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Home ignitability
Jack Cohen’s 2000 paper found that a house ignites in a wildland-urban fire because of the house and its immediate surroundings, about 40 meters. Firebrands can travel kilometers. They only matter if they land on something that will take fire. A FHSZ map is not that 40-meter site. This is not R327 and not Zone 0.
You are looking at a house that stood next to a foundation. People call that a miracle. Cohen says look at the exterior, the roof, and the fuel within tens of meters. Then look at the one that burned.
No g3-passed exhibit matches this object. MECH-008 is still a brief. The scene is the two lots. Do not use a house-level hub graphic as a research figure.
Why it matters
People treat the forest as the problem and the house as scenery. Cohen’s result flips that. If the community site is ignored, wildland fuel reduction has to reach kilometers to stop firebrands. If the house and the next 40 meters are ignitable, a lower-intensity fire can still take it. That is why premiums can move with no nearby fire, and why covering versus assembly and vents are not landscaping.
How the documents split
| Object | Spatial scale | What it is not |
|---|---|---|
| Home ignitability (Cohen 2000) | The house plus about 40 meters | A landscape fuel map |
| Zone 0 | The first five feet | Cohen’s 40 meters |
| Zone bands | 0 / 5 / 30 / 100 feet in current practice | SIAM’s 40-meter flame-to-structure result |
| FHSZ | Mapped fire hazard for construction overlays | Home-loss potential by itself |
| Ember vs flame vs radiant | Exposure types | The 40-meter finding |
Cohen 2000 uses home ignitability, not the later phrase home ignition zone. This page stays with the 2000 paper because that PDF opened.
The paths
1. Read the house first. Cohen: structural characteristics and immediate surroundings determine ignition potential. Roof, walls, openings, attached decks, and the fuel against them. IBHS and R327 are later documents that operationalize pieces of that. They are not this paper.
2. 40 meters is the flame-and-firebrand neighborhood. Modeling, experiments, and case studies in that paper converge on ignitions not being likely unless flames and firebrand ignitions occur within 40 meters. Case studies he cites showed high survival with about 10 to 20 meters of vegetation clearance and nonflammable roofs. Do not treat 40 meters as a planting list.
3. Distant firebrands still need a landing pad. Kilometers of travel do not skip the last meters. If the firebrand hits a clean Class A roof and a hardscape band, the distant crown fire is not a contact ignition on that house.
4. Do not map only wildland. Cohen says W-UI as a geographic location misrepresents home-loss potential. Low-ignitability homes can survive high-intensity wildland fire. Highly ignitable homes can fail in lower-intensity fire. How to read an FHSZ map is still required for California construction overlays. It does not replace this finding.
5. Neighbors occupy the 40 meters. At higher density the adjacent house is inside the immediate surroundings. That is a community problem, not a reason to skip your own exterior.
What people mix up
- Calling Zone 0 “the Cohen zone.”
- Treating a green forest a mile away as the ignition test.
- Treating a FHSZ rating as a burn/no-burn stamp on the house.
- Waiting for firefighters. Cohen notes severe W-UI fires can destroy neighborhoods faster than suppression can arrive. Painted Cave 1990 is his example of hundreds of homes in about two hours.
- Telling the official this paper is R327.
How to check
- Open Treesearch 4688. Find 40 meters. Find firebrands from several kilometers.
- Walk 40 meters from the walls. Count decks, fences, sheds, and the neighbor’s siding.
- Walk five feet. That is Zone 0, a different document.
- Open the FHSZ or the R327 adoption map separately. Do not merge the layers.
- If someone quotes 100–200 feet as Cohen 2000, stop. That figure is not in the 2000 PDF opened here.
Limits
This page is one paper. Later Cohen writing uses home ignition zone and other distances. Those PDFs did not open in this session. FireApproved is not the Forest Service. The finding does not stamp a permit.
Related records
Questions
If the fire is a mile away, is the house safe?
Not as a rule. Cohen (2000) says firebrands capable of ignition can originate from a fire several kilometers away. They threaten the house only if they ignite the house directly or ignite adjacent flammable materials that then ignite the house. Distance to the flame front is not the whole problem.
Is the home ignition zone the same as Zone 0?
No. Zone 0 is the first five feet of hardscape in current defensible-space and insurance language. Cohen’s 2000 paper uses home ignitability and a roughly 40-meter surrounding. Do not shrink his finding to five feet, and do not stretch Zone 0 to 40 meters.
Does a high FHSZ rating mean the house will burn?
Cohen argues that mapping the wildland-urban interface as a geographic location misrepresents the home-loss problem. Homes with low ignitability can survive high-intensity wildland fires. Highly ignitable homes can be destroyed in lower-intensity fires. Read the house, then the map. See how to read a CAL FIRE FHSZ map.
Is this Oregon R327?
No. R327 is adopted construction where a department opted in. Cohen is research on how houses ignite. Use both. Do not tell the building official that a 2000 journal paper is the code.