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Wildfire Defensible Space Calculator

Calculate defensible space zones, clearing areas, and estimated costs per CAL FIRE PRC 4291 and NFPA Firewise USA guidelines. Includes state-specific compliance distances and slope adjustments.

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The math, shown transparently

No black box. Here is exactly what the calculator computes for each zone. (Sources: CAL FIRE PRC 4291, NFPA Firewise USA, USDA Forest Service, IBHS research)

// For each zone (0, 1, 2):

outer_side = structure_side + 2 x outer_distance_ft

inner_side = structure_side + 2 x inner_distance_ft

area_sqft = outer_side^2 - inner_side^2

area_acres = area_sqft / 43,560

// Zone cost adjusted for vegetation and slope

zone_cost = area_acres x per_acre_rate x veg_multiplier x slope_multiplier

Worked example

1,500 sqft house (38.7 ft side), moderate vegetation, moderate slope, California:

// Zone 0 (0-5 ft): ember-resistant zone

area = 875 sqft, cost = $265

// Zone 1 (5-30 ft): lean, clean, green zone

area = 7,373 sqft, cost = $559

// Zone 2 (30-100 ft): reduced fuel zone

area = 47,254 sqft, cost = $1,790

total = $265 + $559 + $1,790 = $2,614 for 1.27 acres

How defensible space zones work

CAL FIRE and NFPA Firewise divide the area around a structure into three zones. Zone 0 (0-5 feet) is the ember-resistant zone. Nothing combustible within 5 feet of the structure. This means no mulch, no plants, no wooden fences touching the house.

Zone 1 (5-30 feet) is the lean, clean, green zone. Vegetation is spaced and maintained so fire cannot easily reach the structure. Trees are pruned up 6-10 feet from the ground. Shrubs are spaced apart so they do not create a continuous fuel bed.

Zone 2 (30-100 feet) is the reduced fuel zone. Trees are spaced and dead material is cleared to slow fire spread. The goal is to reduce fire intensity so firefighters can safely defend the structure.

Steep slopes extend the required distances because fire travels faster uphill. California, Colorado, Oregon, and Montana all have specific distance requirements written into state law. Non-compliance can void your insurance or result in fines after a fire.

Frequently asked questions

What is defensible space?

Defensible space is the area around a structure where vegetation and combustible materials have been cleared or reduced to slow the spread of wildfire. It is typically divided into three zones: Zone 0 (0-5 feet, ember-resistant hardscape), Zone 1 (5-30 feet, lean/clean/green landscaping), and Zone 2 (30-100 feet, reduced fuel). These zones are defined by NFPA Firewise USA and enforced by state regulations like California PRC 4291.

How much does defensible space clearing cost?

Defensible space clearing costs vary by zone and vegetation density. Zone 0 (hardscaping the first 5 feet) runs $500-$1,500 for a typical home. Zone 1 clearing costs $1,000-$3,000 per acre. Zone 2 fuel reduction costs $800-$2,000 per acre. For a 2,000 sq ft home on a 1-acre lot with moderate vegetation, total defensible space clearing runs $2,000-$6,000.

Is defensible space legally required?

It depends on your state and location. California requires 100 feet of defensible space under PRC 4291 for homes in wildfire-prone areas, with a new Zone 0 requirement added in 2023. Colorado SB 21-049 requires compliance in wildland-urban interface areas. Oregon SB 762 has similar requirements. Even in states without legal mandates, insurance companies increasingly require defensible space for coverage in fire-prone zones.

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