“Defensible space” is the buzz phrase for the area of fuels modified around a structure to change fire behavior so that firefighters can safely defend that structure.
There are three basic categories of homes (and other structures) during a wildfire: homes that can stand alone needing little to no protection from firefighters; homes that are safe to defend, but are not likely to survive on their own; and homes that are not safe to protect while a fire is coming through. The amazing statistics of no homes burned in the Weber Fire last year or in the West Fork Complex fire so far this year owe greatly to the defensible space created before and during the fires.
Creating defensible space is a great wildfire preparedness goal for homes that were not built and landscaped with wildfire resistance in mind. Defensible space can also be an intermediate step to enable firefighters to directly defend your home during a fire if one threatens while you are still working toward stand alone.
The reason defensible space works, is because wildfire is not like other natural disasters. Fire requires a heat source for ignition and continued combustion, oxygen and fuel. Remove any of these elements, and fires stop burning. Once a fire is started, its behavior will be determined by the fuels, weather and topography it encounters. In determining fire behavior, fuel modification is the only real tool we have.
Fuel modification is the way you create defensible space. Our homes are made of a mix of materials that will burn easily, and materials that are difficult or impossible to burn. To keep your home from burning during a wildfire, the heat that your home is exposed to cannot be strong enough for long enough to ignite the fuel that is your home. As radiant heat dissipates with distance, so too can the fuel modification as you move out from your house.
The first five feet is ideally free of fuels. Anything in this area that can burn should be treated like part of the structure with the defensible space measured from that pet tree, landscaped foundation, or wooden deck, and the hardened area should extend outward from there. Even small breaks using stone walls, walkways, pavers, or bare dirt can make a difference. Think 360 degrees.
Within the first 30 feet of the home, the landscape should be very lean, clean, and green. Minimize the storage of combustible materials such as wood for projects or firewood in this area, especially during the fire season (which is getting longer). A couple of scattered trees are okay, but they should not overhang the home or touch the trees in the second zone of your defensible space. They should be watered, limbed and the leaf or needle litter removed underneath.
In tests, a typical home exposed to radiant heat from a crowning forest fire 50 feet away did not ignite. But homes can withstand intense heat for longer without damage than the firefighters sent to defend them. This is why the second zone for major fuel modification around a home extends 100 feet from the home (including decks and pet trees) on level ground. The State Forest Service recommends 10-15 feet in the ponderosa pine forest, removing the limbs to about 10 feet, and removing the brush beneath the trees.
While removing these ladder fuels is often effective at keeping a fire out of the treetops in healthy pine forests, special consideration should be given in sage flats, oak slopes, and piñon/juniper forests which naturally burn as stand-replacing fires. Creating space between the trees or shrubs, which often requires the removal of grown trees, may be more critical than extensive limbing in keeping these fires from burning through the treetops to the home.
This spacing may look more like 15-20 feet with only a few feet of limbs and the wild plants removed beneath the trees to create separation from any grasses or organic material that will burn on the ground.
Every defensible space looks different, but as long as it does the job of reducing the intensity of a fire along your access road enough that you can drive out and any crowning fire is dropping to the ground about 100 feet out from the home and any firefighters there protecting it, your defensible space is serving its purpose. Find more resources at csfs.colostate.edu or contact yours truly at 564-4007.
Rebecca Samulski is wildfire prevention and education specialist for the Montezuma County Fire Chiefs Association and Montezuma Chapter coordinator for FireWise of Southwest Colorado.