How do I tell dry rot from wet rot, and how serious is it?

The names are unhelpful. Neither fungus grows in dry timber; both need timber wetted above roughly 20 per cent moisture content, which in practice means a leak, a bridged wall, a blocked airbrick or an unventilated void. What separates them is what they do next: wet rot stays put, and dry rot can move.

Safety

What you'll need

• Torch
• A screwdriver or bradawl for probing
• Particle mask and gloves
• Moisture meter with pins, for timber (optional)
• Camera

Step by step

1. Find the water first.There is always a water source. A leaking gutter or downpipe, a plumbing leak, a bridged damp course, a blocked airbrick, an unventilated underfloor void, a failed flashing. Until it is found, nothing else matters.
2. Probe the timber.Push a screwdriver or a bradawl into the suspect area. Sound timber resists; decayed timber takes the point easily and comes away in fibres or crumbs. Probe the ends of joists at the wall, which is where decay starts.
3. Look at how the timber has broken up.Wet rot typically cracks along the grain and goes dark and stringy or soft. Dry rot produces deep cracking across and along the grain into cubes, and the timber becomes light, dry-feeling and brittle.
4. Look for growth.Dry rot produces white or grey cotton-wool-like sheets, sometimes with lilac or yellow tints, and grey strands that can run over brickwork and plaster. A fruiting body looks like a flat rusty-orange pancake with a white edge, and it sheds a fine rust-red dust that settles on everything nearby.
5. Smell it.A strong damp mushroomy smell in a room with no obvious source is a recognised early sign of an outbreak behind a floor, a panel or a skirting.
6. Establish the extent before deciding anything.This means lifting boards, removing a skirting, opening a panel — and it needs to be done carefully and with the structure in mind. It is often the point at which a competent surveyor earns their fee.
7. Understand what the treatment actually is.For both, the primary treatment is the same: stop the water, ventilate the space, remove decayed timber back to sound material, and replace with treated timber isolated from damp masonry. Chemical treatment is a secondary measure, and the extensive irrigation of masonry once routinely specified for dry rot is now widely regarded as unnecessary when the moisture is properly controlled.
8. Be sceptical of a very large specification.Dry rot has a fearsome reputation and it is sometimes used to justify wholesale timber replacement and masonry treatment far beyond what the outbreak requires. An independent surveyor's report is worth having before agreeing to anything extensive.
9. Deal with the ventilation permanently.Most underfloor rot in UK houses traces back to blocked or buried airbricks and an unventilated void. Clearing and, where needed, adding vents is the cheap change that stops it recurring.

Tips

• Joist ends built into a solid external wall are the classic location, because the wall is damp and the timber is buried in it. A joist end that has been rebuilt should sit on a hanger or a plate with a damp course under it rather than back into the wall.
• A springy floor, a skirting that gives under a thumbnail, or a musty smell in a ground-floor room are the three things most likely to be an early outbreak.
• Photograph and date everything you open up. If this becomes an insurance or a vendor matter, the sequence of photographs is what you will need.

Common mistakes

• Treating the timber and leaving the water — Neither fungus can survive in dry timber. A chemical treatment with the leak still running delays the problem rather than solving it, and the guarantee will almost certainly exclude it.
• Cutting out a joist without support — Floor and roof timbers are structure. Removing decayed sections without propping and without understanding what is bearing on them is how ceilings come down.
• Assuming any white growth is dry rot — Harmless moulds and other fungi grow on damp surfaces routinely. Dry rot has specific features — cubical cracking, strands over masonry, rust-red spore dust — and identification matters because the response differs.

Questions people ask