Lytton wildfire report contains useful insights

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Thanks to Robin Urquhart at RDH Building Science for providing this report from the Institute for Catastrophic Loss Reduction into the Lytton wildfire. It’s chock-full of information into how fires spread and what makes structures more vulnerable or more resistant. Good stuff to know if you’re building in a fire-prone area. Here’s an excerpt:

Key Findings

1. Wildfire conditions were extreme, not from a high intensity fire spreading through tree canopies (crown fire), but from a rapidly spreading surface fire burning through grass, forest litter and shrubs, aided by largely short distance ignition spotting from a profusion of burning embers.
2. Given the Lytton wildfire conditions of rapid fire spread along four different fronts and necessary considerations for life-safety under such conditions, it is unrealistic to expect a successful initial attack wildfire suppression response. Wildfire spread to Lytton community boundaries and initiated community ignitions in less than one-hour. Wildland surface fuel outside the community had burned well before 1800 hrs. and any continued influence from wildfire had ceased. By 1800 hrs., 20 primary structures were heavily fire involved and fire spread primarily from burning structures continued within the Lytton community without wildfire influence for 1.5 to 2 hours.
3. Surface fire and short-range spot fires from burning embers initiated multiple structural ignitions, largely at the edges of the Village of Lytton, IR 18 and IR 17 within 1 hour of the reported wildfire. Within one and one-half hours of the wildfire report (by 1800) more than 20 primary structures were significantly burning, distributed along the 4 wildfire Spread Paths in the Village of Lytton, IR 18 and IR 17. The rapid, simultaneous fire involvement of this many primary structures would have overwhelmed any reasonable municipal structure protection response in that region.
4. Structure ignitions during extreme wildfires are principally determined by the local ignition conditions of the home ignition zone (HIZ); that is, a structure and its flammable attachment’s ignition vulnerabilities to burning embers in relation to burning materials within 30 m. An extreme wildfire provides ignition hazards from flames and burning embers; HIZ ignition vulnerabilities determine structure ignitions and community fire disasters. Hence, WU fire disasters are a structure ignition problem.
5. Lofted, wind-driven burning embers – principally from burning structures – initiated ignitions across streets to adjacent blocks of ignition vulnerable structures thus continuing fire spread through the Village of Lytton and IR 18. Fire spread continued through the Lytton Community was principally determined by the highly vulnerable ignition conditions within individual HIZs, and by high structure-to-structure flame spread potential due to overlapping HIZs. Density of structures in Lytton-area communities was commonly increased by placement of highly ignitable sheds, workshops and other outbuildings close to primary structures.
6. The non-flammable exterior walls remaining until structure collapse prevented flame radiation and contact sufficient for ignition at the adjacent surviving house. This indicates an opportunity to use common building materials that mitigate structure-to-structure community fire spread where there are densely overlapping HIZs.
7. The Lytton WU fire disaster, as with previous WU fire disasters, occurred during extreme wildfire conditions; however, total building destruction does not indicate a high intensity wildfire flame exposure. Most structures are unprotected due to an overwhelmed structure fire response; thus, any sustained structure ignition from burning embers or contact with low intensity surface fire results in total destruction. For individual structures and collectively communities, ignition resistance is the most effective approach for reducing community wildfire risk and preventing WU fire disasters.