Fire professionals size up a location by a few factors: topography, fuels, weather and construction. The first three describe the setting; construction is covered on the next page.

Topography: heat and flames rise, so they dry and preheat the fuels upslope, and fire moves faster going uphill. When a fire starts at the bottom of a brush-covered slope, a home at the top is more exposed than one at the bottom. Burning debris can roll downhill, but that is a lesser effect than the fire's uphill run.

Fuels: for defensible space purposes, Public Resources Code §4291 defines fuel as any combustible material, including petroleum-based products, cultivated landscape plants, grasses and weeds, and wildland vegetation. Different fuels burn at different rates and intensities, so what surrounds a home and how it is spaced both matter. Planted landscaping counts, not just wild brush.

Weather and hazard zones: fire hazard severity zones identified by the State Fire Marshal rest on fuel loading, slope, fire weather and other relevant factors, including areas where winds are a major cause of wildfire spread (Gov. Code §51178). The zones apply consistent statewide criteria to the severity of the expected fire hazard. Insurance claim history is not a zoning factor.