Wet rot vs dry rot, and how to tell which one you have

Most people arrive at this question the same way. A board comes up, a skirting gets pulled back, or a carpet gets lifted before a decorator starts, and underneath is timber that is soft, dark, or crumbling in a way that is clearly not right. The instinct is to search for treatment. The better first step is identification, because the two rots are not two versions of the same problem. They behave differently, and one of them does something the other cannot.

The single most important difference is whether it spreads beyond the damp. Wet rot, usually Coniophora puteana, the cellar fungus, stays where the water is. It rots the timber it is sitting in, and when that timber dries out, it stops. Dry rot,Serpula lacrymans, is the only timber decay fungus in Britain that can carry water along its own strands and push through masonry, plaster and mortar to reach timber several metres away that was never wet in the first place. That is why a wet rot repair is usually about the size of the damage you can see, and a dry rot repair almost never is.

Everything below is arranged so you can work through it with a torch in your own house. If you get to the end and are still not sure, that is a normal outcome rather than a failure, and the last section says what to do about it.

Wet rot and dry rot side by side

This is the comparison in one place. Read the second row first, because it is the one that decides what the job actually is.

How to tell in your own house

You need a torch, ideally a bright one you can hold at a low angle rather than pointing straight at the timber, and something to prod with. A screwdriver or a bradawl is fine. Work through these in order.

Look at the way the timber has cracked

This is the most reliable thing an untrained eye can use. Dry rot cracks wood into cubes. The splits run both across the grain and along it, so the surface breaks into rough blocks anywhere from a few millimetres to a few centimetres across, and the whole piece often looks shrunken and dried out. Wet rot cracks mainly along the grain, in long splits, and the timber stays dark and feels soft rather than brittle. Hold the torch almost flat against the surface and the pattern shows up far better than it does under direct light.

Feel what the timber does when you press it

Wet rot in an active state is soft and gives under a screwdriver like damp cork, and when you pull the point out it brings away dark, stringy fibres. Dry rot damaged timber is lighter than it should be, and it tends to crumble or break into blocks rather than compress. Weight is a genuinely useful signal. A short offcut of decayed timber that feels like balsa when it should feel like pine has usually lost a great deal of its strength, whichever fungus did it.

Look for growth, strands and dust

Dry rot leaves evidence that wet rot does not. White or grey growth like cotton wool, sometimes with a silvery skin over it, means dry rot until proved otherwise. So do brittle grey strands running across masonry, plaster or the back of a skirting, which can be anything from thread width to about the thickness of a pencil. So does a flat, fleshy, red-brown fruiting body, usually with a paler rim. And so does rust-red dust settled on the floor, the sills or the top of a skirting, because that is spore dust and it means the fungus has been established long enough to fruit. Wet rot generally offers none of this. It offers dark, soft timber and not much else.

Use your nose on the first breath

Open the cupboard, the hatch or the cellar door and pay attention to the first two or three seconds. Dry rot smells mushroomy, which is different from the flat, earthy smell of ordinary damp. Your nose adjusts quickly, so the impression you get standing there for a minute is much less useful than the one you get walking in.

Then find the water

Neither fungus starts without it, so the last question is where it came from. Look above and outside the damage rather than at it. In Scottish tenement stock the usual culprits are a cracked hopper or a blocked shared gutter running down the same patch of wall for years, a joist end sitting in solid masonry with no cavity and no damp proof course between it and the weather, an airbrick blocked by a flowerbed, decking or a later extension so the floor void never dries, or a failed paint line on a sash and case window letting water into the sill. If you can identify the water, you are most of the way to knowing whether the timber will dry out, and that in turn tells you a lot about which fungus is likely to be sustaining itself in there.

Why calling dry rot wet rot is the expensive mistake

The two errors are not equally costly. Treating wet rot as though it were dry rot means you have paid for more opening up and more masonry treatment than the job needed. That is money wasted, and it is annoying, but the problem is over.

Treating dry rot as though it were wet rot means somebody has cut out the soft timber, put new timber in, replastered and painted, and left the fungus alive in the wall behind the repair. It carries its own water, so it does not need the original leak any more. It keeps going behind new plaster, and it reappears a year or two later somewhere that looks unrelated: the other side of the wall, the next room, the floor above. By that point the repair you just paid for has to come out again.

That is the entire reason this page exists, and the reason a competent survey spends its time tracing where the fungus has been rather than measuring the patch you found.

What to do next

If the evidence points one way, read the page for that problem. If it does not, the survey page is the sensible next step, because a visit that identifies the fungus and finds the water is what everything else depends on.

• Cuboidal cracking, white or grey growth, brittle strands, red dust or a mushroomy smell: read dry rot treatment, and expect the work to reach further than the damage.
• Dark, soft, stringy timber at an obvious source of water, with nothing growing on it: read wet rot treatment, where fixing the water is most of the job.
• Not sure, or the damage is somewhere you cannot see properly: atimber survey covers both rots and woodworm in one visit, which is how most of these start.

One last thing worth saying plainly. A fair share of what we get called out to turns out to be old damage from a leak that somebody fixed years ago, sitting there dry and dead and doing nothing. Establishing that is a perfectly good outcome, and it is a great deal cheaper than finding out after the timber has come out.