How Homes Ignite
Buildings ignite as a result of embers, radiant heat, and/or direct flames.
Embers are the most common cause of home ignition. They are light enough to be blown through the air and can result in the rapid spread of wildfire by spotting (in which embers are blown ahead of the main fire, starting other fires). Should these embers land on or near your house, they could just as easily ignite nearby vegetation, accumulated debris, or enter the home (through openings or vents).
Recent research indicates that two out of every three homes destroyed during the 2007 Witch Creek fire in San Diego County were ignited either directly or indirectly by wind-dispersed, wildfire-generated, burning, or glowing embers (Maranghides and Mell 2009) and not from the actual flames of the fire. Watch this simulation of a home ignited by embers to see just how dangerous wind-blown embers can be.
Near-home ignitions will subject some portion of your house to either a direct flame contact exposure (where the flame actually touches a home) or a radiant heat exposure (the heat felt when standing next to a campfire or fireplace). If the fire is close enough to a combustible material or the radiant heat is high enough, an ignition will result. Even if the radiant exposure is not large enough or long enough to result in ignition, it can preheat surfaces and thus make them more vulnerable to ignition from a flame contact exposure. With any one of these exposures, if no one is available to extinguish the fire and adequate fuel is available, the initially small fire will grow into a large one.
One of the misconceptions about home loss during wildfires is that the loss occurs as the main body of the fire passes. Research and on-the-ground observation during wildfires have shown that the main flame front moves through an area in a very short time: anywhere from 1 to 10 minutes, depending on the vegetation type (Butler et al. 2003; Ramsay and Rudolph 2003). Homes do not spontaneously ignite—they are lost as a result of the growth of initially small fires, either in or around the home or building.
The wildfires that are clearly remembered by the general public are those where hundreds of homes are lost. During these events, many homes are lost because the wildfire becomes an urban fire, where the home-to-home spread of fire becomes more significant than wildland-to-home spread of fire, especially with decreasing separation between homes (Cohen 2008; Institute for Business and Home Safety 2008).