissioned in that order tends to be smaller than the owner feared. Work commissioned in the reverse order tends to be done twice.
Suspected dry rot deserves a specialist survey before anything is cut, because the extent has to be established rather than guessed, and because getting the diagnosis wrong in either direction is costly. Treating wet rot as dry rot means stripping out sound structure. Treating dry rot as wet rot means leaving live fungus behind a new wall lining.
What has to be cut out and what can stay
This is where the work becomes carpentry rather than damp treatment, and where a good deal of money is either spent or saved. Decayed timber has to come back to sound material, and with dry rot the cut is taken well past the last visible growth rather than to the edge of it, because the strands travel further than the damage shows.
What replaces it is a judgement about the structure. Rotted joist ends, rotted wall plates, decayed lintels and bearing timbers are all structural work, and how they are repaired affects how the floor or the wall above behaves afterwards. Where the decay is confined to external joinery, such as a window sill, a door frame or a porch post, the repair is a joinery job and the replacement can be made to match the original profile rather than bought as a standard section, which is where bespoke joinery earns its place on a period property.
What thirty years of lifting floorboards teaches you
Across 30 years in carpentry and joinery, the pattern that repeats most often is that the decay is found somewhere other than where the water got in. Water runs along a joist, down a wall cavity or across the top of a plate before it soaks in, so the wet patch on the ceiling and the rotted timber are regularly in different rooms. Chasing the stain rather than tracing the fall of the water is why a first repair so often misses the real cause.
In our experience, cutting a decayed joist end back to sound timber and splicing in a new section with a bolted side plate works better than replacing the whole joist because the far end of that joist is normally built into a wall you would otherwise have to open up, taking plaster, skirting and often a chimney breast with it. A splice keeps the intervention at the damaged end, lands the new timber on new bearing clear of the damp wall, and holds the floor level because the sound half of the original joist is still setting the line. Replacing the whole member is occasionally necessary, but it converts a contained repair into a room sized one.
The other thing worth passing on is about ventilation, because it is the cheapest prevention there is. Sub floor airbricks blocked by a raised path, a new patio, a flower bed or insulation stuffed into the void will produce the still humid conditions that both rots prefer. We have found more decay traceable to a well meant improvement that sealed a void than to any dramatic leak. If you have recently had rendering, landscaping or floor insulation done, checking that the airbricks are still clear is five minutes well spent.
The short answer
Wet rot and dry rot are both fungal decay caused by wet timber. Wet rot is far more common and stays within the damp area, showing as darkened, soft or crumbly wood at joist ends, sills and frames. Dry rot is less common and more serious because it spreads through masonry and small cracks to reach dry timber elsewhere, showing as cotton wool growth, square cracking, a fruiting body and a musty smell. For both, the water has to be found and stopped first, the structure has to dry, and only then does cutting out and repairing the timber make sense. Done in that order the repair lasts. Done in any other order it returns.
If you have found decayed timber in a Kent property and want it looked at properly before it is cut about, AGC Carpentry & Building Services is based in Ashford and works across the county, including Maidstone, Canterbury, Folkestone, Hythe and Dover, covering carpentry, joinery and full renovation work. You can see recent projects on our work page or follow us on Instagram, and get in touch for an honest assessment.
