Facing Fire

To understand Firewise building and developing ideas, it helps to take a lesson from Forest Service research physicist Jack Cohen. A former wilderness firefighter himself, Cohen has spent many years examining why and how wildfires destroy houses.

From a forester's perspective, says Cohen, it's a mistake to put out every wildfire. Fire is a fact of nature that helps create the natural landscape we love, he explains; and in the long run, wildfire can't be prevented. But that doesn't mean we need to let our houses burn, too. Speaking like the scientist he is, Cohen argues, “This is simply a combustion ignition problem. Ignition requires three things: fuel, oxygen, and heat. If the house and its surroundings meet those requirements for combustion, then it ignites and burns. And if it doesn't meet those requirements for combustion, it doesn't burn. And in the second case, the interface problem goes away.”

Cohen pioneered the concept of the “home-ignition zone,” which he defines as the house itself, plus its immediate surroundings out to about 100 feet (or in very dangerous terrain, as far as 200 feet). Within that buffer area, he says, it's possible to maintain conditions that will make the house largely immune to ignition from wildfire—allowing foresters to decide how to handle the wildfire itself based on ecological concerns rather than property loss risks.

HOLDING THE LINE: Firefighters scrape away flammable ground debris for a “scratch line” near another house at the Lake Tahoe Angora fire. Once homes catch fire, they become a primary source of the fire's spread through the populated area.

Cohen's research—including test fires on large forest stands as well as extensive post-fire investigations in burned-over neighborhoods—indicates that big, hot “crown fires” aren't the main threat to homes. Those raging-hot blazes in the treetops are fed by fast-burning fine fuels such as needles and twigs, he says, and flame fronts pass by in just minutes. As long as the fire's not too close—no closer than 100 feet, as a rule—the radiant heat subsides too soon to set a wood wall on fire. (Windows are another matter, however. Single-glazed windows may crack and fall out from the heat long before a wall would ignite—that's why codes call for double-glazing.)

So in wildfire country, keeping the woods 100 feet distant from the house provides a good safety zone against the crown fire. “The streets, the houses themselves, driveways, utility easements, all break up the tree canopy,” says Cohen. “It doesn't take very much fragmentation of the canopies to stop the crown fire from spreading at that location. So what I find when I go into a community after a fire is that the crown fires, almost without exception, aren't spreading through the community.”

But that still leaves a second threat: low-intensity, creeping ground fire that approaches houses slowly after the main flame front has passed by. To stop those slowly encroaching flames, Cohen says, property owners have to remove the fuel they feed on. You do that by keeping large trees away from the house at least 30 feet, and thinning the trees and branches to separate tree crowns at least 10 feet apart. “Ladder fuels”—low trees and shrubs that let fire climb from the ground into the trees—have to be removed. Closer to the house, the landscaping should be “lean, clean, and green”—with no shrubs against the building.

NO-IGNITION ZONE: A handful of house and landscape details can make all the difference for homes threatened by wildfire, according to fire science experts. Roof, vent, window, and deck details can toughen a house against flame and embers, while brush clearing and pruning within a 100-foot radius can hold super-hot “crown fires” at bay.

But there's a third way that wildfires attack a home: wind-blown burning embers, or “firebrands.” Says California Fire Marshal Kate Dargan, “We believe the majority of structure ignitions are caused by embers.” On a windy day, these burning twigs can travel as far as a mile from a burning forest, although they might also come from as close as the house next door. Like a blizzard of strike-anywhere matches, the embers pile up in corners and drift under decks. If the spot where they settle out has a fuel source, the embers will start a fire. A broken window will let them into the house where they can ignite curtains or furniture. A low deck with dry leaves under it or a stack of logs against a house gives the firebrands a ready ally for attacking the home. Here is where ignition-resistant building details really come into play: To survive the attack by flying embers, the house needs its Class A roof, its ignition-resistant deck, and its debris-free gutters.

Saving homes from burning requires attention to all three sources of ignition. And that means paying attention to the whole problem: the house, its landscape, and the surrounding area. Says Dargan, “There is a set of responsibilities that travel around. The folks who are approving the site and permitting the structure need to be responsible for making sure that good site design is in place; the builders for constructing the building according to good code; the homeowners for maintaining it; and the firefighters for assessing risk and weighing benefit.”