Damp under the house

What is actually under a South East Queensland house

Three broad types dominate the region. High-set and low-set timber on stumps or piers with a suspended floor. Post-war brick veneer, often with a suspended timber floor on ant caps and a brick base course. And post-1980 slab-on-ground, which behaves completely differently and is covered on our concrete slab page.

The problems live in the low-clearance cases. A high-set you can stand up in ventilates itself and dries after rain. The back corner of a low-set on a sloping block, with 200 mm of clearance, a retaining wall above it and a paved path outside, is a different building entirely. Add a regional water table that sits high in places, ground that has been reworked since the 2011 and 2022 floods in the Ipswich and Logan corridors, and reactive clay that moves with the seasons, and you have the conditions for a subfloor that never quite dries between storms.

A subfloor is doing three jobs at once, and it is worth knowing all three because losing any one of them creates a different symptom.

• Keeping the ground physically clear of the timber, at 150 mm minimum, or 400 mm where termite inspection access is required.
• Moving air across the whole space, so that moisture leaving the ground leaves the building rather than condensing on the underside of the floor.
• Staying inspectable, so that a licensed pest inspector can get eyes on every bearer, joist and ant cap.

Why this usually is not rising damp

Rising damp is the upward movement of groundwater through a continuous connected pore network in masonry, driven by capillary suction. The key word is continuous. A timber floor on stumps has no continuous capillary path from the soil to the floorboards. The stumps are discrete, and in newer work ant caps and damp-proof courses break the path deliberately.

What you have instead is a volume of humid air sitting under a floor, and everything in contact with it, the timber, the underside of the boards, the base of any masonry, comes into equilibrium with that air. That produces symptoms that read very much like rising damp to anyone holding a meter, and are not.

There is a real version, though, and this is where the honest split sits.

What to look at, in order

This list is ordered by cost, cheapest first, which is also roughly the order of how often each one turns out to be the answer. Most of it you can do yourself in an afternoon.

1. Walk the perimeter and count the ventsLook for vents that have been rendered over, painted shut, buried by paving or a garden bed, blocked by an air-conditioning unit, covered by a deck, or built over by an extension. Rendered-over and buried vents are the single most common finding, and reopening them costs nothing but the afternoon.
2. Measure the ground clearanceBetween the ground and the lowest horizontal subfloor member you want 150 mm, or 400 mm where termite inspection access is required. Ground level creeps up over decades as mulch and fill are added, and clearance is the first thing lost.
3. Get under there after heavy rain, not on a dry TuesdayYou are looking for standing water, wet patches that persist between storms, water entering from the high side of a sloping block, and where it eventually drains to. Take a torch and a phone camera, and do not crawl into anywhere you cannot reverse out of.
4. Clear the debrisThe NCC requires subfloor spaces to be cleared of building debris and vegetation and the ground graded. Offcuts, formwork, cardboard, prunings and stored belongings all hold water, block airflow and feed termites.
5. Check the wet pointsAir-conditioning condensate discharging under the house, a leaking tap or hose bib, a slow supply leak found by isolating the water meter and watching it, or a cracked stormwater or sewer line. On reactive clay sites those connections crack with ground movement and leak beside the footings.
6. Follow the water outsideGrading, downpipe discharge and stormwater. Water arriving under the house after every storm is a drainage problem, not a ventilation problem, and more vents will not fix it.
7. Then monitor for a seasonA newly created subfloor moisture problem can take one to three years to become visible, and a fix takes just as long to show. Anything judged at three months proves nothing in either direction.

What the code actually asks for

Subfloor ventilation is covered by NCC Housing Provisions Part 6.2. The figures below are of the right order in both the 2022 and 2025 editions, but the exact table values must be verified against the edition adopted in your jurisdiction before they are relied on in a specification.

One point worth understanding before you read the numbers: the required opening area is stated per metre of wall, so a longer perimeter needs proportionally more. And the number is only half the requirement. Placement matters as much, because a subfloor with the correct total opening area all on one side has no cross-flow at all, and a vent buried behind a garden bed counts for nothing.

Adequate, but not too much

There is a genuine tension here that nobody selling ventilation hardware has any reason to mention, and it is worth stating carefully because it cuts against the intuitive answer.

More air under the floor lowers subfloor humidity, and in any masonry down there it lowers the height that damp rises to, because the steady height of rise falls as the evaporation rate goes up. That is the good half. The bad half is that a higher evaporation rate also means salt crystallises faster inside the masonry, and crystallisation inside the pores is what actually breaks bricks and turns mortar to sand. Ventilate harder and you get a lower damp line on a wall that is decaying faster.

The Australian heritage guide phrases the target as adequate, but not too much, underfloor ventilation, and the same logic applies to mechanical drying generally. Air-conditioning and dehumidification raise the drying rate and so raise the associated decay, and an outdoor unit blowing warm air at an old wall concentrates salt damage directly behind it.

So the aim is code-compliant cross-flow air movement over a dry, graded, debris-free subfloor. It is not the largest fan somebody can sell you.

Two things to be careful about under there

Damp and termite activity travel together, because both follow high subfloor humidity. The 400 mm clearance figure exists precisely so the space can be inspected. If you are restoring clearance and airflow anyway, book a licensed pest inspection at the same time. It is the same access and the same trip, and a termite inspector who can actually see the bearers is worth considerably more than one who cannot.

The second item is a safety matter and is not negotiable. Subfloor soils in buildings treated before about 1990 may carry organochlorine termiticide residues. Do not dig, sweep, vacuum or otherwise disturb that soil casually, and get advice before any excavation, regrading or laying of a ground membrane. Buildings of the same era commonly carry asbestos in sheeting and lead in old paint, both of which have identification and handling obligations under the Work Health and Safety Regulation. These are legal requirements, not optional extras, and they change what the work costs.

We do not install subfloor ventilation, and here is who does

S&S Remedial are waterproofing and concrete repair specialists, and we do two things. Chemical damp-proof course injection into the mortar bed of external brickwork, and Hydropoxy negative treatment to internal concrete slabs. That is the list. Subfloor ventilation is not on it. Neither is drainage, landscaping, excavation, plumbing, restumping or pest control.

When the answer under your house is vents, a ground membrane, grading or drainage, you want a subfloor ventilation contractor, a drainage contractor, a plumber or a licensed pest inspector, and you will get a better job for less money than we could do it for. We would rather tell you that in the first conversation than find a way to be involved.

It is also worth seeing the numbers side by side, because this is frequently the cheapest genuinely effective intervention available in this whole subject area.

One licensing note before you engage anyone: building work above a threshold commonly stated as $3,300 including materials and GST requires a licensed contractor in Queensland and triggers Home Warranty Scheme insurance. Confirm the current figure with the QBCC and check the licence number on the public register before you sign anything.

When it actually is our work

There is a version of the under-the-house case that is genuinely ours, and it looks like this. Solid masonry base walls, piers, or a brick base course below a suspended floor. A level salt line on the outside of that base brickwork somewhere between 1.0 and 1.5 m. Mortar you can rake out with a screwdriver. And drilled samples showing real free water rather than only hygroscopic moisture held by salt, taken after the housekeeping has been done and monitored.

Then a chemical damp-proof course injected into the mortar bed is the right conversation. Note the order, though. In the Seven Key Steps published by the Heritage Council of NSW, Heritage Victoria, the South Australian Department for Environment and Heritage and Adelaide City Council, damp course insertion is step six of seven, and the guide is explicit that it should not be undertaken until the basic housekeeping measures are complete and their effectiveness has been assessed over a period of at least a year. Most of this industry sells step six to people who needed step two.

The other version that is ours is a slab-on-ground extension off the back of an older house, where the slab is wet and new flooring is going down over it.

One last thing about the subfloor specifically. The repair itself can put floor timbers at risk, because injecting the base of a wall can displace saline moisture upward and the bearers and joists sit right there. That is why an underfloor inspection appears twice in our process: once in the diagnosis, and again in the sign-off. Out of sight is exactly where these problems prefer to be.

What it costs

Questions we actually get asked

There is a musty smell under the house after rain. Is that rising damp?

Probably not. A musty smell that comes and goes with the weather is the signature of high subfloor humidity, and the usual causes are blocked or buried vents, lost ground clearance, debris holding water, or water arriving from outside every time it rains. Genuine rising damp does not track the weather much at all; it is fairly constant year round because it is driven by the balance between water arriving from below and evaporation from the wall face, not by last Tuesday's storm. Check the vents and the grading first, and get under there in wet weather rather than dry.

How many vents do I actually need?

The aggregate opening area required is stated per metre of wall, of the order of 2,000, 4,000 or 6,000 mm² per metre for climate zones A, B and C where there is no impervious ground membrane, and roughly half that where one is installed. So measure your perimeter first. But the total is only half the answer. The openings have to be evenly spaced, within about 600 mm of corners, with no dead air spaces, or you get a technically compliant area of vents and no cross-flow. And a vent sitting behind a garden bed contributes nothing at all.

Do I need a mechanical subfloor fan, or will more vents do?

Passive first, in almost every case. Reopen what is blocked, add openings where the placement is wrong, clear the debris, grade the ground, and fix whatever is delivering water. Mechanical extraction earns its place where cross-flow is physically impossible, which usually means very low clearance, an extension built across one side, or a boundary wall. Understand what a fan does and does not do: it moves air, it does not stop water arriving. A fan running over saturated ground that is fed by a stormwater problem every storm is an electricity bill with a warranty.

Someone rendered over my vents, or built a deck across them. Does it really matter?

Yes, and it is the most common single finding under a South East Queensland house. Rendering over a vent removes its opening area entirely, and a deck built hard against the wall can block a whole elevation at once. The building was designed on the assumption that a stated area of openings would be evenly distributed around the perimeter. Take away one side and you convert cross-flow ventilation into a still pocket, and the still pocket is where the cupping boards, the mould in the cupboard and the damp bearer show up first. Reopening them is the cheapest fix in this entire subject.

Should I lay plastic sheeting on the ground under the house?

An impervious ground membrane is one of the three routes the code allows where excessive dampness or flooding occurs, the others being 50% more ventilation and specified timber durability, and it roughly halves the required ventilation opening area. It has to be laid properly, lapped, and kept clear of stumps and ant caps so that termite inspection is still possible under it, which is a real constraint and not a detail. And for any building treated before about 1990, the organochlorine question has to be answered before anyone crawls under there and starts spreading plastic across the soil.

My stumps look damp at the base. Are they rotting?

A damp stump base is a symptom of ground clearance and drainage, not a diagnosis in itself. Timber decays where moisture stays high and air movement stays low, which is the same list of causes as everything else on this page: clearance, grading, vents, debris, and where the water goes. If the timber is soft when probed, that is a structural question for a licensed builder or a restumping contractor, not for us. We do not restump, and we would rather say so now than after you have paid for a report.