Subfloor ventilation: what the NCC requires and how to fix it

Damp, musty subfloor or water lying under the house? Get a quote or call 1300 241 678, and send us photos of the vents, the subfloor and the ground around the house.

Why does a suspended timber floor need subfloor ventilation?

A suspended timber floor needs subfloor ventilation because moisture rising from the soil collects in the air under the house, and without airflow it soaks into the bearers, joists and floorboards. The National Construction Code (NCC) Housing Provisions describe subfloor ventilation as cross-ventilation of the space between the floor and the ground. Its purpose is to reduce water vapour that can lead to timber rot, fungal growth and termite activity.

The NSW salt damp guide, published by the Heritage Council of NSW with interstate partners, explains the same process for older houses. Soil moisture evaporates under the floor and passes out through the vents in the base of the walls. Without that ventilation, the guide says, the moisture in the walls and floor timbers would be much greater, with the risk of fungal rot, borer and termite attack.

How much subfloor ventilation does the NCC require?

The NCC requires vent openings of 2,000, 4,000 or 6,000 mm² per metre of wall, depending on the climatic zone, and half that where the ground is sealed with an impervious membrane. The figures come from Table 6.2.1a of the Housing Provisions:

Climatic zone	Vent area, no membrane (mm² per m of wall)	Vent area, ground sealed with an impervious membrane (mm² per m of wall)
A	2,000	1,000
B	4,000	2,000
C	6,000	3,000

The zones are mapped in NCC Figure 6.2.1a and are based on average relative humidity at 9 am and 3 pm in January and July. Your builder, certifier or engineer can confirm which zone applies to your site.

Clause 6.2.1 adds rules on where the vents go:

• Every wall. Openings go in the external walls and in internal subfloor walls. Internal openings must have the same unobstructed area as the external openings next to them.
• Even spacing. Openings must be spaced evenly as far as practicable, no more than 600 mm in from corners, and leave no dead air spaces.
• Both leaves. In double-leaf (cavity) masonry, openings go in both leaves and line up so air flows through.
• Clean ground. The subfloor must be cleared of building debris and vegetation.
• Clearance. The lowest horizontal subfloor member must sit at least 150 mm above the ground, or 400 mm where a termite management system that needs inspection is installed. On sloping sites the 400 mm may reduce to 150 mm within 2 m of the external walls.

Vent area is the open area, not the vent size. Timber Queensland's data sheet (a Queensland source) calculates it as the number of holes times the area of each hole. Its example clay vent has 15 holes of 16 x 16 mm, which is 3,840 mm². In a 6,000 mm²/m zone, vents of that type would need to be no more than 640 mm apart.

What if the ground under the house is damp?

Where the ground under the house is excessively damp or floods often, the NCC requires one extra measure on top of the normal vents. Clause 6.2.1(5) gives three options:

1. increase the ventilation by 50%;
2. seal the ground with an impervious membrane; or
3. use durable framing: Class 1 or 2 timber or H3-treated timber above ground, Class 1 or 2 or H5-treated timber in the ground, or steel framing to the NASH Standard.

The NCC's explanatory note links this clause to high water tables, poor drainage and flood-prone sites.

What are the signs of poor subfloor ventilation or water under the house?

Signs of poor subfloor ventilation or a damp subfloor include damp or rotting floor timbers, mould in built-in cupboards, and fungal rot, borer or termite activity. Salt damage on walls under the floor points to salt attack, which too much ventilation can also cause. The NSW salt damp guide lists these signs and causes:

Sign	What it can point to
Mould growth in built-in cupboards	The guide says this can be a sign of insufficient underfloor ventilation.
Wet or rotten floor timbers, especially near a damp wall	Damp reaching the timbers; the guide shows a wet, rotten floor plate beside a damp wall, where fallen salt-laden plaster held moisture against it.
Fungal rot, borer or termite activity	The guide says these are associated with high humidity and moisture in the masonry, and that rising damp and termite problems often go together.
Crumbling mortar and salt on walls under the floor	Salt attack; the guide warns that too much underfloor ventilation can also cause this on inside wall faces and dwarf walls.
Vents blocked by paving, planter boxes, dense shrubs, dust or cobwebs	Airflow cut off; the guide says these obstructions should be removed.
Water lying on the ground under the house	Surface water reaching the subfloor; the NCC requires the ground there to be graded so water can't pond.

Changes to the house can cause the problem. The guide notes that additions can block old air passages, and that sealing a timber floor with vinyl, polyurethane or foam under the skirtings reduces circulation. It calls replacing a ventilated timber floor with a concrete slab one of the worst mistakes renovators make, because the soil moisture is then forced up the walls. If white salt is showing on your bricks, see our guide to efflorescence.

How do you fix poor subfloor ventilation?

Fix poor subfloor ventilation by clearing the existing vents first, then removing the source of water, and only then adding vents, a membrane or more durable framing. The order matters because the NSW salt damp guide advises cleaning out existing vents and monitoring the result before deciding to add new ones.

Problem	Fix	Where it comes from
Dirty or blocked vents	Clean the grilles; remove paving, planter boxes or shrubs in front of them	NSW salt damp guide
Garden water or rain running into the vents	Make sure surface water isn't directed through the vents	NSW salt damp guide
Water ponding under the house	Grade the ground under the floor above the outside ground level	NCC Part 3.3, clause 3.3.3
Groundwater seeping under the house	A subsoil drain with a fall of at least 1:300 to an external silt pit	NCC Part 3.3, clause 3.3.4
Air passages blocked by an addition	New vents matching the originals	NSW salt damp guide
Damp ground that stays damp	50% more vent area, an impervious ground membrane, or durable framing	NCC clause 6.2.1(5)
Too few vents for the wall length	Add vents to meet Table 6.2.1a, evenly spaced and near corners	NCC clause 6.2.1

Our guide to drainage around the house covers the gutters, grading and ag pipes that keep groundwater away from the subfloor. For subsoil drains in more detail, see what is a French drain.

Subfloor ventilation rules in Sydney and NSW

In NSW, subfloor ventilation for houses follows Part 6.2 of the ABCB Housing Provisions, under the edition of the NCC in force. The NSW Government announced on 25 March 2026 that NSW will adopt NCC 2025 on 1 May 2027, giving industry a 12-month transition. The NCC 2025 version of Part 6.2 keeps the same vent table, clearances and damp-ground options as NCC 2022.

For older houses in NSW, the NSW salt damp guide adds practical points:

• keep the ground about 200 mm below the damp-proof course, so gardens and paving don't bridge it;
• keep raised paving clear of the vents, since the guide shows it both bridging the damp-proof course and blocking underfloor ventilation;
• check underfloor spaces regularly for fungal rot, borer and termites where the house has timber floors.

When to call a structural engineer

Call a structural engineer when the subfloor shows damage, not just damp, or when a fix means changing the structure or the drainage. Typical triggers:

• bearers, joists or floorboards are soft, rotten or sagging, or termite damage has been found;
• piers, dwarf walls or base brickwork are crumbling, cracking or leaning;
• water keeps ponding under the house after rain, or the ground stays wet;
• an addition or a new slab is planned next to, or over part of, a suspended floor;
• new vents have to be cut into load-bearing brick walls.

On a site inspection and assessment, an engineer checks the framing, the vents, the clearances and where the water comes from. Where repairs are needed, a building defect report records the damage and the fix. For the repair itself, see our remedial engineering service.