Wood rot or termite damage — and is the timber still doing its job?
Two questions get asked in the wrong order. People start with “what caused it” when the one that decides what happens next is “is this member still carrying its load”.
Three things make timber go soft
You have pressed a skirting board, a deck post or a subfloor bearer and it has given way. From the outside, the three usual causes look much the same. And they have completely different answers.
Wood rot is not an insect at all. It is fungal decay: fungi digesting the timber, which they can only do once the moisture content stays high enough for long enough. Rot needs water, always, and it stops advancing when the timber dries. That is the single most useful fact in this article, because it means a rot problem is a water problem with a fungus attached.
Termite damage is timber eaten from the inside out, which is why it so often keeps a thin, sound-looking surface over a hollow. The group that does most of it in Australian houses leaves mud, either packed into the galleries or running up a pier as a tube, because it needs to keep its route humid.
Borers (wood-boring beetles) leave clean round flight holes with fine, flour-like frass. Borers and fungal decay goes through both of those properly.
And there is a fourth answer that gets missed: nothing is eating it at all. Weathered, checked, silvered timber on an exposed deck or fence can be ugly and shallow-degraded without being structurally compromised. And paint failure over a sound member is a painting problem.
Telling them apart from the outside
| What the timber looks like | Usual cause | What it says about water |
|---|---|---|
| Cracked into cubes or blocks, crumbles dry to a powder, may have shrunk | Fungal decay (brown rot) | Wet at some point, possibly no longer |
| Soft, stringy, spongy, darker, often lighter-weight than it should be | Fungal decay (white or soft rot) | Usually still wet |
| Sound thin surface over hollow galleries, mud packed inside, tubes nearby | Subterranean termites | They bring their own humidity. But a damp area drew them |
| Clean round exit holes with fine flour-like powder | Wood-boring beetle | Not necessarily anything |
| Grey, splintered, surface checking on an exposed face only | Weathering | Exposure, not a pest |
Two warnings about that table. It sorts observations into likely groups so you know who to call and what to say: it is not a diagnosis, and the three regularly appear together. Termites and decay favour the same damp conditions, so the wet bearer with rot in it is an entirely ordinary place to also find termites. And we will not identify an insect from a photograph or a description; the species is determined from soldiers under magnification, and it changes the treatment.
Dry rot, wet rot and the words people argue about
The everyday labels are less precise than they sound. “Wet rot” is generally used for decay in timber that is still visibly damp. And “dry rot” for the kind that leaves timber cracked into dry cubes: but the name is misleading, because that fungus needed moisture too. It got wet, decayed, and then the surface dried.
Which is why “the timber is dry now” is not evidence that anything has been fixed. It may mean the water has stopped. It may mean it is summer. The strength that was lost to decay does not come back either way.
Cosmetic or structural: the line that governs everything
This distinction decides urgency, cost and who you need. It is worth being precise.
Non-structural: skirting boards, architraves, door jambs, cabinetry, wall linings, decorative mouldings, a fence paling. Nothing is holding anything up. Unpleasant and sometimes extensive, but replacement is ordinary carpentry once the cause is dealt with.
Structural. Bearers, joists, stumps and piers, studs, top and bottom plates, lintels, rafters, trusses, deck posts and deck bearers, and anything else carrying load. Here the question is not how it looks. It is how much sound section is left, and does the member still carry its design load.
That question is not answered by eye, by tapping or by pressing a thumb into it. And on a significantly damaged load-bearing member it is a licensed structural engineer’s assessment, worth having in writing. This site will not tell you a member is fine. And it will not tell you it is about to fail: neither of those is a call that can be made from a description.
Why you cannot tell by prodding it
The instinct is to dig at the soft spot with a screwdriver to see how deep it goes. There are three reasons not to.
The first is that it does not answer the question. Damage is rarely uniform; the depth at the one accessible point tells you very little about the remaining section across the member, which is what load-carrying depends on.
The second is that it makes things worse. Breaking through the skin of a member that is already carrying load with a reduced section is not a free measurement.
The third is the one people never expect: if there are termites in there, you have just destroyed the evidence and moved the problem. Opening the galleries disturbs the colony into abandoning that route and continuing somewhere concealed, which means the damage carries on where nobody is looking. And the inspector loses the pattern that would have told them where it came from. Why spraying termites yourself makes it worse covers the same mechanism for insecticide.
What not to do
- Do not dig, prise or break the damaged timber open. That is the evidence and, if it is structural, part of what is still holding the load.
- Do not spray anything at it. Nothing over a counter reaches a colony inside a member, and it scatters them.
- Do not fill, seal or paint over it. Filler restores appearance and no strength at all, and sealing damp timber traps the water that caused the decay.
- Do not replace the timber first. If the cause is still live (the leak, the ground contact, the colony) new timber goes into the same conditions. The order the work has to happen in sets out why treatment and assessment come before carpentry.
- Do not prop or jack anything yourself because a member looks marginal. Temporary support on a loaded structure is design work.
What to do
Photograph what you found in place, with something for scale, and note the date and the exact location. If it is damp, work out what is wetting it (a blocked subfloor vent, ground built up over the damp-proof course, a gutter or a wet area letting go). That is the half of the problem that outlives the repair. Moisture and the subfloor covers the usual sources.
Then get it looked at by the right person. It is often two people. A timber pest inspection establishes whether anything is living in it and how far it goes, and what an inspection covers explains the scope and its limits. If a load-bearing member is involved, add a structural engineer for the assessment the inspection is not there to make. Keep the reports. They are what the repair gets designed from, and what the next inspection gets compared against.
Until then, the best thing you can do to the timber is nothing at all.
Frequently asked questions
How do I know if wood rot is structural or just cosmetic?
Start with what the member does rather than how bad it looks. If it is skirting, architrave, a door jamb, lining or cabinetry, it is not carrying load and the repair is carpentry once the moisture is fixed. If it is a bearer, joist, stump, stud, plate, lintel, rafter, truss or deck post, the question becomes how much sound section is left and whether it still carries its design load. And that is assessed by a licensed structural engineer, not by eye and not from a photograph.
Can I tell wood rot from termite damage myself?
You can often tell which group you are looking at, which is genuinely useful for knowing who to call. Decay tends to crumble into cubes or go soft and stringy all the way through; termite damage tends to keep a thin sound surface over hollow galleries, usually with mud packed inside. What you cannot do is confirm it, identify a species, or rule out the other one being present as well. The two turn up in the same damp timber constantly.
Will wood rot keep spreading if I dry the timber out?
Decay needs sustained moisture to advance, so removing the water source is the thing that stops it progressing, and it is the first half of any real fix. What drying does not do is restore strength already lost, or tell you how much was lost. A member that has decayed and then dried can look stable and still have well under its original section.
Should I just cut out the rotten section and replace it?
Not as the first move, and not on anything load-bearing. If the timber is wet, whatever wetted it is still there and new timber goes into the same conditions. If there are termites in it, cutting into the galleries scatters the colony and destroys the evidence. And on a structural member, how the repair is done (sistering, splicing, full replacement or new support) is a design decision that follows the engineer’s assessment rather than preceding it.
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.
- Queensland Building and Construction Commission. Protect against termites — termite management systems and inspections
- Standards Australia. Australian Standards (AS 3660 termite management, AS 4349.3 timber pest inspection)
- Australian Building Codes Board. National Construction Code