I heard an interesting comment recently, that it is “Not a good idea to live in the flame zone.” For many people living on the bush interface, this is not an option.
The highest level of bushfire attack level (BAL) is known as BAL Flame Zone.
Australian Standard (AS 3959) “Construction of Buildings in Bushfire Prone Areas”, have classified different bushfire intensity levels that a home may experience during a bushfire. These are referred to as Bushfire Attack Levels, or BAL’s for short.
If a building is within close proximity to unmanaged vegetation, the flame length from the burning vegetation will directly impact on the building. The NSW Rural Fire Service (RFS) define it notes it as “sustained flame contact to a building”.
Figure 1 Bushfire Attack Levels (Country Fire Authority)
Figure 1 shows what to expect from a bushfire. In simple terms, in BAL Flame Zone, direct exposure and contact to bushfire flames would be expected. These are conditions where fire fighters cannot operate, and the house is likely to be immersed in flame. Houses which have not been built to AS3959 are more susceptible to loss while newer houses built to the Standard will be more resilient.
Research by Risk Frontiers, based at Macquarie University, found that in significant blazes in the past, the probability of loss in the first 50 metres of the bush was about 60 per cent. In the 2009 Black Saturday fires, ”60 per cent of losses occurred within 10 metres of bushland.”
In bad fire conditions, many houses in the flame zone are simply un-defendable. However, management of fuel around the house and the use of materials that are resistant to flame impact and high levels of radiant heat will increase the likelihood of structure survival.
AS3959 provides an accepted recipe or a Deemed-to-Satisfy approach for building in BAL Flame Zone. An expert committee from government, industry, the fire services and research bodies developed the Standard. While the Standard does not guarantee house survival, it does provide an acceptable approach for fires up to a Fire Danger Rating of 100 and importantly, provides a building solution for those houses that are in BAL Flame Zone.