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Defensible Space

Defensible space is the buffer you create between your home and the grass, trees, shrubs, and wildland around it. It slows or stops the spread of fire toward your home, reduces the chance of ignition from radiant heat and direct flame, and gives firefighters a defensible position when they protect your property.

Home on a San Mateo County hillside surrounded by managed defensible space with low, well-spaced vegetation

San Mateo County's hillsides are an ideal environment for a damaging wildfire. As summer temperatures peak, humidity drops, and grasses and brush cure, the fuels around our communities become ready to burn. The 2020 CZU Lightning Complex destroyed nearly 1,500 structures in San Mateo and Santa Cruz Counties and showed how quickly fire moves through our coastal mountains. Homes are lost two ways in these fires: from wind-driven embers that ignite material on or next to the house, and from fire spreading through connected vegetation and structures. Defensible space addresses both, and it works best when combined with home hardening.

What the law requires: California Public Resources Code 4291 requires 100 feet of defensible space (or to your property line) around buildings in the State Responsibility Area and in designated fire hazard zones. Assembly Bill 3074 added a third zone to state law: Zone 0, an ember-resistant zone within 5 feet of structures. The Board of Forestry and Fire Protection is finalizing Zone 0 regulations now, phasing in first for new construction in Very High Fire Hazard Severity Zones. Local fire agencies in San Mateo County may have stricter requirements, so check with your fire district.

The Three Zones of Defensible Space

Defensible space is organized into three zones, and priority runs from the house outward. Research from post-fire investigations is consistent: conditions in the first few feet around a home matter more than anything else on the parcel, because that is where wind-driven embers collect and ignite material against the house.

Zone 0: 0 to 5 Feet

The ember-resistant zone. The goal is nothing combustible within 5 feet of the house, including under and around attached decks.

  • Use hardscape or noncombustible ground cover such as gravel, pavers, or bare soil. No bark or wood mulch.
  • Remove combustible plants, leaf litter, and debris. Keep the roof and gutters clear.
  • Move firewood, lumber, garbage bins, brooms, and door mats out of this zone.
  • Replace the last 5 feet of any wood fence that attaches to the house with a noncombustible section or gate.
  • Do not store anything under decks or stairs.

Zone 1: 5 to 30 Feet

The lean, clean, and green zone. Keep fuels low, watered, separated, and free of dead material.

  • Remove all dead plants, grass, and weeds.
  • Remove dead leaves and needles from the yard, roof, and gutters.
  • Keep tree branches 10 feet from other trees, your roof, and your chimney.
  • Prune or remove plants near windows and under eaves and vents.
  • Move woodpiles to Zone 2.
  • Separate shrubs, trees, and combustible items such as patio furniture and play structures.

Zone 2: 30 to 100 Feet

The reduced-fuel zone. Break up fuel continuity so fire loses intensity before it approaches the home.

  • Cut or mow annual grasses to a maximum height of 4 inches.
  • Create horizontal spacing between shrubs and trees based on slope and plant height.
  • Create vertical spacing between grass, shrubs, and tree canopies to remove fire ladders.
  • Remove fallen leaves, needles, twigs, and small branches. A layer up to 3 inches deep may remain where needed for erosion control.

Spacing for Slope and Wind

The steeper the slope, the more space you need. Fire moves faster and burns more intensely uphill, and homes on steep or windswept sites need greater separation between plants than the minimums above. To find the vertical clearance needed between a shrub and the lowest tree branches above it, multiply the shrub height by three: a 5-foot shrub needs 15 feet of clearance to the lowest branch. Remove all tree branches within 6 feet of the ground.

Defensible Space Works With Home Hardening

During a wildfire, burning embers can travel a mile or more ahead of the fire front and ignite homes far from the flames. Defensible space reduces the fire's intensity and removes the fuels embers land in, but the house itself must also resist ignition. Vents, roofs, decks, and siding are the common entry points. The two measures function as a system, and a weakness in one undermines the other.

Harden Your Home Against Wildfire Fire-Safe Landscaping

Landscaping Your Defensible Space

Creating defensible space does not mean a ring of bare dirt around your home. With planning, you can have an attractive, water-wise landscape and a fire-safe home. The general pattern: keep trees farthest from the house, shrubs in the middle distance, and low groundcovers or hardscape nearest the home, with nothing combustible in Zone 0. See our fire-safe landscaping guidance, our fire-resistant plant list for San Mateo County, and the fire-prone plants to avoid.

Check your local requirements. Defensible space in San Mateo County is enforced by CAL FIRE in the State Responsibility Area and by local fire agencies elsewhere. Several districts conduct annual defensible space inspections in High and Very High Fire Hazard Severity Zones. Contact your local fire agency, or see the San Mateo County Fire defensible space page, for requirements specific to your address.

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