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Ignition of Homes in Wildfire-Prone Areas

Homes do not spontaneously ignite in a wildfire. They are lost when a small fire starts in or on the building and grows with no one there to put it out. Understanding the three exposures that start those small fires is the basis of every mitigation decision on this site.

Wildfire burning on a hillside behind a residential neighborhood at night

The Three Wildfire Exposures

  • Embers (firebrands). Light enough to blow far ahead of the fire, embers spread fire by spotting. Landing on or near your home, they ignite vegetation, debris, or the structure itself, and they enter attics and crawl spaces through vents and openings. Embers are the leading cause of home ignition.
  • Radiant heat. The heat you feel standing near a campfire, scaled up. Close, intense, or sustained radiant heat ignites combustible materials outright, and even when it does not, it preheats surfaces so a brief flame touch ignites them.
  • Direct flame contact. Flames from burning vegetation, debris, fences, or neighboring structures touching your home.

The Fire Front Is Not What Destroys Homes

A common misconception is that homes are lost as the main body of the fire passes. Research and field observation show the flame front moves through in roughly 1 to 10 minutes depending on vegetation (Butler et al. 2003; Ramsay and Rudolph 2003). Most buildings can survive that brief exposure. What destroys homes is the small ignition that follows: the ember in the gutter debris, the mulch fire against the siding, the ignition inside the attic, burning unattended after evacuation.

In the fires that destroy hundreds of homes at once, the wildfire becomes an urban fire. Home-to-home spread overtakes wildland-to-home spread, and the closer together homes stand, the faster it moves (Cohen 2008; IBHS). This is what happened in the 2018 Camp Fire and again in the 2025 Los Angeles fires: the built environment itself became the fuel.

What this means for you: your job is to deny the fire its small beginnings. Our Hardening Your Home Against Wildfire page walks through each vulnerable component, how it ignites, and the materials and design choices that fix it, and our defensible space page covers the fuels around the building. Neither works without the other.

Adapted from Quarles et al., Home Survival in Wildfire-Prone Areas (UC ANR Publication 8393, PDF).