Subfloor Ground Clearance and Ventilation

The clearance you need depends on whether the framing is termite treated. For untreated (non-termite-treated) subfloor timber, keep at least 400mm between the ground and the underside of the bearer. On sloping sites this can reduce to 150mm within 2m of an external wall, as long as access for inspection is kept.

For termite treated subfloor timber, a minimum of 150mm is acceptable. Airflow is tighter at that height, so it often pays to add measures that keep moisture down, such as mechanical ventilation, builders plastic laid over the ground as a moisture barrier, or the foil backed Ultimate board for extra moisture protection.

Good crossflow ventilation keeps the subfloor breathing. The National Construction Code sets ventilation to suit the climate zone, with humid and tropical areas needing more airflow than drier ones. Where crossflow is limited, add mechanical ventilation, and increase ventilation where timber flooring will be laid over the top. Lay builders plastic or use Ultimate to limit ground moisture rising into the space.

Drainage works with ventilation. The ground under and around the subfloor should always slope away from the building so water never pools. Poor drainage leads to decay in bearers and joists and, over time, foundation problems.

Before any floor covering goes down, whether timber, vinyl or carpet, the subfloor moisture content should not exceed about 14%, the common industry limit under ATFA guidance. Test it with an approved meter, and if it reads high, improve the ventilation, lay ground plastic and check the drainage before you proceed. Meeting the NCC and ATFA guidance is what gives the finished floor a dry, stable base to sit on.

Working out what belongs in each room? See what subfloor do I need. Building a new floor? See our particle board flooring range for sizes, or work out your sheets with the estimator and build a quote.