When we picture a house or building being destroyed by a bushfire we often imagine the flames from the fire directly attacking the house overtaking the house.But did you know that houses have been lost that are over 700 metres form the fire front? And that most houses are not destroyed by direct flame contact?
Photo by Thomas Ehling
During a bushfire event a house can be subject to three different modes of attack:
- Embers and burning debris;
- Heat radiation; and
- Direct flame attack.
Of these modes of attack, most houses are destroyed due to attack by embers and burning debris
Following the Ash Wednesday bushfires in 1983, the CSIRO commenced a study of more than 1150 houses lost or damaged (Ramsay, Rudolph 2003). This study revealed that contrary to public opinion, it is ember attack that claims most houses in a bushfire. An ember attack is when burning matter, such as leaves and small twigs, become airborne and swept hundreds of metres in front of the fire. These embers can then land on buildings or open ground, causing small spot fires. If they land on buildings that do not have adequate protection, the embers can work their way into roof spaces or build up against vulnerable building materials, starting a structural fire. This fire spreads throughout the house and if not extinguished in time, destroys the house. Being able to defend against ember attack is crucial to the survival of buildings.
Heat radiation, although not the main cause of building loss, needs to be considered in building survival. Bushfires give out infra-red radiation traveling out in all directions, heating up fuels and aiding the spread of the fire. While the heat radiation helps the fire to spread, the study by CSIRO found it appeared that the radiant heat peaked for as little as 60 to 90 seconds as the fire front passed. This was not enough radiant heat to adversely affect a house (Ramsay, Rudolph 2003). Most buildings, built to the correct standard, can survive the radiant heat of a bushfire.
Direct Flame Attack
Direct flame contact is when an item is close enough to a bushfire to be touched by the flames of the fire. In their investigation of the houses lost in the 2011 Roleystone and Kelmscott bushfires in Western Australia, Fire and Emergency Services of Western Australia (2011) stated ‘of the homes that suffered direct flame contact or radiant heat damage as a consequence of the bushfire did so because the BPZ was not appropriate.’ A BPZ, which is now referred to as an APZ or Asset Protection Zone, ensures that buildings have a wide buffer from bushfire prone vegetation. A building with standard construction methods is unlikely to survive a direct flame attack. Therefore, it is vital that the landscape of the area around the building is modified to ensure the safety of the building and protect against direct flame contact.
A Bushfire Attack Level (BAL) report will indicate which level of attack that your project is likely to come under and prescribe the building requirements from the Australian Standard 3959 (AS 3959), which can help prevent losing a building to bushfire. To learn more about BAL reports, see our article here:
Need more information about bushfire reports and how it may affect your next project? Drop us a line or call on 9555 9444.
Fire and Emergency Services of Western Australia 2011, ‘Final report on – “Investigation of the house losses in the Roleystone/ Kelmscott bushfire 6 february 2011”
Ramsay, C, Rudolph, L 2003, ‘Landscape and building design for bushfire areas’.