Importance of Defensible Space

Jason Weber, Chief, Fire Department

By Fire Marshal Scott Alber

Study after study has shown that the solution to disastrous home loss during wildfires is to prevent buildings from igniting while the wildfire burns surrounding vegetation. This can be accomplished by focusing on two hazard mitigation measures:

  • Reduce the wildfire's intensity and its exposure time to the structure by vegetation management immediately surrounding the structure.
  • Reduce the ignitability of the structure, particularly from wind-blown embers and small flames.

The wildland-urban interface fire home-loss problem can be dealt with independently from other problems associated with wildfires. The historic role of forest fire suppression, past and present forest management practices and prescribed burning are usually not directly related to the extent of building destruction on interface fires. This was clearly demonstrated by the East Bay hills conflagrations (the 1923 Berkeley fire, the 1970 Oakland Hills fire, 1980 Wildcat Canyon Fire in Berkeley and the 1991 Oakland Hills fire), where small vegetation fires quickly spread to so many readily ignitable homes that the fire suppression capabilities of the local fire departments were temporarily overwhelmed.

As mentioned above, what is always directly related to house survival during interface fires is the ignitability of the building and/or the vegetation management immediately surrounding the home. No scientifically based study has ever been published concluding or suggesting that burning vegetation (or related "fire intensity") around a home is not related to interface fire building survival.

The Black Tiger Fire Report is one of many studies supporting this conclusion.  The report states (in part): "Homes in the wildlands that are at special risk usually share several dangerous traits." The first of these dangerous traits was "Combustible vegetation - an approaching fire will ignite surrounding vegetation in a step-by-step attack on a home. A safety zone of low fuel density all around the home offers important protection."

We can reduce a wildfire's intensity and its exposure time to homes by creating landscapes with healthy well-spaced fire-resistant plants and nonflammable physical barriers such as paths and rock walls. We can also reduce the ignitability of homes, particularly from wind-blown embers and small flames, by using fire-resistant siding with screened openings and Class A roofing (watch the video on FIRESafe MARIN).

Firefighters who battle urban wildfires, such as the Angora Fire in South Lake Tahoe, are often forced to choose which houses can be safely and effectively defended from fire. By reducing the amount of fuel on your property, you increase the likelihood that a fire engine could be safely parked in your driveway and that firefighters could work there to protect your home.

No amount of preparation will guarantee survival of a home in a wildfire. Some firestorms, such as the 2003 Cedar Fire in San Diego County, are so intense that many fire prevention measures become ineffective. But for the vast majority of our fires, landscaping with fire resistive plants and materials increases the chance your house will survive the next wildfire in Marin.

Scott Alber is a Battalion Chief and Fire Marshal for Marin County Fire Department. 

Learn More

Information on creating defensible space around homes and planting fire-resistant landscaping is available at FIRESafe MARIN. The Web site also contains information on many fire-related topics, including how to make children more fire smart. Call (415) 570-4376.