Bushfire basics

How do homes burn down?

There is widespread belief that bush fires move at the speed of express trains, exploding houses into flames and burning them down in minutes, and that there is not much that can be done to prevent it. THIS IS NOT TRUE. Scientific research into the behaviour of bush fire attack has shown otherwise.

"The majority of houses destroyed in bush fires actually survive the passage of the fire front, only to burn down during the next few hours due to the fire spreading from ignitions caused by windborne burning debris. Whilst direct flame contact and radiant heat also play a part in the ignition and destruction of buildings, these mechanisms are generally only significant during the few minutes it takes for the fire front to pass. Showers of burning debris, on the other hand, may attack a building for some time before the fire front arrives, during the passage of the fire front and for several hours after the fire front has passed. This long duration of attack, to a large extent, explains why burning debris is a major cause of ignition of buildings."

Extract – CSIRO – Buildings Innovation & Construction Technology / Number 11 Feb 2000

There are three ways in which the bushfire can ignite your home: radiant heat, convection or embers.

Radiant heat:

Radiant heat can ignite your home much the same way a radiator heats your room (and can cause fires as well). Radiated heat is capable of igniting combustible materials from distances of 30 meters or more. It is NOT stopped by windows, making it possible for e.g. your curtains to catch fire even though the windows are closed. In addition the strong winds and extreme temperatures frequently cause the windows to explode leaving a clear path for burning embers and other debris to float right in.

Convection:

Direct contact with flames (convection column) may cause your house to ignite. Typically the convective heat column rises vertically within the smoke plume. The combustible building materials and other debris outside your home are very vulnerable to ignition from a direct contact with flames.

Embers:

Embers are the single most dangerous vehicle for the fire to ignite your home. The are burning materials that detach from the fire during strong convection drafts in the burning zone. Embers can be carried long distances (kilometres, see spot fires) by the winds. They will cause building materials, loose debris and other combustible materials to ignite. If your windows have broken, they have direct access inside your home.

Most homes burn from the inside out, not the other way round. With proper preparation many homes can be saved.