Wet ground under the house days after rain, damp creeping into the boards, a smell that lingers. We get the air moving: vents dug out or cut in, fans to sweep the still air, plastic over the soil, timber checked. All over the Central Coast, Blue Haven to Woy Woy. Tell us what the house is doing and which suburb.
Get a Quote →Most Central Coast homes built before the 1990s sit on a raised timber floor, with a gap underneath called the subfloor. Subfloor ventilation is just keeping air moving through that gap so the timber stays dry. Cover the vents with a deck, paving or bricks and the damp has nowhere to go. On the Coast it gets help from a high water table, salt-laden air from Terrigal to Woy Woy, and sandy ground around Woy Woy that holds water after long rain. That means mould, musty smells through the house and eventually rot in the bearers and joists, the framing timbers that carry the floor.
We have seen what a wet subfloor does when nobody looks: a lounge carpeted twice for a smell that was never in the carpet, and joists under it gone black and soft enough to dent with a thumbnail. That is why we do this work, and why we find where the water comes from before fitting anything.
When we come out we start underneath, moisture meter on the timber, torch on the soil and piers, checking which vents still breathe. Then we open it up: covered vents dug out, rusted ones replaced, fresh openings cut where the breeze can't get in. We size the fans to the floor area and place them to pull air across the middle of the subfloor. We run the cable along the base wall, clipped clear of the ground, to a switch or humidistat on an outside wall. Where the soil is still wet we lay heavy plastic over it, overlapped and pinned, so the ground can't breathe into the timber.
Expect brick dust and noise on the day and a low hum from the fans after. Give it time: soaked timber dries slowly, and the meter picks up the change before your nose does. The chart below shows how the air is meant to move.
Multi-fan systems that pull moist air out and push fresh air through, wired to a discreet external switch or humidistat. Cost depends on whether one fan covers the space or a larger Central Coast home needs a proper multi-fan system, plus how much wiring and ducting the job involves.
Cutting in new brick or timber vents, replacing rusted or crushed metal ones, and unblocking vents that renovations, decking or garden beds have covered over. Priced per vent, supplied and fitted — a simple swap costs less than cutting a brand new opening.
Heavy-duty polythene sheeting laid over exposed subfloor soil to stop ground moisture evaporating up into the timber. Common in sandy, low-lying pockets around Woy Woy and The Entrance. Priced per square metre of ground covered, so it scales with the size of the subfloor.
A proper crawl-through with a moisture meter and torch to find blocked vents, wet timber, mould and any early rot before it becomes structural. 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 on the Coast 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.
The Bureau of Meteorology's Gosford record (Narara Research Station, 1916–2013) averages 1,328 mm a year, and every month from January to June averaged 118 mm or more. A solar-only unit has to keep the subfloor drying through that wet half of the year.
On the Woy Woy Peninsula, before accepting a fans-only quote, run the plastic test. If water forms under the plastic, the fan count should be worked out with a vapour barrier in the plan, not before 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.
Do the torch check twice: a few days after a long wet spell in the first half of the year, and again in September, the driest month on average in the Bureau of Meteorology's Gosford AWS and Norah Head records (63.0 mm and 62.1 mm). If the soil is still dark and damp then, look harder at leaking pipes, downpipes and what the ground itself is doing.
On the Woy Woy Peninsula, keep track of how long water sits after long rain, in the yard's low spots and under the house, and say what you've seen when you send the enquiry.
Elsewhere, Council has published floodplain risk management studies for areas including Davistown and Empire Bay, Killarney Vale and Long Jetty, the Northern Lakes, Tuggerah Lakes, Erina Creek and the Brisbane Water foreshore. If you're in one of them, check what the study says about your area, and if it shows flooding near you, look for a silt or water line on the piers and base brickwork.
Central Coast Council's Woy Woy Floodplain Risk Management Study, approved in April 2023, describes its study area as generally a flat sand plain, with ground levels typically between 4 and 6 metres AHD (Australian Height Datum). The peninsula's soils are coarse sands, but the study says podsol soils can often slow or stop water draining down to the groundwater table, causing ponding and waterlogging at the surface. Sandy ground doesn't mean a dry subfloor.
The study says the low-lying part of the Woy Woy Peninsula was subdivided mainly in the early 1900s on a rectangular grid that ignored the sand dune ridges, so runoff drains to low points between the ridges, in streets and backyards, and some areas have no piped drainage or effective overland flow path. Nuisance flooding usually lingers for a couple of days after long rain. In very wet periods the groundwater table can rise and keep ponded water in place for several weeks. The study also says groundwater flooding can happen after long rain around the Everglades catchment at the foot of the escarpment.
That matters under a suspended floor. Vents and fans move air; they can't drain a subfloor sitting on raised groundwater, or in a low point with nowhere for water to go. The first question is where the water comes from; the answer decides whether the job is ventilation, a moisture barrier, drainage or a mix. If it involves digging, clause 7.1 of the Central Coast Local Environmental Plan 2022 can require development consent on Class 1 and Class 2 acid sulfate soil land.
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 includes a trench, a sump or a drain line, local rules can come into it. Under the Central Coast LEP 2022 (clause 7.1), any works on Class 1 acid sulfate soil land need development consent; on Class 2 land, so do works below the natural ground surface or likely to lower the watertable. An exemption covers works disturbing less than 1 tonne of soil and not likely to lower the watertable, so a drain meant to draw groundwater down from under the house needs the lot's class checked first.
In the former Gosford council area, Part C of Chapter 3.1 of the Central Coast DCP 2022 allows no excavation within 2 metres of a drainage easement without Council approval, so find out where any easement runs before a trench is marked out.
In the DCP's drainage black spots (land around Cogra Road and Rothwell, Moana and Angler Streets in Woy Woy; Shephard, Glenn and Carpenter Streets in Umina), Council says the problems can't readily be overcome and public funds for the works are unlikely in the foreseeable future. Any plan to pipe water off the lot there needs a clear answer on where it goes.
Enquiries are taken from across the Central Coast: Toukley, Budgewoi, Blue Haven, Lake Haven, Gorokan and Wyong in the north; Tuggerah, Berkeley Vale, Bateau Bay, Long Jetty and The Entrance; Gosford, Wyoming, Narara, Lisarow, Ourimbah and Point Clare; Erina, Terrigal, Wamberal, Avoca Beach and Copacabana; Kincumber, Saratoga, Davistown and Empire Bay; Woy Woy, Umina Beach, Ettalong Beach and Booker Bay; and Kariong, Somersby and Peats Ridge out west. If you're on the Woy Woy Peninsula, say whether water sits under the house after rain, because it changes what the job involves. Suburb not listed? Send the enquiry anyway and say where you are.
Tell us what you need and where the property is.