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The basics of defensible space and the “home ignition zone”
The concept of the home ignition zone was developed by USDA Forest Service fire scientist Jack Cohen in the late 1990s, following some breakthrough experimental research into how homes ignite due to the effects of radiant heat. For more than 15 years, NFPA’s wildfire safety recommendations have been shaped by this fire science and because of it, is able to provide actionable guidance for homeowners to help them prepare homes/home landscapes to resist wildfire.
Using the Zone Concept
The primary goal for Firewise landscaping is fuel reduction — limiting the amount of flammable vegetation and materials surrounding the home and increasing the moisture content of remaining vegetation. The home itself and everything around it up to 100 – 200 feet is known as the ‘home ignition zone.’ In areas across the country where the risk of wildfire is high, the home ignition zone extends up to 200 feet beyond the actual home structure. Within this 200 foot area, there are three zones:
Zone 1 encircles the structure and all its attachments (wooden decks, fences, and boardwalks) for at least 30 feet on all sides. Note: the 30-foot number comes from the very minimum distance, on flat ground, that a wood wall can be separated from the radiant heat of large flames without igniting. In this area:
Plants should be carefully spaced, low-growing and free of resins, oils and waxes that burn easily.
Mow the lawn regularly. Prune trees up six to ten feet from the ground.
Space conifer trees 30 feet between crowns. Trim back trees that overhang the house.
Create a ‘fire-free’ area within five feet of the home, using non-flammable landscaping materials and/or high-moisture-content annuals and perennials.
Remove dead vegetation from under deck and within 10 feet of house.
Consider fire-resistant material for patio furniture, swing sets, etc.
Remove firewood stacks and propane tanks; they should not be located in this zone.
Water plants, trees and mulch regularly.
Consider xeriscaping if you are affected by water-use restrictions.
Zone 2 is 30 to 100 feet from the home, and plants in this zone should be low-growing, well irrigated and less flammable. In this area:
Leave 30 feet between clusters of two to three trees, or 20 feet between individual trees.
Encourage a mixture of deciduous and coniferous trees.
Create ‘fuel breaks’, like driveways, gravel walkways and lawns.
Prune trees up six to ten feet from the ground.
Zone 3 is 100 to 200 feet from the home and this area should be thinned, although less space is required than in Zone 2. NOTE: Because of other factors such as topography, the recommended distances to mitigate for radiant heat exposure actually extend between 100 to 200 feet from the home – on a site-specific basis. In this area:
Remove smaller conifers that are growing between taller trees. Remove heavy accumulation of woody debris.
Reduce the density of tall trees so canopies are not touching.
Property owners need to address the "little things" first: NFPA advises property owners to start with the house and work their way out. Having a nonflammable roof covering and assembly adds an enormous safety measure. Keeping roofs and gutters clean and clear of leaves or needles is critical to minimizing ignition from embers. Flammable attachments (e.g., untreated wooden decks) are very vulnerable to ignition and can carry fire to the main structure. Keep flat surfaces clear of debris. Clean out any leaves, needles or stored material that could burn from under decks or porches. During this high fire danger season, remove large potential heat sources such as piles of firewood, spare building materials, vehicles - anything that could catch embers or ignite by flames in the grass needs to be as far away from dwellings as possible. Download NFPA’s
Firewise Tips Checklist for Homeowners that includes these and other actionable steps residents can start working on today.
Fuel: Remove fuel sources close to the house: The perimeter of the home and attachments out to about 5 feet is vulnerable if there is anything there - organic mulch, woody shrubs and plants, juniper bushes - that could ignite and thus allow flames to touch the house. Wind-driven fire will create a blizzard of embers that will pile up in corners where you might normally find accumulations of leaves or needles around your home. These corners, nooks and crannies should be clear of any flammables. If there are any limbs or branches overhanging the roof, or any branches close to/touching the house, trim back to at least 10 feet from the house. Keep grass mowed low and well watered if possible.
Walkway: Larger projects to reduce potential fuel: Our tips for homeowners also cover projects that can be done when fire is not imminent. Download our Firewise Tips Checklist for Homeowners that includes tips for landscaping to create space between trees, removing heavy accumulations of brush or trees out to 100-200 feet depending on slope and topography (because radiant heat ALSO causes homes to burn), creating a low-water (xeriscape) landscape, adding hardscape (rock or concrete patios, walkways, etc.) to break up the path of flames, screening vents or openings with fine metal mesh, and replacing windows with double- or triple-paned alternatives or tempered glass.
Embers and small flames are major culprits:Jack Cohen’s work and further analysis and studies, including experiments sponsored by the insurance industry, show that not only should the radiant heat exposure be mitigated in the home ignition zone, but exposure to embers and surface fire as well. In fact, all the research around home destruction and home survival in wildfires point to embers and small flames as the main way that the majority of homes ignite in wildfires. For that reason, NFPA recommends methods to prepare homes to withstand ember attack and minimize the likelihood of flames or surface fire touching the home or any attachments (fences, decks, porches) as the first place for homeowners to start working to prepare their properties.
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