Can Construction Keep Up with Australian Weather Extremes?

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Monday, September 28th, 2015
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Our history of intense bushfire seasons looks set to continue with predictions for the approaching summer season citing extreme El Nino effects, bringing with it drier conditions and unprecedented high temperatures.

Australian climate change projections based on the CSIRO and BOM State of the Climate annual reports suggest more intense cyclones and flooding, more frequent hailstorms, more intense rainfall events, storm surges and higher risk of bushfires.

According to the third biennial State of the Climate report from the Bureau of Meteorology and CSIRO, since the 1970s extreme fire weather has increased and the fire season has lengthened across large parts of Australia. These days, it is reported that over 100 Australian homes on average are destroyed by bushfire each year, and the trend is rising.

Despite these warnings, and in part because a growing number of houses are zoned as at risk, it has been historically difficult for fire authorities to make existing homeowners take the threat of bushfire seriously.

The aftermath of Black Saturday on February 7, 2009, however, prompted stricter building regulations specifically designed to identify and protect homes within bushfire-prone areas. And this is not just relevant for new builds; retrofitting older, existing homes that now find themselves in designated bushfire zoned areas to improve the fire resistance of the building envelope will become more and more important if homes and lives are to be saved in future.

AS3959:2009 is the most recent version of the Australian Standard for construction of buildings in bushfire prone areas (a new review of the standard has commenced). The standard is primarily concerned with improving the ability of buildings in designated bushfire prone areas to better withstand attack from bushfire, thus giving a measure of protection to the building occupants (until the fire front passes) as well as to the building itself. It covers the calculation of the bushfire attack level (BAL) of a building, and all construction and retrofitting requirements based on the calculated BAL.

Six bushfire attack levels have been developed, along with their predicted risk levels, from low to extreme:

  1. BAL Low – Very low risk
  2. BAL 12.5 – Low risk
  3. BAL 19 – Moderate risk with radiant heat exposure of between 12.5 and 19 kW/m2
  4. BAL 29 – High risk with radiant heat exposure of between 19 and 29 kW/m2
  5. BAL 40 – Very high risk with increasing radiant heat exposure of between 29 and 40 kW/m2, and increased likelihood of exposure to flames
  6. BAL Flame Zone – Extreme

Building in a bushfire-prone area introduces a number of additional design, specification and construction parameters that cannot be ignored. It has been widely stated that these additional requirements can add extra costs to a new home, though it is important to note that there is usually more than one option available to designers and builders to meet the requirements of the building regulations and standards.

Take the example of windows, which have enormous significance in terms of the integrity of the building envelope when under attack from embers, radiant heat and flames. There are various options for windows and doors at BAL Low and up to BAL 29. Inevitably though, it is more challenging to meet BAL 40, where the radiant heat exposure is greater, having implications for all components of the window system.

Construction or retrofitting to achieve compliance with BAL 40 therefore requires the highest level of protection against ember attack and radiant heat. When specifying windows for a building in a BAL 40 rated area, designers, specifies and builders may believe metal window frames and the use of non-combustible metal bushfire shutters are the only option.

However, AS3959:2009 recognises other means of compliance for windows and it does not mean that traditional building materials such as timber and uPVC are prohibited.

Any window system that has been tested and complies with the requirements of AS 1530.8.1 for BAL 40 is acceptable for use in any BAL zone up to and including BAL 40. Note that the openable portion of the window is still required to be screened with corrosion-resistant metal screens as it is assumed the window may be left open at the time of a fire.

Recent developments and manufacturing of BAL appropriate construction materials have advanced the building industry in terms of safety and widened the options for homeowners. For example, a number of uPVC double-glazed window systems have been independently tested and comply with AS 1530.8.1 for BAL 40.

Anyone who lives in a bushfire prone area is expected be aware of their local bushfire attack level zoning and implement the requirements of AS 3959:2009. While constraints inevitably do exist in the range of product choices, there is nevertheless some flexibility in materials that can be used under the standard, so long as the product meets the requirements of AS 1530.8.1 for the appropriate BAL zone.

Manufacturers are also waking up to the fact that there is a growing market for products that can be used in bushfire prone areas. Constant development of key materials in order to provide the most fire resistant and fire-retardant alternatives will help safeguard homes and widen the options for construction.

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