This section describes a range of design and construction principles to maximise the bushfire resilience of your new wall system. Walls are vulnerable to the ignition of combustible wall elements and the passage of flames and embers through the building envelope and into the living areas of the home. When designing a wall system, your main objective should be to prevent embers from entering into the cavities and interior of the building, and to prevent ignitions, collapse, displacement, breach, and damage caused by wind-born debris.
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Preventing external wall ignition
Flame, radiation, debris and adjacent combustible elements can either individually or as combined actions to impact wall cladding. When constructing a house is can be difficult to anticipate all of the possible adjacent combustible elements1 that may be near a wall over its lifespan as well as the amount of debris that can blow up against it during a bushfire. It is recommended that only non-combustible materials be used for cladding of the walls. The wall cladding should also be tight fitting to tolerances of less than 2mm to minimise the rate of debris build up behind the wall cladding. The construction requirements in AS 3959:2018 anticipate direct action from the bushfire front but does not consider potential additional exposure from other adjacent combustible elements.
The requirements in AS 3959:2018 allow for combustible cladding; however, this can ignite during a bushfire, spreading fire to other parts of the house and threatening the effective egress of occupants.
Preventing wall cavity ignition
Wall cladding may contain many gaps between the wall cavity and the outside world. Gaps in cladding that are less than 2mm are effective in minimising ember and debris entry, however any flame local to this gap could pass through a gap of this size. When this happens small flame sources can occur in the wall cavity which can either burn out or ignite adjacent materials leading to a larger fire and possible house destruction. Cavity fires are difficult to monitor and suppress, thus contributing to the possible entrapment of occupants and a loss of life.
Efforts to limit the amount and size of embers and debris that can enter a wall cavity helps to limit the size of small flame sources. Hence, it is essential to use non-combustible material for all elements within the wall cavity including the framing, gas supply piping and insulation. It is also essential to design any penetrations in the internal wall lining with gaps finer than 2mm.
The requirements in AS 3959:2018 allow for cladding to be installed over combustible framing materials. Cladding is often damaged through the normal wear and tear of a building. When this cladding is damaged it can compromise the entire structure, especially when installed over a combustible frame, making the home more vulnerable to bushfire.
- Use non-combustible cladding, solid masonry walls and earth walls are best.
- Use non-combustible wall cavities.
- Seals all gaps (larger than 2mm) in the wall system to prevent ember attack.
- Protect against wind attack and tree strike by siting the home away from hazards (if possible) and by using a design that can resist high wind loads.
- Use a simple design profile which avoids re-entrant corners.
- Avoid combustible cladding and frames.
- Do not rely on sarking as an ember barrier to a combustible wall cavity.
- Do not rely on non-combustible cladding to protect combustible framing materials. A break in the cladding will allow flames and embers to penetrate the exterior of the house and ignite the vulnerable framing materials.
Construction and materials
The following table contains information on different wall systems and the level of bushfire protection they provide. Consider the Bushfire Attack Level (BAL) at the site of your build and design a wall system that will protect against the predicted level of exposure.
|Type of frame||Cladding||Additional fire protection||Other features||Exposure protection – BAL|
||None||NA||NA||BAL-FZ, consequential fire and wind attack|
|Non-combustible cladding. For renders, check the flammability and ignitability of the paint.||BAL-FZ, consequential fire and wind attack|
||Thick non-combustible cladding (e.g., brick veneer)||30min fire rated cladding||NA||BAL-FZ|
|60min fire rated cladding||NA||BAL-FZ|
|Thin non-combustible cladding (e.g., corrugated steel)||Seal and cover joints||NA||BAL-29|
|External fire barrier (e.g., plasterboard)||BAL-FZ and surface fire|
1 AS3959:2018 wall prescriptions only attempt to anticipate the direct action from a fire front and do not consider potential additional exposure from other adjacent combustible elements.