Once the current tragic and devasting bushfires occurring in Australia are brought under control, the focus will turn to reconstruction. Questions will be asked as to what products can be used and, in particular, whether timber products can be used… Yes, timber products can be used in all Bushfire Attack Levels (BALs).
The National Construction Code 2019 (NCC) references (calls up) AS 3959–2018: Construction of buildings in bushfire-prone areas, as a Deemed-to-Satisfy means of compliance, when building in a designated bushfire-prone area i.e. design and build your home in accordance with AS 3959 and you comply with the Performance Requirements of the NCC. AS 3959 has been developed in conjunction with the broadest range of stakeholders including material organisations (e.g. timber, brick and steel), window manufacturers, HIA, MBA, architects etc. ensuring public transparency and following Standards Australia’s consensus processes.
AS 3959 defines 6 levels of risk under the Bushfire Attack Level (BAL) classification system – these are described in Table 1. In relation to the use of timber products, the following comments can be made:
- Structural and non-structural timber-framing products can be used in all BALs e.g. timber wall framing, roof trusses, beams, bearers, joists, battens when used within the building’s compliant external envelope.
- All traditional internal timber products (flooring, linings, skirtings, architraves, kitchen cabinetry etc) can be used to fit-out homes in all BALs.
- For external applications, such as timber cladding, decking, windows, doors, carports, pergolas, timber can be readily used in BALs up to and including BAL–
- In the highest BALs (BAL–40 and BAL–FZ), external timber products can be used as part of a “building system” e.g. fire-rated wall, fire tested windows.
- The first BAL–FZ fire tested and approved timber framed-roof system is incorporated in AS 3959 as a deemed-to-comply system (Appendix H3).
Table 1: BAL levels relate to the potential radiant heat flux, a measure of heat intensity.
Fire affects all materials –steel melts, concrete and masonry spall and crack, and timber burns. However, as timber burns it develops a protective char layer which enables it to continue carry load over time. The standard fire test “Time-Temperature Curve” below shows the loss of strength of aluminium, steel and timber during a standard fire test. Note that after 5 mins of testing, the steel beam has lost about 40% of its strength and the timber beam would be near full strength.
Getting a site assessed prior to building under the Standard
Site assessment is typically conducted by a Bushfire Assessor but can be undertaken by an experienced architect, designer or building practitioner prior to submitting for a permit.
A BAL assessment is based on the fire danger index (FDI) for the region, the surrounding vegetation type, effective slope of the ground under the classified vegetation and distance from the vegetation.
“The best bushfire resistance in high risk areas results from a combination of design and material choices, building practices and keeping vegetation clear of the structure. Research shows that keeping hot air and embers out of a building is the important factor; whatever its material of construction.” (Boris Iskra – Forest and Wood Products Australia).
- Design with a simple footprint to minimise external nooks and crannies, corners and spaces in which debris (which can become fuel) or embers from a bushfire can accumulate.
- Have a simple roof (avoiding roof valleys and skylights). Specify gutter guards to help keep the gutters clear and free of debris.
- Where the site is on a vegetated slope, the bottom of a slope is safer to build on than the top.
- Reduce the risk of embers getting underneath a house by ensuring the house perimeter is fully enclosed below the floor level, with vents protected by mesh typically made of steel, aluminium or bronze (2 mm maximum aperture) depending on the BAL.
- Use “clever” garden design and paving to leave a “firebreak” between nearby vegetation and the house.
WoodSolutions is currently finalising an update to its Technical Design Guide #04 Building with Timber in Bushfire-prone Areas which will be available as a free download shortly via the WoodSolutions website.
This article was written by Boris Iskra, National Codes & Standards Manager, Forest & Wood Products Australia. 17 January 2020.