Sub-floor Ventilation Requirements

Sub-floor ventilation is often a forgotten part of termite management. The BCA (Vol. 2) requires the sub-floor between a suspended floor and the ground to be ventilated. This minimises sub-floor moisture which helps to prevent timber decay, and creates an environment that discourages termite attack. To be effective, acceptable sub-floor construction must:

• Be free of building debris and vegetation
• Provide cross ventilation,
• Contain no dead air spaces
• Be graded to prevent ponding and be above the external ground level
• Have evenly spaced openings

Unless full underfloor physical or chemical barriers are provided (refer Fig 4 to 8), sub-floor clearance must be provided to enable regular inspection.

Where required, sub-floor clearance must be a minimum of 400 mm to the underside of bearer, except on sloping sites where part of the area may be a minimum of 150 mm clearance as shown in Fig. 9.

The amount of ventilation depends on the location of the site in Australia. This is governed by relative humidity; the relevant zone can be read off Map 1. Using Table 1, select the appropriate climate zone and read off the required ventilation area per metre of subfloor wall. Amounts vary depending on whether a sealed impervious ground membrane is used or not.

The sealed ground membrane option assists where special consideration is required for sub-floor ground which is subject to excessive dampness or frequent flooding. Care must also be taken to ensure patios, paving and similar construction does not limit the effectiveness of ventilation. Further details are in the BCA.

Map 1: Climatic zones based on relative humidity (Source: BCA 2007)

Fig 9: Minimum underfloor clearance - Refer to AS3660.1