Subfloor Ventilation FAQ — Central Coast

It depends where the moisture is coming from. A fan sized to the space, with clear vents feeding it, moves humid, stale air out. A fan pulling against vents covered by soil, pavers or deck skirting just stirs stale air around, which is why vent coverage gets checked first. What no fan can do is drain water sitting on the ground. Central Coast Council's Woy Woy Floodplain Risk Management Study says that in very wet periods the groundwater table on the Woy Woy Peninsula can rise so that ponded water stays for several weeks. A subfloor sitting in that water needs the water dealt with, not just the air.

Inside, a musty smell in the rooms over the lowest part of the block is worth taking seriously. Underneath, look for dark staining on the bearers, wet or pooled soil, and vents blocked by soil, pavers or decking. Central Coast Council's Woy Woy floodplain study says nuisance flooding on the peninsula usually lingers for a couple of days after long rain, so water still under the floor weeks later, in a very wet period, may be groundwater. Mention it when you enquire. Check again after paving, landscaping or deck work beside the house, when vents tend to get covered, and every couple of years otherwise.

The price follows the cause. Clearing or replacing blocked vents is the smallest job. A single mechanical fan costs more to supply and install, and a multi-fan system with ducting for a larger or wetter subfloor costs more again. A moisture barrier over bare soil is priced by the square metre covered. If the moisture is groundwater rather than humid air, drainage may come into it, and digging on Class 1 or Class 2 acid sulfate soil land can need development consent under the Central Coast Local Environmental Plan 2022.

Check the acid sulfate soil rules before anyone digs. Clause 7.1 of the Central Coast Local Environmental Plan 2022 requires development consent for any works on Class 1 acid sulfate soil land. On Class 2 land it requires consent for works below the natural ground surface or works likely to lower the watertable. Works that disturb less than 1 tonne of soil and are not likely to lower the watertable don't need consent under the clause, so clearing vents or fitting a fan within those limits isn't caught by it. Digging out for clearance, trenching a drain or putting in a sump pump that draws down groundwater can be. New vents should meet the National Construction Code's subfloor ventilation requirements.

The ground itself can be the reason. Central Coast Council's Woy Woy floodplain study says the low-lying part of the peninsula was subdivided mainly in the early 1900s on a rectangular grid that ignored the sand dune ridges. Runoff drains to the low points between the ridges, in streets and backyards, and some areas have no piped drainage or effective overland flow path. A house in one of those low points can collect water that a house on the ridge sheds. The study also says podsol soils can often slow or stop water draining down through the sand, leaving it ponded at the surface. Vents alone can't clear water that has nowhere to go.

Damp doesn't bring termites by itself, but a wet, still subfloor is the kind of place termites and timber rot prefer. Timber that stays damp softens, which makes it easier to attack and harder to inspect properly. That is why sorting out moisture under the floor is about protecting the structure, not just getting rid of a smell. On the Woy Woy Peninsula, where Central Coast Council's floodplain study says ponded water can stay for several weeks in very wet periods, it is worth knowing whether the ground under your floor holds water after rain.