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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-resistant 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.

bushfire aftermathAustralia's updated Standard AS3959-2009 'Construction in bushfire-prone areas' has the benefit of a number 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. It was included in the Victorian Building Regulations 2006 in March 2009 and has also been adopted in the ACT. Other states are likely to adopt the standard into their building regulations in 2010.

keep framing in timber

rebuilding after a bushfireAS3959-2009 makes no reference to internal house framing in any of its six Bushfire Attack Levels (BAL). 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 maximum 2mm aperture in higher BALs.

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.

Table 1   timber exterior cladding
* 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 and AS3959-2009

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 AS3959-2009. 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.

Table 2

AS3959-2009 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:

Table 3

AS3959-2009 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.

A compliant roof solution for use in BAL-FZ has been achieved for a steel sheet roof. It allows for timber or engineered wood product roof framing and plywood is used as a roof membrane and to provide support for other components.

These strategies, and more, embedded in the new Victorian building regulations and based on AS 3959-2009 Construction of buildings in bushfire prone areas, have been developed scientifically by experts and Australian Standards over a number of years. They are about creating a fire-resistant building envelope. Building out bushfires does not necessarily alter how attractive a home can look. You can still commitment to sustainability and use naturally renewable, responsibly grown and harvested timber.

Contact us for the brochure: Timber Housing in Bushfire-prone Areas for more general information.

The guide, Building with Timber in Bushfire-prone areas provides a detailed overview on what is required in each BAL to design and build with timber. Visit www.timber.org.au/bushfire to order your copy and to find out about other compliant systems and products as they are successfully tested and become available.

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