Finding mushrooms growing along a skirting board is alarming, and it usually means a wood-rot fungus has been growing in the timber for some time. Most of what's written about wood rot describes overseas fungi in overseas houses. This guide covers what actually rots timber in New Zealand homes, based on research done here and what we see in our lab, and what to do if you find it.
What wood rot is
Most of the mould we identify indoors, such as Aspergillus, Penicillium, Cladosporium and even black mould (Stachybotrys), grows on and into the surface layers of materials: paint, the paper facing on plasterboard, and dust. It stains and damages finishes, but it doesn't destroy the strength of structural timber. Wood-rot fungi do. They are basidiomycetes, the same group of fungi as mushrooms and bracket fungi, and they digest the wood fibres that make a beam or a stud strong.
There are two main kinds. Brown rot leaves timber dark, shrunken and cracked into small cubes that crumble to powder. It weakens wood early, while the timber can still look sound. White rot leaves wood pale, soft and stringy. Both are caused by a network of fine fungal threads, called the mycelium, spreading through the timber, and both are usually well advanced before anything shows in the room.
The white sheets across this timber are the mycelium, the main body of the fungus and the part that does the damage. It can spread unseen for months or years before any mushroom appears.
The overseas names you'll see online
Most guides to timber decay list the same fungi: Serpula lacrymans (true dry rot), Coniophora puteana (cellar rot) and a few relatives. That list comes from British, European and North American houses, which have a different climate, different timber and different building methods. Many New Zealand remediation and pest-control websites repeat it as if it describes Kiwi homes, but the local evidence points elsewhere.
What New Zealand research shows
New Zealand's leaky-building crisis produced a large body of evidence about wood decay. For more than a decade, framing timber, mostly untreated radiata pine (Pinus radiata), stayed wet behind the cladding of homes that looked fine from the street, and slowly rotted.
The key study comes from BRANZ, which examined 421 samples from leaky buildings and identified 68 different fungi. Four brown-rot fungi stood out as the ones doing the damage:
- Gloeophyllum sepiarium: a hardy brown rot that copes well with repeated wetting and drying, which suits a leaky wall.
- Gloeophyllum trabeum: a close relative, and a common cause of framing decay worldwide.
- Oligoporus placenta (also called Postia or Rhodonia placenta): known for tolerating some timber preservatives.
- Antrodia sinuosa: another brown rot that tolerates preservatives and is hard to control with chemicals.
So in New Zealand, structural wood rot is mainly brown rot caused by Gloeophyllum, Oligoporus and Antrodia on untreated radiata pine. Other sources agree. The New Zealand Fungarium (Manaaki Whenua – Landcare Research) holds one of the world's leading collections of NZ wood-decay fungi, and Scion has published research on how readily these fungi rot radiata pine. This local evidence, rather than a British list, is the best guide to what rots timber here.
Serpula, or "dry rot": often named, rarely confirmed
No fungus is named more often in New Zealand rot discussions than Serpula lacrymans, the "true dry rot" of the northern hemisphere. In cool, damp British buildings it can spread aggressively and force large amounts of timber to be removed, and that reputation is why its name often gets attached to rot here.
The BRANZ study, the closest thing we have to a systematic survey of building decay fungi, points to Gloeophyllum, Oligoporus and Antrodia, not Serpula. In our own lab, across more than 2,000 Bio-Tape samples from decayed timber and water-damaged buildings around New Zealand, we have never identified Serpula lacrymans. That doesn't mean it could never turn up here, but the fungus most often named hasn't appeared in our samples, while the fungi doing the real damage often go unnamed. If a report says "dry rot" or names Serpula, it's reasonable to ask how it was identified. Naming the exact fungus needs a lab result, not a photo.
What wood rot looks like under the microscope
Under the microscope, wood-rot fungi look quite different from ordinary mould, and their spores are the giveaway. The sample below, from decayed timber, shows a dark tangle of fungal threads releasing tough, dark, oval spores. Spores this heavily coloured and thick-walled point to a wood-decay fungus rather than an everyday indoor mould. We only report what we can actually see.
A decayed-timber sample under our microscope: a dark tangle of threads releasing tough, thick-walled oval spores. This looks quite different from the common surface moulds.
Testing for hidden rot
Wood-rot fungi release their own distinctive spores. A few of them in an ordinary indoor air sample mean very little, because outdoor air carries them from mushrooms and rotting wood all around.
Sampling the air in a hidden space is different. When we draw air from inside a wall, under a floor or from a closed-up roof space, and the sample holds far more of these spores than the outdoor air, it suggests rot is growing inside the structure. This is one of the few ways to find hidden rot without opening up the building, and it can pick up a problem long before any mushroom appears. Where growth is visible, a Bio-Tape lift can confirm it directly, and it's a sample you can take yourself in about five minutes.
What a mushroom indoors tells you
Look again at the photo at the top: a row of mushrooms where the skirting meets the carpet, with black mould in the corner. People who find this often think it appeared overnight. It didn't. A mushroom is how the fungus reproduces, and it only appears once the fungus is well established in the timber. What looks like a sudden problem has usually been building, out of sight, for months or years.
That's why wood rot is worth investigating early rather than watching and waiting. The fungi weaken timber gradually and out of sight, but they can be identified from the right sample well before anything shows. If you have already found a mushroom, get the timber checked promptly: the sooner the moisture source is fixed, the less timber is likely to need replacing.