Building inspectors check a home's structure, weathertightness and code compliance, but not what is growing in it. Stained subfloor timber, white powder on the soil under the house, a carpet with a faint damp smell: each can be a sign of active mould that a standard building inspection won't pick up. These are the situations we come across most often when testing homes before purchase.
Subfloor timber: staining or wood rot?
Subfloors are often outside the scope of a standard building inspection, or are checked only for structural problems. To the untrained eye, mould on subfloor timber looks much like ordinary wood staining, and most building reports won't comment on it.
Under the microscope, the story is often different. Some of the fungi we find on subfloor timber are wood-decay fungi, from the same group as mushrooms and bracket fungi. They break down the timber itself, causing the brown rot or white rot that engineers call "wood rot". By the time the timber looks discoloured, the fungus may already be well established inside it and the timber weakened.
We sample subfloor timber with a tape lift or swab. If we find wood-decay fungi, we recommend that a structural engineer takes a look, and our microscopy backs that up. It changes the job: it is no longer just about removing mould, but about replacing damaged timber. To learn more about these fungi, and why New Zealand rot rarely matches the overseas idea of "dry rot", see our guide to wood rot in New Zealand buildings.
Efflorescence: the moisture sign that looks like mould
Efflorescence on subfloor soil: salts left behind as rising moisture evaporates. Homeowners often report it to us thinking it is mould.
Efflorescence on brickwork: the same process on a different surface. Brick and block walls hold moisture close to the structure and can stay damp for long periods.
Efflorescence is not mould. It is a white, crystal-like deposit left behind when rising moisture carries salts to a surface and then evaporates. It appears on subfloor soil, brickwork, concrete block and foundations. It is one of the more common things we find on pre-purchase inspections, usually reported by clients who thought they had found mould, when what they were seeing was a sign of a failed damp-proof membrane.
That still matters. Efflorescence is a reliable sign that moisture is rising through the ground or masonry, and it almost always points to a damp-proof membrane that has failed or was never installed. Rising damp raises the risk of mould indoors in two ways. First, it causes the musty smells that people notice but can't trace. Second, it raises the humidity in the rooms above, and humidity that stays above 70% is the main driver of mould growth. Aspergillus and Cladosporium, the moulds we find most often in New Zealand indoor air, grow readily in these conditions, which can affect sensitive occupants over time.
Case study. The new owners of a home noticed a persistent mouldy smell with no obvious source inside, and had ongoing breathing problems. A mould assessment found efflorescence across the subfloor soil and a failed damp-proof membrane. The repairs, which included replacing structural timber and subfloor joists, cost about $20,000. A pre-purchase mould assessment, at a fraction of that cost, would have picked up the high indoor spore counts before settlement and given the buyers the evidence to negotiate or walk away.
Hidden water damage under the carpet
The underside of a carpet lifted during an inspection. The carpet looked normal from above; the water had come from the en-suite next door.
Old water damage under carpet usually can't be seen from above. Sellers may not know it is there, and it may date from before they owned the home. The carpet looks and feels normal, because the damage is hidden between the carpet and the floor below, and nothing shows unless the carpet is lifted.
Case study. A pregnant occupant, one of the groups more sensitive to mould, had symptoms whenever they slept in one particular bedroom. There was no visible mould anywhere in the home. A spore trap from the room returned about 2,500 spores/m³.
More telling than the count was the mix of moulds: Chaetomium, traces of Stachybotrys and raised Aspergillus, a combination strongly linked with long-term water damage rather than spores drifting in from outside. When the carpet next to the en-suite was lifted, there was extensive old water damage underneath, with black mould actively growing. The carpet was removed, and the symptoms cleared up.
A tape lift from the tops of door frames and other places where dust settles often picks up spores that reflect the room's mould history, even when nothing is visible. It is one of the more reliable ways to find signs of hidden water damage without pulling anything apart.
Mould hidden before a sale
Visible mould is often cleaned off before a property goes on the market, usually with bleach. That removes the stain but not the mould inside the material or the moisture feeding it. Our lab work shows what bleach actually does: the surface clears, but the mould inside the material survives, and it usually grows back within weeks once moisture returns.
We see this in homes that have recently changed hands, and in new builds where condensation problems during construction were covered up rather than fixed. Previous occupants may also have under-ventilated the home for years, leaving a build-up of spores in dust, soft furnishings and building materials that you can't see when you move in but that shows up in testing within days. No type of building is immune: modern airtight homes can trap moisture just as older homes can be cold and damp.
A tape lift from the tops of door frames and window surrounds, where spores settle over time, gives a record of the mould history that a quick clean before sale won't remove. If you'd like a first look before booking a full assessment, you can take a tape lift yourself with our sampling supplies and instruction sheet (PDF).
Pre-purchase testing as a record for later
Even when no mould is suspected, a pre-purchase mould assessment gives you a record of the property's normal mould levels: which moulds are present, how much, and in which rooms, before the building changes hands.
That record is useful in two ways. First, if there is a flood or leak after you buy, it shows you and your insurer what normal looked like for this property. Without it, arguments about how far the clean-up needs to go come down to guesswork rather than data. Second, if a future tenant raises a mould complaint, you have a record of the property's condition when you took ownership.
We recommend booking pre-purchase mould testing alongside your other building inspections, before your due-diligence period ends. It adds one site visit and a few days for lab results, and gives you a lasting record for as long as you own the property.