Musty rooms, damp sand under the floor, vents lost behind a new deck: that is a subfloor that has stopped drying. We open it up again with new and cleared vents, a fan on the wall the breeze cannot reach, a barrier over the sand and a check of every bearer and joist, from Nelson Bay and Soldiers Point to Medowie and Raymond Terrace. Tell us what the house is doing.
Get a Quote →The older beach houses around Shoal Bay and Soldiers Point and the weatherboards through Raymond Terrace stand on timber floors over a subfloor space. It is designed to have air moving through it all the time: in through vents on one side, across under the floor, out the other. Subfloor ventilation is keeping that air moving so the damp rising off the ground goes out with it.
In Port Stephens the ground under the house is sand, free-draining but often sitting on a high water table near the bay and the estuary, so moisture rises off it steadily. The wet months here run from February to June, and June is also the slowest-drying month of the year, so a subfloor that loses its airflow in autumn stays wet right through winter. Block the vents with an extension, landscaping or a new deck, and that moisture condenses on the underside of the floor, feeds mould and starts on the bearers and joists.
We have crawled under enough Port Stephens houses to know what wet sand does to timber that was never meant to be wet: bearers that look fine from the side and crumble at the top where the damp air sat against them. That is why we treat the sand as part of the job, not just the vents. Dry the ground, move the air, and the timber looks after itself.
When we come out, we go under with a moisture meter and a torch: timber first, then the sand, then every vent. Covered vents are cleared, rusted ones replaced, and new ones cut where the breeze comes from. A fan sized to the floor area goes on the far wall to pull the air through, wired to a switch or humidistat on an outside wall. Over damp sand we lay a plastic barrier, overlapped and pinned, so it stops feeding moisture up into still air. Then we give it a few weeks and check the timber with the meter again. The chart below shows how the air is meant to move under the floor.
Multi-fan systems that draw moist air out and push fresh air through, wired to a discreet switch or humidistat. Cost depends on whether one fan covers the space or a larger Port Stephens home needs a full multi-fan system, plus how much wiring and ducting the job involves.
Cutting in new brick or timber vents, replacing corroded metal vents (common this close to salt water), and unblocking vents covered by decking or garden beds. Priced per vent, supplied and fitted — a simple swap costs less than cutting a brand new opening.
Heavy-duty polythene sheeting laid over exposed sandy subfloor soil to stop ground moisture evaporating into the timber above - especially useful in low-lying blocks near Salamander Bay and the estuary. Priced per square metre of ground covered, so it scales with the size of the subfloor.
A crawl-through with a moisture meter and torch to check vent coverage, timber condition and mould before it turns into rot. There's usually a modest inspection fee, and many operators credit it back if you go ahead with the work, so it's worth asking when you book.
Prices are a general guide only — your exact quote comes after an under-house look.
There isn't one product called "subfloor ventilation" — there are a few different setups, and which one suits your place depends on how big the space is, how much air already moves through it, and how wet the ground underneath is. Here's what a contractor is choosing between when they get under your house.
No power, no fans — just openings in the base brickwork letting the breeze do the work. It's the cheapest fix, and it's the right one when the house simply hasn't got enough vents or the ones it had have been covered over. It only works if air can get in one side and out the other. If your subfloor is wide, boxed in, or the vents all face the same way, passive on its own usually won't shift it.
One fan pulling damp air out, with the existing vents letting fresh air in behind it. Suits smaller subfloors, or a house where the airflow is nearly there and just needs a nudge. Simplest powered option and the least to go wrong.
Fans pushing fresh air in on one side of the house and pulling stale air out on the other, so the whole space gets swept instead of just the bit near the vent. This is the setup for wider homes where the air in the middle sits still and never dries. Most multi-fan jobs end up being some version of this.
Flexible ducting run to the corners air never reaches — behind a bathroom addition, under an enclosed deck, or wherever piers and walls break the space into pockets. Usually added to a cross-flow system rather than used on its own.
A small solar panel runs the fan instead of mains power, so there's nothing on your electricity bill and often no switchboard work. The neat part is the timing — it runs hardest on hot, bright days, which is exactly when the air is driest and moisture actually evaporates. Some setups add a mains or battery backup to keep going through a run of grey weather.
A plain switch means you have to remember. A timer runs set hours. A humidistat is the set-and-forget option — it starts the fans only when humidity under the house climbs past a set point and stops them when it drops. Worth asking about, because it's the difference between a system you manage and one you forget about.
If you get more than one quote you'll hear different fan types named, and they aren't interchangeable. What suits your place depends mostly on whether the air has to travel down a duct or just move across an open space.
Propeller-style, the type usually fitted straight into a vent opening or through the base wall. They shift a lot of air cheaply, which is what you want across an open subfloor. What they don't do well is push air along a length of ducting — the resistance stops them.
Squirrel-cage style, sat in the ductwork rather than in the wall. They move less air than an axial for the same size, but they'll push it a long way against resistance — so they're the choice when air has to be carried to a far corner or around obstacles. Usually the quieter option too, since the unit sits away from the vent.
Fan and panel supplied as a package, with the panel mounted where it catches sun and the fan under the house. Runs on daylight rather than mains, and no ongoing cost. Sizing matters more here than with mains fans, because output rises and falls with the light.
Solar needs a plan for winter. The Bureau of Meteorology's Nelson Bay (Nelson Head) station averaged rain on about 100 days a year, and its wettest months were May and June. If a quote is for solar, ask whether it's sized for winter.
A humidistat earns its place here. A timer runs the same hours in June as in January. A humidistat keeps the fans going while the air under the house stays damp and stops them once it dries, so it follows the seasonal swing without anyone resetting it.
Most people notice it inside the house first and never connect it to what's underneath. Here's what to look for, upstairs and down.
You don't have to crawl under to learn a lot. Get down at a vent with a decent torch on a still morning and look for these.
Tape a square of clear plastic flat onto the soil under the house, sealed on all four edges, and leave it a couple of days. Water forming underneath the plastic means the ground is still giving off moisture and a vapour barrier is likely part of the answer. Water forming on top means the air down there is damp and needs shifting.
Pick your mornings. The Bureau of Meteorology's Williamtown RAAF records put the July mean minimum at 6.5°C, and beads on the underside of the boards can show on a still winter morning. Look again between July and September, which average about six rain days a month; September is the driest month. Soil still dark through that stretch can point to moisture from the ground or a leak. It's a good time for the plastic test.
Check where the lot sits. Council's 2017 flood study for Anna Bay and Tilligerry Creek notes that swamplands have formed where groundwater is high. On a low block, check the lowest corner of the subfloor for water lying on the ground. Council's online Land Check shows whether a lot is flood prone. If yours is, look for a flood line on the piers or silt on the ground.
In Port Stephens the wet months run straight into the slowest-drying time of year. Bureau of Meteorology averages for Williamtown RAAF show more than 110 mm of rain in every month from February to June. June also has the year's lowest average daily evaporation, 2.5 mm against 6.9 mm in January, and its highest average 9am humidity.
Hunter Water describes two sand aquifers in Port Stephens: the Tomago Sandbeds, running about 25 km from Tomago towards Lemon Tree Passage and recharged by rain soaking straight into the sand, and the Tomaree Sandbeds at Anna Bay and Nelson Bay. Port Stephens Council's 2017 Anna Bay and Tilligerry Creek Flood Study says large sections of its area lie on flat ground less than 2 m above sea level (AHD), and that runoff can pond in pockets between dunes before soaking in. Under Anna Bay's town centre it notes a water table potentially just 1 m deep.
The same study says its flood maps do not show ponding from groundwater seeping up after rain, so a flood map alone won't tell you whether ground moisture reaches your subfloor. In the Anna Bay catchment, NSW DPI assessments found acid sulfate soils, and the recommended works aim to keep groundwater high, because acidic groundwater moving out has caused fish kills and poor water quality affecting oyster growing. Where the ground stays damp, the fix is clear vents, fans where needed and a barrier over bare sand.
Fans are sold in hardware stores and plenty of people have a go themselves. Sometimes that works. Here's what a contractor is actually being paid to get right.
Musty smells and mould on the walls don't always come from underneath. A leaking shower waste, a blocked downpipe, a roof leak tracking down a cavity, or rising damp in the brickwork all look much the same from inside. Ventilating a subfloor that was never the problem is money gone.
Two fans mounted the wrong way round will short-circuit — pulling air straight back out the vent beside them and leaving the middle of the house untouched. Where the fans go matters as much as how many there are, and that gets judged by looking at the actual space, not a floor plan.
A fan won't outrun water that keeps arriving. Ground sloping back toward the house, a downpipe emptying beside the footings, a garden bed built up over the vents, or a dripping pipe underneath all need sorting or the damp comes straight back.
Mains-powered fans are electrical work and need a licensed electrician. Solar setups avoid a good deal of that, which is part of why they suit older homes with tight switchboards.
Bearers and joists are what hold your floor up. Ventilation work is cheap next to replacing structural timber, and the gap between those two problems is usually a couple of wet seasons.
Termites want moisture as much as they want timber, and a damp subfloor is an invitation. Anyone going under with a torch and a moisture meter is looking at both at once.
If the fix goes past fans and vents to a drain along the footings, a sump or regrading, check the land's class on the Acid Sulfate Soils Map first. Under clause 7.1 of the Port Stephens LEP 2013, Class 1 land needs development consent for any works, Class 2 for works below the natural ground surface or likely to lower the watertable, and Classes 3 and 4 for the same tests at more than 1 m and 2 m down. Consent needs an acid sulfate soils management plan. Works disturbing under a tonne of soil skip that consent if they aren't likely to lower the watertable, so a drain meant to draw groundwater away is the part to ask about.
Hunter Water also asks for plans for building or excavation work to be stamped before work starts, and work close to or over its pipes can need a Build Over Asset Application. On a flood-prone lot, a Port Stephens Council flood certificate gives the flood planning level and, where available, the house's surveyed floor level, which can show how the floor sits against Council's flood levels before money goes under it.
Enquiries are welcome from anywhere in Port Stephens, including Nelson Bay, Shoal Bay, Salamander Bay, Soldiers Point, Anna Bay, Boat Harbour, One Mile, Fishermans Bay, Bobs Farm, Salt Ash, Taylors Beach, Tanilba Bay, Mallabula, Lemon Tree Passage, Medowie and Raymond Terrace.
Tell us what you need and where the property is.