Subfloor Ventilation

High levels of moisture in the subfloor area of a home can lead to a number of issues including fungal decay and increased risk of termite attack.

In sub floor areas with poor ventilation water can pool or moisture can escape from the soil which increases the humidity in the subfloor space. This in turn increases the moisture content of timbers in the sub floor area. When the moisture content of timber is above 20%, it becomes more susceptible to fungal decay and termite attack.

Vents

In Gippsland (Climatic Zone C), the minimum subfloor ventilation openings is 6000mm2 per meter of wall. Vents should be evenly spaced and a vent should be placed no more than 600mm in from each corner.

A practical example of this is a common Pryda 230×165 Vent. As this vent provides 14157mm2 of ventilation, the maximum spacing for this vent is 2.36 meters.

Clearance Requirements for Subfloor Ventilation 

For inspection purposes, a ground clearance of 400mm is required between the finished ground level and any structural components such as bearers. For sloping sites, the minimum clearance may be reduced to 150mm as long as it is within 2m from an external wall.

Drainage 

The ground under a suspended floor must be graded to prevent water pooling under the home. This means low areas must be filled so they are higher than the external adjacent finished ground level.

Other Impacts on Subfloor Ventilation

Materials stored under the home impact the efficiency of sub floor ventilation. As well as blocking airflow in the sub floor, stored items containing wood increase the risk of timber pest attack.

Vegetation and garden beds against the home commonly impede the subfloor ventilation of homes. While garden beds against the home can enhance the visual appeal of the home, it is important to ensure sub floor vents are not impacted.

Where the subfloor space is excessively damp (from high water tables) or subject to flooding, the BCA offers 3 options.

• Subfloor ventilation is increased by 50% (to 9000mm2 per meter of wall); or
• The ground in the subfloor space is sealed with an impervious membrane; or
•  Durability class 1 or 2 timbers or H3 (above ground) and H5 (below ground) treated timbers must be used.