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Where to build on hilly land

Where a home sits on the land matters as much as what it is made of. A fire coming up a brushy hill moves faster and burns hotter than the same fire on flat ground, and some spots on a hillside pull fire and heat straight toward the house.

Why fire runs uphill

Why fire climbs a hill faster than it crosses flat ground Left: on flat ground, the flames stand up and most of their heat rises into the air, so the fire moves slowly. Right: on a hillside, the flames lean into the plants uphill, drying and heating them before the fire arrives, so the fire runs up the hill much faster. A home at the top of the slope is in its path. Flat ground: slower Most of the heat rises into the air Uphill: much faster Flames lean in and dry the plants above
On a slope, the flames lean into the plants above them and dry them out before the fire gets there.

Heat rises. On flat ground, most of a fire's heat goes up into the air. On a hillside, the plants above the fire are in the way of that heat. The flames lean into them, dry them out and warm them up, so they catch fire faster, and the fire speeds up the hill.

The steeper the hill, the faster it goes. The rule of thumb wildland firefighters are taught: when a slope gets three times steeper, a fire spreads about twice as fast, and three times steeper again makes it four to six times as fast.

That is why a home at the top of a brushy slope, or partway up one, is in the path of the fastest, hottest part of a fire, and why the zones around a home on a slope should reach farther down the hill.

Safer and riskier places to build

Safer and riskier places to build on hilly land A side view of land: a flat valley, a brushy slope, a flat hilltop and a wide dip (a saddle) in the ridge. Numbered homes: 1, on flat ground with the area around it cleared, is safer. 2, partway up the brushy slope, is at risk. 3, right at the top edge of the slope, is at risk. 4, set well back from the edge on the flat hilltop, is safer. 5, down in the saddle, is at risk because wind and fire are funneled through it. Set back from the edge 1 2 3 4 5 SaferAt risk
Green numbers are safer spots, red numbers riskier ones.
  1. Flat ground, with the area around it cleared. Fire moves slowest here, and there is room for the three zones all the way around.
  2. Partway up a brushy slope. Fire coming up the hill reaches the house at full speed, with fuel below it.
  3. Right at the edge of a slope or ridge. The flames and heat from the whole slope rise into the house.
  4. Set back from the edge. Building back from the top of the slope gives the fire's heat room to rise before it reaches the house. FEMA's guidance says to leave at least 50 feet from the wild plants on the downhill side, and more where the brush is thick.
  5. In a narrow draw, gully, canyon or saddle. These low notches work like a chimney: they funnel wind, heat and flames through them.

Building or buying on a hill

Sources

More about preparing for wildfire