How Topography Influences Wildfire Risk

Fire runs uphill. The slope of your land and the direction your home faces are major factors in your wildfire exposure, and homes on or above steep slopes need more defensible space than the standard minimums.

Why Slope Drives Fire Behavior

Wildfires burn faster and more intensely up a slope than across flat ground. Flames lean into the hill, preheating the vegetation above them, and the steeper the slope, the faster the spread and the longer the flame lengths. A home at the top of a vegetated slope sits in a chimney: it receives the fire's most intense run, plus the convective heat and embers the slope funnels upward. In San Mateo County, where homes line ridgetops from Skyline to the coastside canyons, slope is often the difference between a survivable exposure and an extreme one.

A Quick Field Test for Steepness

1. Pick a mark on the slope and walk 10 paces downhill.
2. Look back. If your head is below the mark, you have a steep slope.

Protecting Homes on or Above Slopes

If your home sits less than 15 feet (single story) or 30 feet (two story) back from a slope, take additional precautions:

• Be more aggressive with vegetation modification and maintenance on the downslope side, and pay closer attention to the materials in your building, deck, and outbuildings. Decks overhanging slopes are a particular vulnerability; see fire-resistant decks.
• Extend fuel modification beyond the standard 100 feet where possible. On steep slopes, 150 to 200 feet is a reasonable target, subject to property lines and permission.
• Consider a noncombustible retaining wall to increase the effective setback, and incorporate ignition-resistant materials into future improvements.