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Defensible Space

What is Defensible Space?

Defensible space describes the area adjacent to a structure where vegetation and other combustible materials are carefully located and maintained to provide defense from an approaching wildfire burning or to minimize the spread of a structure fire to wildlands or surrounding areas. The purposes of a defensible space are:

  • Minimize fire pathways to the house
  • Reduce potential heat exposures to house components
  • Reduce the potential for embers to ignite materials adjacent to the house

  • Provide a safe place for fire personnel to defend the home and allow evacuation 

     

Diagram of a single-family home surrounded by dense vegetation like shrubs and trees.
Interpretation of a three zone defensible space strategy.

The images above show before (left) and after (right) implementation of a three zone defensible space strategy. While these figures are in a forested setting, the same concepts apply to other vegetation types and more residential settings.

 

It is recommended to start at the house and work outwards giving the greatest effort to avoid combustible vegetation and other materials within five feet of the house and any attached decks. The California Department of Forestry and Fire Protection defines three zones of a defensible space, respectively Zone 0 (0-5 ft), Zone 1 (5-30 ft), and Zone 2 (30-100+ ft). The table below describes the intent behind each zone as well as recommended strategies for implementation.

 

Zone

Purpose

Recommendations

Zone 0 (0-5 ft)

"The Ember Resistant Zone"

Zone 0 can reduce the likelihood of structure ignition by reducing the potential for:

  • Direct ignition of the structure from flame contact
  • Embers that accumulate at the base of a wall
  • Indirect ignitions when embers ignite vegetation or other combustible materials located close to the structure that result in either a radiant heat and/or a direct flame contact exposure to the structure.
  • Remove and dispose of woody vegetation and combustible mulches.
  • Relocate wood piles, planters that contain woody plants or are made of combustible materials, and other flammable material.
  • Consider concrete, gravel, or rock mulch around buildings to keep the area free of vegetation.
  • For fences attached to the building, upgrade the attachment point with a noncombustible panel or gate.
  • Remove tree limbs that overhang structures.
  • Replace natural-fiber doormats or move them inside pre-evacuation order.
  • Relocate garbage and recycling cans pre-evacuation order or store in a non-combustible enclosure (metal or fiber-cement).
Zone 1 (5-30 ft)

"The Lean, Clean, and Green Zone"

Zone 1 emphasizes fuel modification such that there is a disconnection between planting groups and trees so that fire does not travel directly to the structure.
  • Remove and dispose of dead and dying vegetation.
  • Space shrubs two times the height of mature plants, increasing spacing on steeper slopes.
  • Use noncombustible rock mulches and avoiding shredded bark mulch – can incorporate organic soil amendments to increase water holding capacity.
  • Remove limbs of tall trees from 6-10 ft off the ground. 
  • Remove tree branches within 10 ft of a structure.
  • Remove combustible materials from around and under detached decks and overhead structures.
Zone 2 (30-100 ft)

"The Reduced Fuel Zone"

Zone 2 is focused on changing the behavior of fire to help personnel defend a structure safely.  

  • Fire personnel can work effectively with flames 4 feet and less. Equipment such as bulldozers can work near flames up to 8 feet.  For flames greater than that height, indirect methods such as retardant and water drops are the safest approach.
  • Remove and dispose of dead and dying vegetation.
  • Thin trees so all branches or groupings of trees are separated by at least 10 ft.
  • Limb lower branches of taller trees up to a height of at least 10 ft.
  • Mow or graze grasses to a maximum height of 4 inches.
  • Aboveground fuel tanks or exposed woodpiles should be kept on a noncombustible pad and clear of all vegetation.

 

UC ANR publication #8695

If any of your defensible space zones extend into a neighbor's yard, work with them to enhance collective efforts. For larger properties, consider creating a stewardship plan (i.e., Forest Management Plan, Rangeland Management Plan, Oak Woodland Management Plan, etc.) for the property. These plans will help you determine your goals and objectives for that space and what tools and methods you might want to use. For more information about land stewardship and plans, visit the UCCE Forest Stewardship page.

 

First responders access

Road access is crucial for your personal safety as well as those of first responders. Ensure that there is enough space for firefighting equipment to move onto your lot, as close as possible to your home and multiple access points to your parcel. During incidents, power lines or trees falling across roads are not uncommon. Work to develop:

  • Two or more roads in and out of your parcel provides an alternate route in case of emergency. Dead-end roads should have a turnaround as approved by the local fire authority.
  • Each road should be accessible year round and at least 20 feet wide.

  • Road grades should be less than 5% (5 foot rise for each 100-foot distance) are more accessible for larger fire equipment.

     

slopeDistance

Implementing Defensible Space on Steep Slopes

If the home is located on a slope, in a drainage, in a windy area, or an area surrounded by unusually dense, tall, or combustible vegetation, thinning and spacing requirements increase. Additionally, if the home is in a vegetation type that is especially prone to wildfire or has an active fire history, a greater clearance and separation between plants and plant groupings may be beneficial. However, the areas cleared from vegetation should be monitored and maintained to prevent the spread of invasive and flammable grasses. 

When the home is at the top of a slope, keep in mind that fire and heat rise, allowing for pre-heating of the upslope fuels, resulting in the potential for more intense fire behavior. In these cases, greater effort should be directed at the area downslope of the home with even higher levels of fuel treatments given to the area below a deck.

 

Additional Resources

Check out these defensible space-related resources from UC ANR Fire Network Members and our colleagues!