Wildfire can threaten a home in three ways. Learn how and actions you can take to reduce your risks.Article Written by:
Glenn Nader, University of California Cooperative Extension, Yuba City, CA
Ed Smith, University of Nevada Cooperative Extension, Reno, NV
Stephen L. Quarles, University of California Cooperative Extension, Richmond, CA
Knowing how wildfire threatens a home can effectively reduce the wildfire risk. Wildfire can threaten a home in three ways:
- Flying embers
- Contact by flames
- Radiant heat.
Flying Embers
Embers are the leading cause of home loss during wildfire. Embers are burning pieces of vegetation or construction debris that can be lofted high into the air, carried by wind, or transported by fire whirls for hundreds of feet to more than a mile from the actual fire. Burning embers landing on or near your house or under your deck can ignite near-home vegetation or other combustible debris that could then threaten your house. Poor defensible space could allow embers to ignite vegetation in outlying areas and spread fire through continuous vegetation that leads to your house. Developing a 5-foot noncombustible zone immediately adjacent to your home — through the placement of lawns, rock gardens, or cement walkways in that area — is critical. Embers can also be carried into your house through unscreened vents and then start a fire inside the attic or crawl space. They can also land on combustible roofs (like wood shingles) and ignite the roof.
More houses burn due to flying embers than any other cause. If conditions are right, pieces of burning material, including small branches, pine cones, and construction materials, will result in even more embers.
Contact by Flames
Contact by flames occurs when a fire is burning close enough to allow flames to touch an object. Direct contact with flames will heat the building materials of your home and, depending on the exposure (i.e., the time and intensity of the flames), materials can ignite or break. For example, in a high-intensity fire, your combustible siding could ignite, or the glass in your windows could break and fall out. Direct contact by flames is probably what most homeowners visualize when they think of a house burning during wildfire.
Radiant Heat
Radiant heat is the energy that is transferred through the air to other objects when materials burn. If a house receives enough radiant heat for a sufficient time, it will ignite without flames contacting it. Sometimes radiant heat can break windows and allow glass to fall out. This could allow burning embers to enter the house. Even if the radiant exposure isn’t large enough or long enough to result in ignition, it can pre-heat surfaces, making them more vulnerable to ignition from exposure to a direct flame contact and embers. Consequently, even vegetation and other fuels located away from the house are a concern.
Homeowners living in high fire hazard areas must address all three types of threat through proper design, management, and maintenance of their houses and surrounding landscapes. The key is to have the fuel (vegetation) arranged or landscaped to protect your house (also fuel if it ignites) from igniting by one of these three exposures. Also, an ignition-resistant house (noncombustible roof, screens on attic and foundation vents, etc.) is important to your home's attack from all three forms of fire.
The diagrams shown below illustrate the vegetation around a home before and after a defensible space has been created. An effective defensible space can reduce the threat to your house from embers, direct flame contact, and radiant energy.
