Beverly B. Voelkelt
Defining Defensible Space
Significant wildfire hazards exist in most wildland urban interface / intermix communities. Thus it is important to implement reasonable defensible space around homes, structures and communities in order to mitigate these hazards. Doing so greatly helps a home or structure survive a wildfire and also reduces the risk of a structure fire igniting the forest. Defensible space makes it easier for firefighters to determine which homes can be defended (triage) and helps increase safety for firefighters while they protect that home or structure. Furthermore, defensible space lessens the impact of wildfire on a community by reducing total damages and losses to that community.
The Colorado State Forest Service defines defensible space as an area around a home or structure, which is either man-made or natural where the vegetation is modified and maintained to slow the rate and intensity of an advancing wildland fire. It also provides room for firefighters to work and helps protect the forest from becoming involved should a structure fire occur.
The Auburn Fire Department in California defines defensible space as an area within the perimeter of a parcel, development, neighborhood or community where basic wildland fire protection practices and measures are implemented, providing the key point of defense from an approaching wildfire or defense against encroaching wildfires or escaping structures fires. The perimeter, as used in this definition, is the area encompassing the parcel or parcels proposed for construction and or development, excluding the physical structure itself. The establishment and maintenance of emergency vehicle access, emergency water reserves, street names and building identification, and fuel modification measures that characterize the area.
Note: This sensible definition goes beyond vegetation management to include requirements for defending a community or a section within, which is similar to Area Risk Mitigation (A.R.M.) also mentioned in this website.
Regardless, it is very difficult to apply a universally applicable definition or standard to what constitutes reasonable or adequate defensible space for a home or structure. Too many varying factors enter the question of what constitutes reasonable defensible space. These factors include topography (flatland or slope), climate (dry or wet), weather conditions (hot or cool, windy Santa Ana conditions or the absence of winds) and vegetation types (plants retaining high moisture, hard woods, soft woods or plants that are extremely volatile and combustible). Furthermore, the way a home is constructed (flame resistant materials like roofing, siding and windows, and fire wise architecture) and access to the home (can fire engines get to and from the target safely) also determine whether or not a home can be defended.
The creation of defensible space requires the review and modification of vegetation within a certain perimeter around a house or structure. CAL FIRE, the firefighting division of the California Department of Forestry and Fire Protection, and California legislation (Public Resource Code 4291) call for the creation of 100 feet of defensible space or to the property line. In some cases, and only after review by a CAL FIRE authorized fire expert, this zone may be extended beyond the 100 feet definition. This 100 feet space can be further broken down into three zones … the Home Ignition Zone (the first 10 to 15 feet), the Lean Space Zone (extending to 30 feet from the home or structure), and the Reduced Fuel Zone (extending from 30 feet to 100 feet or the property line).
Note: Some wildfire researchers, like Jack D. Cohen with the U.S.F.S., consider the area of 30 to 45 feet around a home or structure the Home Ignition Zone. This may indeed be a prudent space definition, especially when the home or structure is located on a steep slope. In this case the fuel modification effort below the home or structure needs to focus on wider spacing of individual plants to prevent the fire from gaining intensity.
The creation of reasonable and adequate defensible space focuses on measures to modify or breaking up the hazards emanating from the continuity of fire fuels, both horizontal (across the ground) and vertical (from the ground up into the crowns of brush and trees). Fuels that exhibit a large degree of both vertical and horizontal continuity are the most hazardous; in particular when they are on slopes. Heavier fuels (brush and trees) are more hazardous and likely to produce a more intense fire than light, flashy fuels such as grass. Thus, mitigation of wildfire hazards within the defensible space perimeter focuses on breaking up the continuity of horizontal and vertical fuels, while also addressing environmental concerns like habitat and watershed protection. It does not require the clear cutting of trees and brush.
Furthermore, defensible space discussions must include a review of entire neighborhoods, as pointed out in the requirements by the Auburn fire department. This is especially true for communities in which homes are build close together and thus creating overlapping home ignition zones. Defensible space legislation typically requires the creation of defensible space only to the property line. This implies that a home with adequate defensible space might still be at risk if a neighbor’s property does not include defensible space; or, as the saying goes - a chain is only as strong as its weakest link. This problem has been recognized and many regional codes now require fuel modifications also on vacant, unimproved lots. Furthermore, the owner of a property without defensible space can be held liable if it can be demonstrated that his property caused flames to spread to a neighbor's home or structure and caused damage or total loss.
Fire science, in both theoretical and field research, has determined that defensible is the single most valuable factor in helping to protect a home or structure from being impacted by wildfire. Thus, the home or property owner's effort to create defensible space is the first line of defense when a wildfire approaches. Please also see the section on "Wildfire Science".
It is not necessary to strip an area around a home or structure of all vegetation to create defensible space. Rather it is important to break up horizontal and vertical continuity of fuels. In addition it is important to reduce the vulnerability of the home or structure, like replacing wood shake shingle roofs, using fire resistant siding, tempered glass windows and other measures.
Neighborhoods and communities must focus on creating defensible space collectively and also address factors of Area Risk Mitigation (A.R.M.), safe passage on roads, access roads or driveways, sufficient availability of water, etc. to make properties and the community defensible during a wildfire.
While it is important to establish basic guidelines for the creation of defensible space and vegetation management on a property, one must look at each property in a holistic manner, that is evaluate all factors that contribute to a fire's behavior and intensity, in order to implement the best possible solution for creating defensible space.
This image of Defensible Space around a single home highlights the concept of creating three distinct zones to protect a home.
Image courtesy of CalMast.org -
San Bernardino County
This image of Defensible Space around multiple homes highlights the concept of overlapping zones to create more fire safe neighborhoods and communities.
Image courtesy of CalMast.org -
San Bernardino County