How Zone 0 stops wildfire: the science of ember ignition.

The finding that changed defensible space

For decades, California defensible space rules focused on creating a buffer that the flame front couldn't cross. Cut back the brush, separate the trees, mow the grass — keep the active fire away from the structure.

Post-fire research over the last fifteen years has rewritten that mental model. The dominant finding, confirmed across California fires and post-disaster studies by IBHS, NIST, and CAL FIRE: most homes that burn in wildfire are not consumed by direct flame contact. They are ignited by embers landing near or on the structure.

Embers are small burning fragments — pieces of bark, leaves, twigs, construction debris — lofted into the air by the fire and carried downwind. In a wind-driven California fire, those embers travel as much as a mile or more ahead of the active flame. They land on roofs, in gutters, in mulch beds, against wooden fences, on doormats, under decks. If they find a fuel they can ignite, the resulting small ignition becomes the seed of a structure fire.

The ember storm

Under wind conditions, the volume of embers produced by a wildfire is enormous. Researchers describe it as an ember storm — a sustained, dense cloud of wind-driven embers passing over and through neighborhoods.

Three California fires illustrate the ember-storm failure mode at scale:

• 2017 Tubbs Fire (Sonoma County). Embers driven by 45+ mph winds ignited homes in the Coffey Park neighborhood — a suburban subdivision separated from the wildland by a six-lane freeway and roughly half a mile of urban infrastructure. Over 1,400 homes were lost in Coffey Park alone, almost entirely to ember-driven ignition.
• 2018 Camp Fire (Butte County). The deadliest and most destructive California fire in modern history. Wind-driven embers ignited homes faster than the fire front itself could travel; the town of Paradise lost ~85% of its structures.
• 2025 Eaton and Palisades Fires (Los Angeles County). Wind-driven embers ignited homes throughout dense urban neighborhoods — Pacific Palisades and Altadena — that had decades of mature defensible space but were not designed for the ember-storm failure mode.

The pattern across all three: the flame front was not the primary threat to individual homes. The threat was an airborne ember load that bypassed every traditional defensible space layer and landed against the structure itself.

The five-foot principle

Zone 0 — the first 5 feet from any wall — is the answer to the ember-storm failure mode. The logic:

1. Embers are going to land somewhere on or near the structure during an ember storm. You cannot prevent this; the load is too high.
2. What you can prevent is ignition at the landing site. If an ember lands in bare gravel, on a concrete patio, on a fiberglass doormat, or on a non-combustible vent screen, it cools and goes out without igniting anything.
3. The critical distance is 5 feet. Inside 5 feet, an ignition has a direct path to the structure — bark mulch touching the foundation, a wood fence touching the wall, a doormat at the door — and can transfer to the home itself. Outside 5 feet, the same ignition is usually too far away to make the jump.

The 5-foot perimeter is therefore not arbitrary. It's the empirical distance at which most ember-driven home ignitions transition from a small ground fire to a structure fire.

The specific ignition modes Zone 0 interrupts

Each of the 12 Zone 0 requirements targets a specific, documented ember-ignition mode:

• Combustible mulch within 5 ft — bark mulch is one of the most observed ignition fuels in post-fire investigations. Embers smolder in mulch for long enough to develop into a small fire against the foundation.
• Wood fencing attached to the structure — an ignition at any point along an attached wood fence can carry directly to the structure's siding via continuous fuel. Effectively a wick.
• Pine needles in gutters and on roofs — concentrated dry fuel in close proximity to attic vents and roof eaves. Embers land in gutter debris, smolder, and ignite the eave.
• Uncovered attic, foundation, and eave vents — embers can enter the structure through standard vent openings and ignite the interior. 1/8-inch metal mesh stops embers larger than this size, which is most structurally-significant embers.
• Combustible doormats — coconut/coir doormats are unusually effective ember-ignition surfaces. They smolder slowly and develop into a fire at the entrance.
• Stored combustibles within 30 ft — firewood piles, propane tanks, and stored lumber are high-density fuel loads that become ember-ignited fires adjacent to the structure.
• Combustible furniture in Zone 0 — wooden patio furniture, decorations, and other combustibles within 5 ft of walls behave the same way as wood fencing — they ignite from embers and carry fire to the wall.

Why traditional defensible space wasn't enough

California's pre-AB 3074 defensible space rules focused on Zones 1 and 2 — the 100-foot buffer of managed vegetation around the home. Those zones work well against direct flame contact from a fire approaching through the wildland.

They work poorly against ember storms. An ember-driven ignition starting in mulch against the foundation is already inside the 100-foot buffer — the buffer's job was to keep the flame front out, and the ember bypassed that job entirely.

Zone 0 is the missing layer. By eliminating the things that ember-ignite within 5 feet of the structure, Zone 0 cuts the loop that traditional defensible space couldn't.

The research that supports it

Major sources behind the ember-driven home-loss finding:

• Insurance Institute for Business & Home Safety (IBHS) — runs full-scale ember and wind testing at its South Carolina research center. Published findings include the empirical demonstration that ember-resistant landscape design is the single most effective home-protection intervention.
• National Institute of Standards and Technology (NIST) — post-fire investigations of major California fires consistently document ember-driven ignition as the dominant mode of home loss.
• CAL FIRE — operational guidance and post-fire investigations reinforce the same finding and informed AB 3074's drafting.
• University of California Cooperative Extension — fire-resistant landscape research and homeowner guidance specifically for California fire ecology.

What this means for your house

The practical takeaway from the ember-ignition research is that passive defenses matter more than active firefighting in a wind-driven California fire. When the fire arrives, you may not be home; firefighters may not be able to defend your property; the active flame front may move too fast for any individual home to be actively saved.

The home either survives the ember exposure or it doesn't. Zone 0 — combined with Zone 1, Zone 2, and home hardening (vents, roof, siding) — is what determines which side of that outcome you're on.

See also: California Defensible Space Zones: Zone 0, Zone 1, and Zone 2 Explained for how all three zones work together.

What to do today

Run the free 60-second Zone 0 readiness check to see how many of the 12 ember-driven ignition modes are eliminated on your property right now — and which ones still need work.

Start the check →

Sources: Insurance Institute for Business & Home Safety (IBHS) research publications; National Institute of Standards and Technology post-fire investigations of California wildfires; CAL FIRE operational guidance; University of California Cooperative Extension fire-resistant landscape research.