Wet rot and dry rot
Both are fungi eating wet timber. One stays where the water is. The other travels, which is why it frightens people.
Neither happens in dry timber
Worth stating plainly at the outset, because it determines the whole remedy: fungal decay needs water. Timber below roughly 20% moisture content does not rot. So every case of rot in a building is the second half of a story whose first half is a leak, a blocked vent, a bridged damp-proof course, a failed gutter or a lack of ventilation.
Treating the fungus and leaving the water is spending money to buy time. Removing the water is the actual repair.
Wet rot
Much the more common of the two. A group of fungi that decay timber which is persistently wet — typically above 30% moisture content — and which stay confined to that wet timber.
- Timber that is dark, soft and spongy when wet, cracking along the grain as it dries
- A localised loss of strength — a window sill you can push a screwdriver into, a soft skirting, a spongy joist end where it enters a damp wall
- Classic locations: window and door frames, sills, fascias and bargeboards, joist ends built into external walls, timber against leaking plumbing, cellar and subfloor timbers
The remedy is proportionate: stop the water, cut out and replace what has lost strength, and let the rest dry. Wet rot does not require the extensive works that dry rot sometimes does.
Dry rot
Serpula lacrymans, and the name is misleading — it needs water like anything else, around 20% and above to establish. What makes it different is that once established it can generate and conduct its own moisture, sending strands through masonry, plaster and behind finishes to reach and attack dry timber some distance from the original wet spot. That is why it spreads and why it has its reputation.
What to look for
- Timber that has shrunk and split into deep cube-like cracks across and along the grain
- White or grey cotton-wool sheets of mycelium, sometimes with lilac or yellow tinges
- Brittle grey strands running over masonry, plaster or brickwork
- A flat, fleshy rust-red fruiting body, often with a white margin, shedding fine rust-coloured spore dust onto surfaces nearby
- A strong damp, mushroomy, earthy smell — frequently the first thing noticed, before anything is visible
What it needs
Dry rot needs finding in full, which usually means opening up to establish its actual extent rather than guessing at it — it is often more extensive than the visible outbreak suggests, and behind plaster or under floors is exactly where it likes to be. Then: eliminate the moisture source, remove decayed timber with an adequate margin, introduce ventilation, and treat or isolate adjacent masonry where strands have run through it.
It also needs a level head. Dry rot has historically been used to justify a great deal of unnecessary work. The extent should be established by inspection, and the specification should follow from what was found.
Both usually come with company
Where there has been enough water for long enough to rot timber, there is often insect activity in the same softened wood, and there is always an underlying damp defect to identify. We look at all three together — see timber surveys and repairs.
Common questions
Is dry rot as serious as people say?
It can be, because it travels beyond the originally wet timber and can affect structural members. It is also frequently over-diagnosed and over-specified. Both things are true, which is why establishing the real extent by inspection matters before agreeing to any programme of work.
Can I just treat the timber with a preservative?
Preservative has a place as part of a specification, but on its own it does not address either the water that caused the decay or timber that has already lost structural strength. Spraying a rotten joist end does not make it capable of carrying a floor again.
How do I know if a joist end has gone?
Common signs are a floor that feels bouncy or has dropped near an external wall, skirting that has moved, and springiness in one area. Confirming it means lifting a board or inspecting from below, which is part of what a timber survey does.
We can smell something musty but cannot see anything. Is that worth investigating?
Yes. That earthy, mushroomy smell is often the earliest indication of fungal decay in a concealed space such as under a floor, in a cellar or behind panelling, and it is much better investigated early.
Not sure what you are dealing with?
That is normal, and it is what a survey is for. Ring and describe it — we will tell you whether it needs a visit and what that visit costs before anyone comes out.