Timber remains a valid construction material under new standard
The tragic events of Black Saturday on the 7 February 2009 have resulted in revisions to building codes and standards in Australia to ensure new homes, alterations and additions are located and constructed with greater bushfire protection.
Prior to the issue of the new Australian Standard, AS 3959–2009: Construction of buildings in bushfire-prone areas, there had been concerns raised around timber’s viability as a building material in bushfire-prone areas. These concerns have proven to be unfounded and timber remains a suitable and viable building material in all traditional applications.
The new Australian Standard has now defined 6 levels of risk under the heading of Bushfire Attack Levels (BALs) – see table. Key insights post publication of AS 3959–2009 include:
- There is no limitation on the use of timber for any building element that is in BAL–LOW, which covers the majority of all residential buildings.
- All traditional internal timber products can be used to fit-out homes across all BALs.
- Requirements for internal use of timber under AS3959–2009, such as framing and flooring, have not changed.
- In external applications where there has been some change, both hardwood and softwood options are available for use in the majority of residential building applications across all BALs, including sensitive applications such as cladding and fascia elements.Solely using timber for traditional applications up to BAL–29 is possible. For example in BAL–29 options available include naturally bushfire resisting timbers such as Blackbutt, Kwila (Merbau), Red Ironbark, River Red Gum, Silvertop Ash, Spotted Gum and Turpentine for decking. Alternately, softwood can be combined with other fire retardant materials or treated with fire retardant products to produce compliant systems.
There are also timber solutions available for use in the highest BAL i.e. BAL–FZ (Flame Zone). These include external timber cladding wall systems (e.g. incorporating fire grade plasterboard) and new sheet metal roof systems incorporating plywood. Details of these tested systems are available (refer www.chhwoodproducts.com.au/bushfire-protection).
The new Australian Standard AS 3959–2009 now applies in Victoria, Queensland and the ACT. It is anticipated that other states will adopt this Standard in May 2010 following amendments to the Building Codes of Australia (BCA2010). Where AS3959–2009 is yet to be adopted, the previous standard AS 3959–1999 remains in force.
Getting a site assessed prior to building under the new Standard Site assessment is typically conducted by an experienced architect, designer or building practitioner prior to submitting for a building permit. A BAL assessment is based on the surrounding vegetation type and effective slope of the ground under the classified vegetation and distance from the vegetation. Design Tips “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 – Wood Products Victoria). • 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 it is fully enclosed below the floor level, fitting vents with spark-proof metal screens. • Use “clever” garden design and paving to leave a firebreak between nearby vegetation and the house. More information • Carter Holt Harvey http://www.chhwoodproducts.com.au/bushfire-protection/ To assist you in identifying the accredited products you require to build safer housing, “A guide to Bushfire Compliant Construction using Carter Holt Harvey Wood Products” can be downloaded for free from Carter Holt Harvey. Other resources are also available: • Forest and Wood Products Australia http://www.timber.org.au/bushfire • Wood Products Victoria http://www.wpv.org.au