What is Defensible Space?
Defensible space refers to the area surrounding a building that is mitigated to protect it from wildfires. This space acts as a barrier between a structure and an advancing fire. Along with the quality of a building’s roofing material, adequate defensible space is one of the most important factors in determining a building’s ability to survive a wildfire. Much was learned from the 2003 San Diego Wildfires.
Defensible space performs the following functions:
- Ideally, a carefully maintained defensible space will not contain enough fuel to allow a wildfire to reach a house. Even if the space is breached, the fire will have been slowed and weakened, helping firefighters to defend the house.
- A defensible space provides an accessible area for fire trucks to park and firefighters to work during a structure fire.
- If there is a pond or pool near a burning house, it can be used to replenish a fire truck’s water supply. The perimeter of the pond should be thinned of trees and brush sufficiently so that firefighters can access it.
Defensible Space Requirements:
The size requirements for defensible space vary by jurisdiction because the potential for wildfires varies by region. Buildings in forested areas of the Southwest need a much larger protective space than in Minnesota for instance. As of January 2005, California state law mandates a minimum of 100 feet of defensible space for houses in rural locations. Your local fire agency may require you to clear additional vegetation by a written letter. Trees and shrubs surrounding a house should be trimmed and spaced apart a safe distance from one another. Chainsaws can be used to remove trees and branches, pruning shears to trim plants, and rakes for removing pine needles and other ground-level combustibles. Trees that are very close to the house should be removed because this is where fire-prevention is most critical. Any trees you remove shall have the stumps cut no higher than 8 inches above the ground. The only exception would be an orchard. Orchard trees may have their stumps completely removed. Vegetation can be plentiful towards the perimeter of the space if it is green and pruned.
Cal Fire divides defensible space into two zones in the following manner:
Zone 1: The first 30* feet from a home should be devoid of all flammable vegetation. Firewood and other flammable materials should not be stored in this region. Relocate wood piles to Zone 2.
Zone 2: This area of fuel reduction should extend from Zone 1 outward to 100 feet from the structure. Cut or mow annual grass down to a maximum height of 4 inches. Create horizontal spacing between shrubs and trees (See diagram). Create vertical spacing between grass, shrubs and trees. Remove fallen leaves, needles, twigs, bark, cones, and small branches. However, they may be permitted to a depth of 3 inches
*San Diego County requires 50 feet of clearance in Zone 1.
Check with your local fire department for any additional defensible space or weed abatement ordinances.
Precautions That Homeowners Should Be Aware of When Creating Defensible Space:
- Homeowners should obey all environmental protection laws while creating and maintaining defensible spaces. In particular, removal of vegetation should not interfere with the well-being of endangered species, air and water quality, or archaeologically significant resources. Homeowners may need to obtain a permit to cut down trees over a certain size, depending on local jurisdictions.
- Vegetation removal can cause soil erosion, especially in steep terrain. Your home inspector advises that in areas that are prone to wildfire and soil erosion, it can be helpful to replace highly flammable plants and trees with less-flammable alternatives.
Summary of Defensible Space for San Diego
In summary, buildings can be spared from wildfire damage through the removal of surrounding flammable vegetation. Defensible spaces are critical in hot, dry, forested regions, although their presence is recommended everywhere. More detailed information can be found in this Pdf available on the Cal Fire website . A Homeowner Checklist Pdf for fire safety can also be found on the Cal Fire website