Wet rot in timber: where it starts and why it stays put
Unlike the fungus popularly called "dry rot", ordinary wet rot never travels far from the water that is feeding it: which is exactly what makes it the more fixable of the two.
What wet rot actually is
Wet rot is timber decay caused by a fungus that needs the timber to stay saturated. Or very close to it, for as long as the fungus is active. Take the water away and the fungus stops: it does not travel into drier timber nearby the way the fungus behind so-called "true" dry rot sometimes can.
That single trait separates the two informal groups covered in the article on "dry rot" and why the name misleads people. It is also why wet rot, despite doing real damage, is usually the easier of the two to deal with. Find the leak, and you have found the entire cause.
Where it typically starts in a house
Wet rot needs a specific, ongoing source of moisture. So it turns up in the same handful of places again and again rather than anywhere timber happens to exist.
| Location | Typical source | Timber usually affected |
|---|---|---|
| External wall near a downpipe or gutter | A blocked or leaking gutter overflowing against the wall, or a cracked downpipe | Window frames, fascia, wall plates behind cladding |
| Window and door frames | Failed sealant or flashing letting rain track in around the frame | Sills, frame timber, adjoining architraves |
| Under a bathroom or kitchen | A slow plumbing leak. A waste pipe joint, a shower base, a tap | Subfloor bearers and joists directly beneath |
| Around stumps and low bearers | Garden bed, mulch or paving built up against or above the timber | Stump caps, bearer ends, low subfloor framing |
| Poorly ventilated subfloor | Condensation from ground moisture with nowhere to escape | Joists and bearers generally, not just one spot |
Every row has a specific, findable cause. That is the defining feature of wet rot. It is never a mystery the way "true" dry rot occasionally is, where damage can appear some distance from the original leak.
How to recognise it
Wet rot leaves no holes and no dust (that rules out borers) and no mud lining, which rules out termites. What it leaves is timber that has lost strength.
- The screwdriver test: push a flat-blade screwdriver gently into the surface. Sound timber resists; decayed timber gives with little pressure, often without splintering.
- Appearance: darker than the surrounding timber, sometimes stringy or fibrous, occasionally with visible fungal growth if the moisture is current.
- Location logic: it is confined to timber that is actually wet or has been recently. A joist three metres from any moisture source is very unlikely to be wet rot.
That last point is the practical difference from "true" dry rot worth remembering. With wet rot, if you can see the soft timber, you are already standing close to the cause.
Why it stays contained
The fungi responsible for wet rot cannot carry water to timber that is not already damp. Some dry rot fungi can move moisture through masonry. Wet rot fungi cannot, so the decay stops where the timber dries out. That is good news structurally. It does not make the affected timber safe, though. A bearer that has lost strength stays weak whether or not the fungus can spread further.
What not to do
Do not paint, seal or cover soft timber to hide it: trapping the moisture in makes the decay worse, not better. Do not assume timber that now feels dry is fine: the strength loss happened while it was wet, and drying it out afterwards does not restore it.
Do not treat a wet rot finding as evidence one way or the other about termites or borers. The three are unrelated and are commonly found in the same damp subfloor. Finding one is a reason to check for the others, not a reason to stop looking.
When to get someone in
A small, contained patch on non-structural trim, with an obvious and already-fixed cause, is rarely urgent. Book a licensed timber pest inspection when the affected timber is structural (bearers, joists, stumps, wall plates) when you cannot find or fix the moisture source yourself. Or when softness turns up in more than one spot. A standard timber pest inspection to AS 4349.3 covers decay alongside termites and borers, so one visit answers all three questions at once.
Frequently asked questions
What causes wet rot in timber?
A fungus that needs sustained, ongoing moisture in the timber to survive. In Australian houses that almost always traces back to a specific source: a leaking gutter or downpipe, a failed window or door frame seal, a slow plumbing leak, or garden soil and mulch built up against timber.
How is wet rot different from dry rot?
Both need moisture to start, but wet rot stays confined to timber that is actually wet and stops as soon as the source is fixed. What is sometimes called "true" dry rot can, in the right conditions, spread beyond the original wet timber through masonry using root-like strands, reaching timber some distance from the leak.
How do you fix wet rot?
Find and fix the moisture source first: the leak, the seal, the drainage or ventilation problem. Any timber that has already lost strength is then assessed and dealt with separately. Treating the timber while the water source stays in place does not solve the underlying cause.
Does wet rot mean I have termites too?
Not necessarily, but the same damp conditions suit both, so they are often found together in the same subfloor or wall cavity. A standard timber pest inspection checks for decay, borers and termites in the same visit for exactly that reason.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- Department of Health, Victoria. Pest control — termites
- Standards Australia. Australian Standards (AS 3660 termite management, AS 4349.3 timber pest inspection)