By the time a mushroom pushes through your skirting board, the decision has already been made for you. That mushroom isn't the start of a wood-rot problem — it's the fungus signalling that it has already taken over the timber behind the wall. Wood rot gets written about endlessly, but almost always in terms of overseas fungi in overseas houses. This is about what actually rots timber in New Zealand homes — from the research done here, and what we observe in the lab.
What wood rot actually is
Most of the mould we identify indoors — Aspergillus, Penicillium, Cladosporium, even black mould (Stachybotrys) — lives on the surface, feeding on the film of dust and grime on a wall. It stains and disfigures, but it doesn't eat the structural timber holding the house up. Wood rot does. The fungi behind it are basidiomycetes — the same family as the mushrooms and bracket fungi on a forest floor — and they digest the wood fibres that make a beam or a stud strong. Ordinary mould is graffiti on the wall; wood rot eats the wall.
There are two kinds. Brown rot leaves timber dark, shrunken and cracked into little cubes that crumble to powder — and it's dangerous because it destroys the wood's strength early, while the beam still looks intact. White rot leaves the wood pale, soft and stringy. Both are the work of a hidden web of fungal threads — the mycelium — spreading inside the timber, well advanced before anything shows in the room.
The white sheets across this timber are the mycelium — the actual body of the fungus, and the part that does the damage. It spreads unseen for months or years; the mushroom only appears once it's fully established.
The fungi that eat buildings
Open any overseas guide to timber decay and the same names come up: Serpula lacrymans (true dry rot), Coniophora puteana (cellar rot) and a handful of relatives. But that list describes British, European and North American houses — different climate, different timber, different building methods. New Zealand remediation and pest-control websites copy it wholesale and present it as if it describes Kiwi homes. It doesn't.
What New Zealand actually documents
New Zealand ran its own painful experiment in wood decay: the leaky-building crisis. For over a decade, framing timber — mostly untreated radiata pine (Pinus radiata) — sat wet behind the cladding of homes that looked fine from the street, and quietly rotted. That gave laboratories a real body of evidence about which fungi actually attack New Zealand structural timber.
The key dataset comes from BRANZ, which processed 421 samples from leaky buildings and identified 68 different fungi. Four brown-rot fungi stood out as the ones actually eating the timber:
- Gloeophyllum sepiarium — a tough brown rot that shrugs off wetting-and-drying cycles, ideal for a leaky wall.
- Gloeophyllum trabeum — a close relative, and a repeat offender in framing decay worldwide.
- Oligoporus placenta (also called Postia / Rhodonia placenta) — notorious for tolerating some timber preservatives.
- Antrodia sinuosa — another preservative-tolerant brown rot, and hard to control chemically.
So the New Zealand reality is brown rot driven by Gloeophyllum, Oligoporus and Antrodia on untreated radiata pine. The wider record agrees: the New Zealand Fungarium (Manaaki Whenua – Landcare Research) holds one of the world's foremost collections of NZ wood-decay fungi, and Scion has published on how readily these fungi rot radiata pine. This — not a British list copied out of habit — is what tells you what rots timber here.
The Serpula problem — the name everyone cites and no one confirms
No fungus gets named more often in New Zealand rot conversations than Serpula lacrymans, the "true dry rot" of the northern hemisphere. It's a formidable organism — in cool, damp British buildings it spreads aggressively from a single point and can force complete timber removal — and that reputation is exactly why its name gets attached to NZ rot by reflex. But reputation isn't evidence.
The BRANZ survey — our closest thing to a systematic look at building decay fungi — points to the Gloeophyllum, Oligoporus and Antrodia group, not Serpula. And in our own lab, across more than 2,000 Bio-Tape samples from decayed timber and water-damaged buildings throughout New Zealand, we have never once identified Serpula lacrymans. That doesn't mean it could never appear here — but the fungus everyone names as the villain is nowhere to be seen, while the fungi doing the real damage go unnamed. So if a report reaches straight for "dry rot" and Serpula, ask how that was worked out. Naming the exact fungus is a lab result, not a guess from a photo.
What the decay looks like down the microscope
This is where a laboratory earns its place. Under the microscope, wood-rot fungi look nothing like ordinary mould — their spores give them away. The sample below, from decayed timber in our lab, shows a dark tangle of fungal threads shedding tough, dark, oval spores; heavily coloured and thick-walled, they point to a wood-eater rather than an everyday indoor mould. We describe only what we can actually see — and that discipline is what separates a solid identification from a marketing label.
A decayed-timber sample under our microscope — a dark tangle of threads releasing tough, thick-walled oval spores. This is what a wood-eater looks like: a different picture from the everyday surface moulds.
The one time hidden spores really mean something
Air sampling is where this gets practical. Wood-rot fungi shed their own distinctive spores, but a background level in a normal indoor sample means nothing — the outdoors is full of them, drifting in from every mushroom and rotting log for kilometres around.
The exception is genuinely useful. When we draw air from a hidden space — inside a wall, under the floor, a closed-up roof cavity — and it comes back loaded with these spores, far above the outdoor level, that points to rot growing out of sight inside the structure. It's one of the only ways to catch hidden rot without opening the building up — an early warning long before any mushroom. And where growth is visible, a Bio-Tape lift confirms it directly, lifting the fungus straight off the material.
By the time you see the mushroom, you are late
Look again at the photo at the top: mushrooms in a row where the skirting meets the carpet, black mould already in the corner. People who find this are sure it appeared overnight. It didn't. A mushroom is just how the fungus reproduces, and it only shows once the hidden web inside the timber is well established. What looks like a sudden invasion is the final chapter of something that's been underway, unseen, for months or years — and by then the timber damage is done.
That's the case for treating wood rot as something to investigate early, not watch and wait. These fungi work out of sight, strip the strength from timber quietly, and can be identified from the right sample well before anything shows — but by the time a mushroom appears, how much timber you're replacing has largely been decided for you.