What Makes a Home More (or Less) Likely to Burn in a Wildfire?

home wildfires


Wildfires are an inevitable environmental concern when living in an area with high temperatures and low humidity. While they are a powerful, destructive force that can engulf entire communities, your property does not have to go up in flames. What causes one home to burn while a neighbor’s home remains largely undisturbed? Read on to learn about safeguarding your property from wildfires.

The wildness of fire

The U.S. Forest Service spends roughly $2 billion each year in preventing and suppressing wildfires, more than half its federal budget, but even with that level of commitment, an average of 1,300 homes burn yearly. As the demand for housing expands, particularly in dry, warm areas, understanding the complexities of wildfires is of paramount importance to urban planners in developing safe communities and for property managers looking to invest in the right place.

Unfortunately, no one algorithm exists to predict how well a home will withstand a wildfire. The variables affecting the likelihood that a home burns include topography, vegetation conditions, and housing density, among others, but a pattern emerging in one area may not apply for another.

A study of two fires—the Cedar Fire northeast of San Diego, California, in 2003 and the Four Mile Fire west of Boulder, Colorado, in 2010—exemplifies the difficulty in pinning down the perfect conditions for structural integrity in a home during a wildfire. In the Four Mile Fire, the homes most likely to burn were located on slopes and ridges and were often on the outskirts of neighborhoods, but these trends did not hold for the Cedar Fire.

Research performed by Colorado State University has determined two major factors in determining whether a home will withstand the destructive force of a wildfire: defensible space and structural ignitability. The former refers to the area around the home that has been modified to reduce the spread of wildfire, and the latter refers to the materials that comprise a home and their ability (or lack thereof) to catch and spread fire. Combined, they create a measure known as the Home Ignition Zone (HIZ).

Defend your space

Defensible space exists in three zones. The first extends from the home in a roughly 15-to-30-foot radius. The second extends beyond the first zone up to 100 feet around the home, and the third zone expands upon the second up to the property line. Property owners should be aware of dry brush, branches, and grasses in the yard and clear them from their first zone, keeping any lawn at a height of six inches or less. Any firewood should be stacked in in the second zone and uphill or at the same elevation of the home.

Within the second zone, the most important task to undertake is the removal of fuels, or sources for fire to spread. Vertical, or ladder, fuels are those such as shrubs and short trees that allow a fire to spread from the ground into the tree canopy; these should be pruned up to ten feet from the ground. Surface fuels, including grass, dry pine or evergreen needles, logs, branches, wood chips, and slashed branches and brush, spread fire pervasively. Rake up and properly dispose these fuels.

The third zone requires less maintenance as it is largely indistinguishable from wilderness, but you might consider clearing firefighter paths of branches and brush. A further benefit of creating defensible space as described above is that any structural fire will be less likely to spread to the surrounding forest or any other homes.

Ignite watch

The second half of the HIZ, structural ignitability, is best examined during the design and build stage of a home, but regardless of whether the house is new or decades old, property owners can reduce ignitability in several key areas.

A thorough survey of home construction and topography is essential to protecting a home. The roof, because of its huge surface area, has a great impact on ignitability; asphalt or slate shingles, (clay) tile, concrete, metal sheets, and other fire-rated Class A materials are widely preferred to wooden or shake shingles. Be sure eaves, soffit, and fascia are all ignition-resistant, and vents and other openings should be designed to prevent embers from entering the house. These vents should be covered with metal no thicker than 1/8”.

Explore the landscape surrounding the home: is the home near hillsides or on steep slopes? Does the home have fences, awnings, or trellises attached, and are they made of wood or another material? Conventional wooden decks are also highly combustible, so consider using safer building materials, or isolate the deck with a patio or wall. A list of noncombustible or fire-safe building materials, as well as building techniques that can reduce ignition potential, can be found here.

Snuffing out hazards

Property managers should take proactive steps to reduce the likelihood of wildfires destroying their property, including cleaning leaves from gutters and replacing loose or missing shingles. Wherever possible, property owners should remove items stored under decks or porches and any flammable items – like propane tanks and dead or dry vegetation – to beyond the first zone of defensible space. Replacing mulch with rock, gravel, or stones is a wise choice. Use indoor, closeable shutters for large glass windows—radiant heat can ignite sheer curtains straight through a window!

For further protection, property managersshould ensure that they have smoke detectors and fire extinguishers on every floor of the house and that the extinguishers are regularly serviced. Make sure your house number and last name are clearly visible from the street and marked in a non-flammable material. Firefighters will need to find homes in smoky and compromised conditions, and they must be able to drive their trucks up the driveway. If your driveway is greater than 150 feet in length, it should be 20 feet wide and clear of branches, preferably with a turnaround of a 30-foot radius near the home (for driveways shorter than 150 feet, trucks can reach the home from the street).

Other best practices for reducing fire hazards include: storing any gasoline, kerosene, oily rags, and other flammable materials in approved cans inside a garage or fire-resistant structure; checking that your motorized garden equipment (lawnmowers, weed whackers, chainsaws, etc.) has functioning spark arrestors; disposing of ashes only when they are cold to the touch; only mowing the lawn when humidity is high and temperature is low, as a spark from striking a rock can ignite a lawn-based fire; using fire-safe pits for recreational fires; and composting where possible in lieu of burning dry vegetation and debris.


Shannan Mills is the division manager for the National Fire Fighter Wildland Corp. Mills focuses on helping the company serve the wildland firefighters who protect lands and homes.

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