The Home Ignition Zone Explained for Montana Homeowners

What is the home ignition zone?

The Colorado State Forest Service defines the home ignition zone, or HIZ, as the home and the area around it. Its guide explains that the two primary determinants of a home's ability to survive a wildfire are the structure's ignitability and the quality of the defensible space surrounding it, and that together those two factors make up the HIZ. In other words, the HIZ is not just the yard. The roof, gutters, vents, deck and siding are part of it, and so is everything within reach of them.

Montana DNRC uses the same framing. Its Reduce Your Wildfire Risk page points landowners to a home assessment video for actions they can take in the home ignition zone, and its west-central Montana landowner guide lists HIZ actions starting at the walls: a 5-foot area free of anything that will burn, a clean roof and gutters, and no branches over the roof.

Where did the home ignition zone idea come from?

From Forest Service fire research. The best-known statement of it is a 2000 Journal of Forestry paper by Forest Service fire scientist Jack Cohen, which brought together structure ignition modeling, crown fire experiments that placed wooden wall sections near burning forest plots, and case studies of California fires that destroyed hundreds of homes. All three pointed the same way: home ignitability during wildland fires depends on the characteristics of the home and its immediate surroundings.

He concluded that home ignitions are not likely unless flames and firebrand ignitions occur within 40 meters, about 130 feet, of the structure. He also drew the policy conclusion that matters most to owners: because ignition is decided at that scale, homeowners in and next to the wildland fire environment must take primary responsibility for their homes' ignitability, with fire services acting as partners. How the three ignition pathways work is covered in our guide to how homes catch fire in a wildfire.

How far does the home ignition zone extend?

The commonly used figure is 100 feet on flat ground. The CSFS guide divides the space around a home into three management zones: 0 to 5 feet, 5 to 30 feet, and 30 to 100 feet, and says the outer zone extends 100 feet from the home on relatively flat ground. It adds that on steep slopes or with certain topography this zone may be larger, and that if 100 feet crosses a property line, owners are encouraged to work with neighbors.

Slope changes the numbers substantially. An older Colorado State University Extension fact sheet on defensible zones gives the example of a home on a 20 percent slope, where the minimum defensible space would be 90 feet uphill and to the sides of the home and 104 feet downhill. Fire runs uphill, so a house above a slope needs more room on the downhill side. Our guide to slope, aspect and wind explains why.

Where do you measure the zones from?

From the outside of the building, including anything attached to it. The CSU fact sheet measures its innermost zone from the outside edge of the home's eaves and any attached structures, such as decks. The CSFS guide says to develop zones around each building on the property, including detached garages, storage buildings and barns, and treats fencing that attaches to the home as a direct extension of the structure.

This is where many plans go wrong. A deck that sticks out 12 feet moves every zone out 12 feet. A wood fence that runs from the corner of the house to the shop means the fence is part of the house for fire purposes. A propane tank, a woodshed and a trailer parked against a wall are all inside the zone and have to be planned for. Our guides on sheds and outbuildings and propane tank clearance cover those.

What happens in each part of the home ignition zone?

Each zone has a different job. The CSFS guide describes the area nearest the home, 0 to 5 feet, as requiring the most vigilant work to reduce or eliminate ember ignition and direct flame contact. The 5 to 30 foot zone is designed to minimize a fire's intensity and its ability to spread while significantly reducing the chance the structure ignites from radiant heat. The 30 to 100 foot zone focuses on keeping fire on the ground and getting fire that is active in the crowns to drop back to the surface, where it is less intense.

That gives a simple way to read your property. Close in, the question is: what could an ember light? In the middle, the question is: what could burn hot enough, long enough, near the house? Farther out, the question is: could fire climb into the trees and run through the crowns toward the house? The full worklist for each band is in defensible space zones for western Montana, and the first band has its own guide, Zone 0: the first five feet.

Is the home ignition zone the same as defensible space?

Not quite. CSFS defines defensible space as the area around a home that has been modified to reduce fire hazard by creating space between potential fuel sources. The home ignition zone includes that space plus the structure itself. A property can have well-thinned trees out to 100 feet and still be highly ignitable because of needle-filled gutters, open vents and a stack of firewood on the deck. It can also have a well-hardened house with a dense thicket of young fir against it. Both halves count.

Defensible space also has a second purpose that the HIZ concept does not stress. CSFS notes that firefighters may not be present at your home during a wildfire, because they protect structures only when it is safe, and that defensible space gives them room to work when they are there. In 2000, the Bitterroot National Forest reported that more than 1,700 homes in the valley were successfully protected during the fires of that summer, while 70 homes were destroyed.

Is the home ignition zone the same as the WUI?

No, and the difference matters. The wildland-urban interface is a map concept: Montana law defines it as the line, area or zone where structures and other human development meet or intermingle with undeveloped wildland or vegetative fuels, and counties map it in their wildfire plans. Cohen argued that the threat to homes is not a function of where a home sits relative to wildlands but of how it ignites, and that home ignitability, at the scale of the home and its surroundings within 40 meters, should be the principal mapping characteristic.

So two neighbors inside the same WUI boundary can have very different risks. Being in the WUI tells you fire can reach your area. The home ignition zone tells you whether your house will catch when it does. Our guide to the wildland-urban interface in Missoula and Ravalli counties explains how each county draws its map.

Why does a neighbor's home ignition zone affect mine?

Because the zones overlap wherever houses are less than about 200 feet apart, and a burning house is a much larger heat source than a shrub. NIST's research on sheds near homes found that even small sheds needed at least 10 to 15 feet of separation in its tests, and noted that some WUI zones currently permit homes as close together as 6 feet. Its lead researcher said that to stop fire running through a community, fuel spacing has to be considered at the community level, not just property by property.

That is the logic behind neighborhood programs such as Firewise USA, and behind CSFS's advice to work with adjoining owners when the 100-foot zone crosses a property line. On smaller subdivision lots, coordinating with the next lot may be the only way to finish the outer zone.

How do you start working on your home ignition zone?

Start at the house and work outward, which is the order CSFS recommends and the order the physics supports. Get a free assessment first if you can. Montana DNRC offers home risk assessments through its forestry pages, and our guide to free wildfire risk assessments in western Montana explains the options. An assessment will usually tell you that the job is smaller and more specific than you feared.

Then split the work by who should do it. Structure items such as roofs, vents and windows belong with a building contractor. Vegetation, trees and cleanup are what HJ Property Care does through our fire mitigation service: clearing needles and debris, removing ladder fuels, limbing to arborist standards and selective thinning across Missoula and Ravalli counties. This guide is part of our Montana Wildfire Guide.

Call HJ Property Care & Tree Service at (406) 215-7319 or request a free estimate and we will walk the property with you.

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