Building Out Bushfires
Building with timber in bushfire-prone areas
Everyone enjoys the aesthetic of timber, especially in a bushland setting – the traditional weatherboard look is also a beloved part of the rural landscape. But given the increase in bushfires, can we go on building timber framed and clad homes? Yes, we can.
Building out bushfires is about creating a fire-resisting building envelope. Making the right decisions means you can still have an attractive timber home, using naturally renewable, responsibly grown and harvested timber both inside and out.
Australia’s updated standard AS3959-2018 (Amdt No. 1) ‘Construction of buildings in bushfire-prone areas’ has the benefit of many of years of scientific development by experts. It provides an extensive guide to building homes to minimise risk for different levels of potential bushfire attack. The first edition of the Standard was included in the Victorian Building Regulations 2006 in March 2009 following the tragic Black Saturday. The current edition has been nationally adopted by all states and territories as of the 1st May 2019 via the National Construction Code 2019.
Keep framing in timber
AS3959 makes no reference to internal house framing in any of its six Bushfire Attack Levels (BALs). You can continue to use traditional timber framing in any new home or extension.
Keeping both embers and radiant heat out of a home is the important factor, whatever it is built from.
Off the ground is still a good idea
Where there is a slope, even for a high bushfire attack level site, it is still advisable to consider a timber sub-floor, rather than the more expensive and environmentally damaging cut-and-fill that is required for a concrete slab.
The Standard specifies that when a timber sub-floor is fully enclosed, all traditional timber sub-floor framing products (stumps, bearers, joists) continue to be appropriate. But to guard against burning embers, vents in an enclosed sub-floor need to be covered with an appropriate metal mesh with a maximum 2mm aperture.
When well off the ground, a pole house for instance, these measures may not be required – check the Standard.
Exterior Cladding Options Include Timber
While there are no special requirements for external cladding when a home is fully brick veneer, where light-weight cladding sections are used, for instance above windows and doors and upper storeys, the same regime as for a totally light-weight clad building may apply to those elements.
External wall cladding: Light-weight materials such as weatherboards and ply sheets
|Normal timber and other light-weight cladding materials
|Bushfire-resisting timbers or timber species listed in AS3959 Table E1 (species with density of 750kg/m³ or greater)
|Bushfire-resisting timber and sarking to the outside of the frame
|FRL* of 30/30/30 required. This can be achieved with a moisture resistant fire-grade plasterboard and timber system which will actually give a higher rating (16mm fire grade plasterboard provides 60/60/60).
* FRL: The Building Code of Australia (BCA) defines a Fire Resistance Level (FRL) as the grading period when under fire attack, in minutes, for three criteria: structural adequacy, integrity and insulation.
Timber Use & AS3959-2018
Fortunately, Australia has a number of high density timbers that provide an inherent natural bushfire resistance. Seven of these are defined as bushfire-resisting timbers and are specified in AS3959. They are solid, dense hardwoods that performed well in extensive fire testing. While the Standard specifies them for some applications, they can be used more extensively for peace of mind.
AS3959 specifies the use of special timbers in some applications. The full lists of timber species with specified densities is given in Appendix E of the Standard. The most common construction timbers from these lists are:
AS3959 specifies timber used for construction in more bushfire-prone areas should not burn readily and that “timber may be ‘bushfire resisting’ by means of one or more of: the inherent properties of the material itself; being impregnated with fire-retardant chemicals; or the application of fire retardant coatings or substrates.”
Some fire-retardant treated timber products have been tested to AS3959 requirements and there are plenty of natural hardwood options readily available.
Up to BAL-19: external timber wall cladding and decks with a density of 750 kilogram per cubic metre or greater and window frames with a density of 650 kilogram per cubic metre or greater or bushfire-resisting timber (see above).
At BAL-29: external timber wall cladding, decks and timber window frames made of bushfire-resisting timber with various precautions.
For BAL-40 & BAL-FZ: fire resisting timber wall systems (e.g. incorporating a moisture resistant fire-grade plasterboard and timber system), modified deck design and materials, bushfire shutters over timber window assemblies.
Compliant roof solutions for use in BAL-FZ has been achieved for both steel sheet and tiled roof systems as are detailed in AS3959. Both allow for timber or engineered wood product roof framing with the steel sheet roof system incorporating plywood used as a roof membrane and to provide support for other components.
These strategies, and more, embedded in the Victorian building regulations and based on AS 3959 Construction of buildings in bushfire prone areas, have been developed scientifically by experts and Australian Standards over many years. They are about creating a fire-resisting building envelope. Building out bushfires does not necessarily alter how attractive a home can look. You can still have your commitment to sustainability and use naturally renewable, responsibly grown and harvested timber.
WoodSolutions Bushfire Publications & Resources
With the appropriate design considerations, you can create a safe and desirable timber home in bushfire prone areas using sustainably sourced renewable timber products both inside and out. The WoodSolutions Technical Design Guide #04, Building with Timber in Bushfire-prone areas provides a detailed overview on what is required in each BAL to design and build with timber.
Timber Framed Housing Options in Bushfire-prone
A 4 page brochure overviewing the range of residential timber construction solutions for brick-veneer and lightweight clad homes for different bushfire attack level (BAL) sites.
Building with Timber – Construction requirements for Bushfire Attack Levels as per AS 3959-2018 (Amdt No. 1) Construction of buildings in bushfire-prone areas
An A3 matrix succinctly summarising the timber construction requirements for each bushfire attack level (BAL).
Level 6, 10-16 Queen St, Melbourne. VIC 3000