The space under a suspended timber-framed floor must be ventilated. A minimum of five air changes per hour should be provided (and double that for wet subfloor spaces). A clear opening area of 3500 mm2 (100 x 35 mm) should be provided for every square metre of floor area. Openings should be provided within 750 mm of corners, and evenly spread around the building at 1.8 metre maximum centres.

Remember that the clear ventilation opening requirement does not refer to vent dimensions. Rodent protection or decorative work on some vents means that the actual clear open space can be less than one quarter of the vent dimensions. If you are buying new vents, ask the supplier about the open space measurement as well as overall dimensions.

If the existing area of clear opening under a house does not meet the minimum requirement, install extra or larger openings.

A building with baseboards should have 20 mm minimum gaps between the boards to allow sufficient ventilation.

Houses built during the 1970s were sometimes built with a floor structure that extended past the foundation line so ventilation could be provided over the top of the foundation wall and between the floor joists. If necessary, install mesh over this gap to stop vermin from getting under the house.

It is very common for subfloor ventilation to be blocked over time as a result of:

  • soil levels being built up in gardens around the base of the house
  • dense planting in gardens around the base of the house
  • raised concrete pathways or driveways reducing the ventilation spaces
  • thick paint reducing the airflow in metal gratings
  • storage of building materials or other items under the house.

Where this has happened, reduce the blockage to restore good ventilation. In some cases this may be as simple as lowering the level of a garden.

Where blocked vents cannot be unblocked, or there were insufficient vents to begin with, you may need to create new vents. This may require cutting through timber or drilling through concrete.