The 30-second version
- Ember ignition dominates loss data — 60-90% of destroyed homes in every major post-2010 CalFire post-fire report.
- Attic ignition is the largest single sub-category — embers through unprotected vents smoldering into flaming ignition inside the attic.
- Zone 0 vegetation and mulch is the second-largest — landed embers finding fine fuel adjacent to walls.
- Direct flame ignition is a minority story — usually less than 20% of losses even in the most extreme fires.
Where the data comes from
After every major California wildfire that destroys 25+ structures, CalFire, the Insurance Institute for Business & Home Safety (IBHS), and the National Institute of Standards and Technology (NIST) conduct post-fire investigations. Investigators walk destroyed and surviving properties, document ignition patterns, and publish detailed reports. Reports for the Tubbs Fire (2017), Camp Fire (2018), Woolsey Fire (2018), Marshall Fire (2021), and Palisades Fire (2025) are all publicly available.
Pattern 1: Attic ignition through vents
Across post-Tubbs, post-Camp, post-Woolsey, and post-Palisades investigations, "attic ignition via unprotected vents" appears as the most-cited single failure mode. Homes with standard soffit vents (1/4-inch mesh or worse) had 3-5× higher loss rates than homes with WUI-rated vents in the same neighborhoods. This is why Chapter 7A's vent requirements are load-bearing, not decorative.
Pattern 2: Zone 0 combustibles
Post-Camp Fire IBHS reporting identified combustible mulch within 5 ft of the structure as the ignition source in a large share of losses. Combustible fencing connecting to the structure and stored firewood in Zone 0 were also frequently cited. This body of investigation directly informed California AB 3074's Zone 0 hardening law.
Pattern 3: Wood-shake and lightweight roofs
Every major fire investigation lists wood-shake or lightweight asphalt roofs as disproportionately represented among losses. In the Palisades Fire, wood-shake roofs represented under 5% of housing stock but a much larger share of the destroyed structures — a pattern documented in post-fire reporting.
Pattern 4: Cluster effects and neighboring structures
Once one home ignites, the radiant heat and ember release ignites adjacent homes at 2-3× the rate of homes with no burning neighbors. This is why community-scale hardening (through CWPPs and neighborhood defensible space) has such outsized impact — a single hardened home in a block of non-hardened homes still faces high loss risk.
Pattern 5: Water infrastructure failure
Multiple major-fire reports document municipal water pressure loss during peak-demand fire response. Palisades reports specifically noted upper-elevation pressure failures. This is why wildfire sprinkler systems designed with independent water sources (dedicated storage tanks or pool backup) are considered essential for canyon and ridgeline properties.
What to actually read
If you want to read the primary sources rather than summaries:
- IBHS post-fire investigation reports — publicly available at ibhs.org, focused on structure survival factors
- CalFire post-fire damage assessments — available through CalFire and county fire departments
- NIST post-fire building code recommendations — available at nist.gov, focused on regulatory implications
Halo Build Co's Chapter 7A and sprinkler installation practices are informed by this body of investigation. Our specifications are designed to close the specific failure modes these reports document. See our wildfire defense overview or Chapter 7A contractor page.
Frequently asked questions
Where can I read primary post-fire investigation reports?
IBHS (ibhs.org), CalFire post-fire damage assessments, and NIST post-fire building code studies are the three main public sources. IBHS reports are the most homeowner-accessible; CalFire reports are the most comprehensive for LA-area fires; NIST reports are the most useful for understanding regulatory implications.
What percentage of homes lost in wildfires ignite from embers vs direct flame?
Every major post-2010 CalFire and IBHS investigation places ember ignition at 60-90% of destroyed structures. Direct flame ignition is typically less than 20% even in the most extreme fires. The remaining loss share is a mix of ignition modes and cases where the ignition source couldn't be definitively identified.
Do post-fire investigations look at surviving homes too?
Yes — IBHS in particular focuses heavily on survival factors, comparing destroyed to surviving homes in the same neighborhood to identify which specific features made the difference. This comparative work is where much of the guidance in Chapter 7A and Safer from Wildfires ultimately comes from.