Fire science

The candle wick on your property line: how fences spread wildfire to homes

Fact-checked August 19, 2026·Sources at the end of this article

Embers first: how homes actually ignite

Picture a wildfire destroying a home and you probably picture a wall of flame rolling over the roofline. That image is mostly wrong. Most homes lost in wildfires are ignited not by the flame front but by wind-driven embers that land on or near the house. An ember is small. To become a house fire, it needs fuel where it lands. Mulch, firewood, dry plants, and combustible fences in the first five feet of the wall are that fuel.

This is the entire logic of Zone 0, the ember-resistant zone covering the first five feet out from a structure. It is also why Governor Newsom's Executive Order N-18-25, the order that directed the Board of Forestry to write the statewide rule, names fences specifically:

"Scientific research has shown that the greatest risk of embers directly or indirectly igniting a home... stems from combustible materials built or planted within five feet of a home, including perpendicular wood fences and combustible landscaping."

Executive Order N-18-25, February 6, 2025

Note which fences the order singles out: perpendicular ones, the fences that run into the house. That word choice is the fire science in miniature.

The wick: how a fence carries fire to the wall

Fire researchers at the National Institute of Standards and Technology (NIST) describe a fence line fed by mulch or dry vegetation as a "wick." The image is doing two jobs, because a wick does two things: it carries flame, and it holds flame in place.

A wood fence is a continuous line of dry fuel, and it rarely stands alone. There is often bark mulch, dry grass, or leaf litter along its base. An ember does not need to land on your siding. It only needs to reach some point along the fence line, or the material at its foot. Once the fence is burning, the fire travels the line. If the line ends at your house, that is where the fire is delivered.

The second job is the worse one. A burning fence attached to a wall does not pass by the way a shower of sparks does. It holds flame against the structure at the point of attachment, for as long as there is fence left to burn. The California Department of Insurance puts the same mechanism in plainer words:

"Replacing wood fencing connecting to your home with metal is critical because it can act like a candle wick leading fire straight to your home."

California Department of Insurance

CAL FIRE has watched a wooden fence do it, on purpose

The wick description is not a metaphor somebody reached for. It is what the state fire agency reports seeing in a controlled burn it runs itself.

"In an ongoing collaboration, CAL FIRE – Office of the State Fire Marshal (OSFM) and the Insurance Institute for Business & Home Safety (IBHS) continue to conduct live burn demonstrations using a mitigated and nonmitigated structure."

The two houses are built to differ in specific ways, then burned side by side. On the fence, CAL FIRE's own summary of what the demonstration shows:

"Zone 0 Landscaping – Non-mitigated home had wood mulch, plants, and a wooden fence within five feet, acting as a wick for flames."

"The mitigated home used gravel, hardscaping, and a metal fence, removing fuel for embers."

CAL FIRE, Zone 0 At-A-Glance, linked from the department's defensible space page; the file itself is undated on its face and carries a September 17, 2025 date in its PDF metadata

Why we lead with this rather than the insurance line. The Department of Insurance describes a wood fence as acting "like a candle wick leading fire straight to your home", and it is a vivid sentence that we have quoted. But it is consumer marketing copy from a regulator promoting a discount. The paragraphs above are the fire agency describing the outcome of a burn it conducted with a research institute. Same mechanism, better provenance, and the difference matters on a site that asks you to take its sourcing seriously.

One limit worth naming: a demonstration burn is a demonstration, not a controlled experiment with a sample size. It shows the mechanism operating in a realistic setting. It does not, on its own, tell you the probability that your fence ignites your house.

What surveyors found after the Palisades and Eaton fires

After the January 2025 Palisades and Eaton fires, surveyors from the Insurance Institute for Business & Home Safety (IBHS) walked the burn areas parcel by parcel. Three numbers from that survey describe how common the fence-against-the-house configuration is:

Read those numbers for what they are: prevalence data. They show that the wick configuration (a combustible fence touching the house or standing inside the ember zone) was the norm, not the exception, in two of the most destructive fires in California history. They do not show how many homes were lost because of a fence. The survey did not measure that, and neither has anyone else.

The attachment point and the gate: the two critical nodes

If the fence is a wick, the attachment point is where the wick meets the candle. Everything about the mechanism concentrates there: the fire the fence has carried arrives at that spot, and that is the spot where flame is held against the wall. The adopted state regulation measures from exactly that point: in the Local Responsibility Area text its five-foot noncombustible span runs "from the point of attachment or the point closest to the Occupied Structure," and the State Responsibility Area text says the same of a "Building or Structure."

The gate deserves equal attention. The regulation names gates alongside fences in its attachment and new-fence provisions, and a gate is where the details hide: hinges, a latch, hardware, and often wood blocking where the frame is fastened to the wall. The Board of Forestry's own staff memo singles out "an attaching combustible gate" as one of the two situations that would bring the Zone 0 fencing standards into play at all.

Two practical checks follow from this. First, look at the whole assembly, not just the panels. A metal panel hung on a wood post fails the intent, so check the post and the footing too. Second, at the wall itself, check the gate hardware, the latch, and any wood blocking behind the attachment. Those are the pieces in direct contact with the structure.

What a five-foot noncombustible break physically does

A noncombustible span in the last five feet before the wall does one simple thing: it removes the final five feet of fuel. A fire traveling the fence line stops being fed five feet short of the structure. An ember that lands beside the wall finds steel, masonry, or bare ground instead of dry wood. The wick is cut before it reaches the candle.

Here is the fencing text as it stands in the package the Board of Forestry approved on August 19, 2026. This is the Local Responsibility Area version, and it is unchanged from the July 2026 draft:

"(8) Combustible fences and gates shall not be attached to an Occupied Structure.

(9) Fences. (A) All fences and gates that are attached to an Occupied Structure, or come within one (1) foot of contact with an Occupied Structure, shall have a five (5) foot Noncombustible span from the point of attachment or the point closest to the Occupied Structure. (B) After the effective date of this regulation, no new Combustible fences or gates (whether attached or parallel to the Occupied Structure) are permitted within five (5) feet of an Occupied Structure. (C) Fence repairs to an existing Combustible fence may be made using Combustible replacement materials."

Zone 0 rule text, §1298.04(b), the Local Responsibility Area track. The State Responsibility Area version at §1299.03(d)(9) is a single undivided paragraph, refers to a "Building or Structure", and carries no parallel-fence clause. The certified "as adopted" text has not been published yet.

"Noncombustible" in the regulation has a precise meaning: materials that pass ASTM E136, a vertical tube furnace test at 750 °C that measures whether the material itself can burn. In practice that means aluminum, steel, wrought iron, chain link, masonry, concrete, or stone. Wood does not pass. Neither do vinyl, composite, or fire-retardant treated wood (FRTW). Our guide to noncombustible fence materials covers the options, and our piece on the five-foot fence transition covers how the retrofit is actually built.

Just as important is what the regulation does not do. It does not clear existing fences off your property:

"The purpose of the regulations is not to require wholesale replacement of existing wood fences. Existing combustible fences may remain, repairs using existing materials may continue, and only where an attaching combustible gate exists or a fence is fully replaced would the applicable Zone 0 standards apply."

Board of Forestry staff, July 2026 clarification memo to the Board

The same memo is blunt about distance: "Fences more than five feet from the structure are not impacted by the draft regulation." The full breakdown of what the regulation does and does not require is in our Zone 0 fence requirements guide.

The evidence the Board says it relied on

One document on the Board of Forestry's Zone 0 page is just a bibliography, headed "The Science Behind Zone 0" and dated June 13, 2025. It is the most checkable thing in the whole rulemaking, because it names the studies the regulation rests on and invites you to read them instead of taking anyone's word. Fifteen sources, and several are squarely about the mechanism this page describes.

• Wood in the first five feet, tested directly. Johnsson and colleagues at NIST, "Wind-Driven Fire Spread to a Structure from Landscape Timbers," Technical Note 2307 (2025). Landscape timbers are not fences, but they are the same thing in the same place: milled wood lying within a few feet of a wall in wind.
• Why mulch keeps coming up. Lin, Li, Conkling, Huang, Quarles and Gollner, "Smoldering Ignition and Transition to Flaming in Wooden Mulch Beds Exposed to Firebrands Under Wind," Fire Safety Journal 148 (2024). Smoldering to flaming is the transition that turns a landing ember into a fire at your wall, and it is why the rule treats bark and wood chips the way it does. Our mulch and ground cover guide covers what that means in a yard.
• How far apart things have to be. Maranghides and colleagues, "NIST Outdoor Structure Separation Experiments (NOSSE) with Wind," Technical Note 2253 (2023), and the same group's hazard mitigation methodology, Technical Note 2205 (2022).
• What survived, and why. Knapp, Valachovic, Quarles and Johnson on single-family home survival in the 2018 Camp Fire, Fire Ecology 17:25 (2021); Syphard, Brennan and Keeley on building construction materials relative to other factors (2017) and on the role of defensible space (2014). The Syphard papers are the ones worth reading if you want the honest version of how much defensible space actually buys.
• The conflagration framing. IBHS, "The Return of Conflagration in Our Built Environment" (2023) and "Noncombustible Zone 0: Minimizing Pathways to Home Ignition" (2023); the UL Research Institutes Lahaina Fire Incident Analysis (2024); and IBHS field observations from the January 2025 Los Angeles fires.

The number the Board put in its own filing: 45 percent

The bibliography above names the studies. The Board's Notice of Proposed Emergency Action, the document it files with the Office of Administrative Law, goes further and quotes them, which is where the most concrete figure about attached wood fences turns up. We read the notice on August 25, 2026.

On the fence itself, the Board cites NIST Technical Note 1635:

"NIST TN 1635 found that within the Witch and Guejito Fire footprints, 45% of homes within the fire footprints with an attached wood fence were destroyed; significantly higher than the baseline loss rate."

And on the combination this page keeps returning to, a fence with fuel at its base, it quotes NIST Technical Note 2228:

"Combustible fences with mulch at their base were found to be high hazard, transporting fire through the community and providing a steady source of firebrands."

Both quoted in the Board of Forestry and Fire Protection's Notice of Proposed Emergency Action, Zone 0 Defensible Space. Technical Note 2228 is the 187-experiment study our mulch and ground cover guide covers in detail.

Read the 45 percent carefully, because it is easy to over-read in either direction. It is a loss rate among homes inside a fire footprint that had an attached wood fence, not a probability that any house with a wood fence burns down. And it is an association drawn from post-fire survey work, so it does not on its own prove the fence caused the loss; homes with attached wood fences may differ in other ways too. What it does establish is that the attachment case, which is exactly the case the regulation reaches, is the one where the field data is most pointed. The Board plainly thinks so, since this is the number it chose to put in front of the agency reviewing its rule.

It is also worth noticing what these two quotations are doing in a filing. An emergency rulemaking has to demonstrate its necessity by substantial evidence, so the notice is where the Board has the strongest incentive to put its best material. If you want to argue with the fence provisions, this is the paragraph to argue with, and it is more specific than anything in the press coverage.

We list them because a regulation that cites its evidence can be argued with on the evidence, which is better than arguing about it on press releases. If you think the science is thinner than the rule implies, these are the papers to read; if you think it is stronger, same list.

What this means for you

As of August 19, 2026, the statewide regulation is adopted but not yet in effect; Office of Administrative Law review and publication by the Secretary of State come first. The fire mechanism does not wait on rulemaking, and neither do the local ordinances already in force in at least thirty-five California jurisdictions. See our local ordinance roundup and check whether Zone 0 would apply to your home.

• Walk your fence line. Find every point where a fence or gate touches the house or comes within about a foot of it. Those are the points the regulation reaches, and the points the fire science flags.
• Look down, not just along. The wick is fed by what sits at the base of the fence: mulch, dry vegetation, debris. Clearing the base matters even before any fence work happens.
• You do not need to replace the whole fence. The regulation targets the first five feet at the attachment. Existing fences that pass through the zone without attaching may remain, and repairs may use combustible replacement materials. More on this in our wood fence myths article.
• If you do the work, it already counts with insurers. Since October 14, 2022, the Safer from Wildfires regulation (10 CCR §2644.9) has required insurers that price California homeowners policies using wildfire risk to offer a rating credit for noncombustible materials within five feet of the home, fences and gates included. The measure appears as the first of twelve on carrier mitigation forms. That regulation binds insurers, not you: they must offer the credit, and nothing in it mandates fence replacement. Details in our fence insurance discounts guide.
• No statewide deadline is running. Once the regulation is effective, fences and gates for existing homes sit in the final compliance phase, and the two tracks run opposite clocks: three years from the effective date in the Local Responsibility Area, extendable to five by the local fire agency, against five years in the State Responsibility Area, which the Director may shorten but not below three. No calendar dates exist yet, because there is no effective date yet. Follow the Office of Administrative Law review on our status tracker, and start with the California Zone 0 fencing guide for the full picture.