How Wildfire Threatens a House

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, Insurance Intitue for Business & Home Safety, Richburg, SC

Knowing how wildfire threatens a home can effectively reduce the wildfire risk. Wildfire can threaten a home in three ways:

  • Wind-blown embers (firebrands)
  • Contact by flames
  • Radiant heat.

Wind-blown Embers

Embers are the leading cause of home loss during wildfire. Embers are burning pieces of vegetation or construction materials 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 (or low-combustible) 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.

Embers from the fire landing on the wood single roof and vegetation and starting spot fires.

More houses ignite and burn as a result of exposure to wind-blown embers than any other cause. In the case of a wood-shake or shingle roof, embers can result in a direct ignition (i.e., fine fuels such as pine needles won’t be necssary). In other cases, embers will ignite vegetation or other combustible materials located near the home, which in turn will result in either a flame contact or radiant heat exposure to the home. The ignition of vegetaion or buildings will create even more wind-blown embers.

The importance of avoiding a direct flame contact exposure to your home cannot overstated. An effective vegetation management plan in creating defensible space around your home and property will reduce the chance of a direct flame contact exposure coming from the wildfire itself. Creating a noncombustible zone within 5 feet of your home, removing debris from your gutters and roof, and avoiding storage of combustible items under your deck or next to your home will minimize the chance for ignition and subsequent flame contact exposure from wind-blown embers.

Contact by Flames

Contact by flames occurs when a fire is burning close enough to allow flames to touch an object. Direct contact by flames will heat the building materials of your home and, depending on the exposure (i.e., the time and intensity of the flames), combustible materials can ignite or, in the case of window glass, 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 from the wildfire is probably what most homeowners visualize when they think of a house burning during wildfire.

Burning trees and shrubs resulting in direct flame contact of the siding and roofing on a house.

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 the glass in windows, allowing wind-blown 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 flames and embers. Consequently, even vegetation and other fuels located away from the house can pose a threat.

Radiant energy from burning trees being transferred to the side of this home.

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.

Before defensible space actions are taken.

After defensible space actions are taken.