Masonry Cavity Wall Construction in Australia

What it is

A masonry cavity wall is two leaves of brick or block separated by a continuous air gap. The outer leaf takes the weather. The cavity drains any water that gets past the outer leaf. The inner leaf carries the floor and roof loads in load bearing cavity construction or sits as a non load bearing veneer behind a timber or steel frame in masonry veneer construction. The whole system relies on the cavity staying clear, the wall ties tying the two leaves together and the weep holes draining at the right level.

In Australia the design standard is AS 3700 Masonry structures. The NCC Volume Two pathway sits under Part H1 for structure and Part H2 for damp and weatherproofing, with Part 5 of the ABCB Housing Provisions giving the Deemed-to-Satisfy detail for residential masonry. Block work follows the same AS 3700 framework with the added provisions of AS 3700 Section 4 for concrete masonry.

Cavity widths and detailing

AS 3700 sets a clear cavity of at least 40 mm between leaves for standard residential work. NCC Housing Provisions tighten this in some climate zones and for some wall classifications. The cavity must stay clear of mortar droppings because a bridged cavity transfers water from the outer leaf to the inner leaf and to the timber frame or plasterboard sitting against it.

A correctly built cavity has:

• Clear gap of at least 40 mm full height.
• Continuous flashing at the base, over openings and at parapets.
• Weep holes at not more than 1200 mm centres along base courses and over openings.
• A clean cavity tray under door and window sills directing water back out.

Weep holes

Weep holes are open perpends in the outer leaf. They sit immediately above the flashing line at the slab edge, above lintels and above any intermediate flashing. AS 3700 calls for weep holes at not greater than 1200 mm centres. A common defect is weep holes installed too high above the flashing or mortared closed by the bricklayer during pointing. Water then ponds on the flashing instead of draining out.

The weep hole insert chosen matters too. Open perpends without inserts work but allow vermin and embers in. In bushfire prone areas under AS 3959 a weep hole insert with a vermin and ember mesh is required for the construction level BAL nominated for the site.

Wall ties and reinforcement

Wall ties connect the two leaves so they act together under wind load. AS 3700 sets a minimum tie spacing of 600 mm vertical by 600 mm horizontal as the baseline for typical residential cavity walls, tightened at openings and free edges. Ties must be the right durability class for the exposure environment. Coastal sites within one kilometre of breaking surf need stainless steel ties to AS 3700 R5 class because galvanised steel will fail within years.

Reinforcement of cavity walls is required at openings, at control joints and where AS 3700 calculates demand for lateral loads or seismic loads. A residential single storey wall in a low seismic and N2 wind area may need no horizontal reinforcement at all. A two storey wall on a hill in a cyclonic region needs bond beams and reinforced piers designed by the engineer.

Inspection hold points for TradeLens

Four hold points cover the critical path:

1. Damp proof course and base flashing inspection before the first course is laid.
2. Cavity inspection at sill height: tie spacing, cavity clear, flashings continuous over openings.
3. Lintel and head flashing inspection before the top leaf goes on.
4. Pre render or pre paint walk: weep holes open and at the correct level, control joints sealed, top flashing or parapet capping in place.

Common defects

The recurring defects are mortar dropped into the cavity bridging the leaves, weep holes mortared up at pointing, flashings stopped short of the cavity face allowing water past the flashing line, wall ties at the wrong centres or omitted near openings and galvanised ties used in coastal exposure where stainless steel is required. Each of these defects becomes a moisture problem within five years of handover.