Cavity closures to cantilevered joists

By - , Build 151

Drained cavities to external walls need to be constructed correctly to keep wind-driven rain and vermin out and allow drainage and ventilation. One overlooked area is when external walls are supported on cantilevered floor joists.

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Figure 1 Cavity closures to cantilevered floor joists.

DRAINAGE AND VENTILATION is achieved in drained cavities by leaving the bottom of the cavity open and fitting a cavity closure to prevent the entry of vermin.

Cavity closures at base of all cavities

Cavity closures are required at the base of all cavities including:

  • above window and door heads
  • at the base of all walls
  • at inter-storey flashed junctions.

They should have 3–5 mm drainage holes or slots to provide an opening area of 1,000 mm² per metre length of wall.

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Walls on cantilevered floor joists

A sometimes overlooked location for cavity closures is at the base of an upper level external wall supported on cantilevered floor joists (see Figure 1).

Although the cantilevered floor joist space must be closed off, the base of the cavity to the wall above must remain open, so cavity closures should be installed.

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Position above bottom of cladding

A drip edge is needed at the base of all walls, above door and window openings and above horizontal flashings that bridge the cavity (E2/AS1 9.1.8.3).

This is done by positioning the cavity closure 10–15 mm above the bottom edge of the cladding.

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Figure 1 Cavity closures to cantilevered floor joists.

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Figure 1 Cavity closures to cantilevered floor joists.

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