In the last twenty years, wildland-urban interface (WUI) fires have grown in severity and size. The structures destroyed by WUI fires have devastated entire communities and have cost billions of dollars while significantly impacting the social fabric and economic well-being of entire regions. Structure losses are attributed to exposures from both embers (firebrands) and fire (radiation and/or convection). As structure losses continue to increase, there is a growing need for a comprehensive hazard assessment and mitigation methodology to harden appropriate structures and parcels effectively and efficiently against ember and fire exposures. To address this need, the National Institute of Standards and Technology (NIST), the California Department of Forestry and Fire Protection (CAL FIRE), and the Insurance Institute for Business & Home Safety (IBHS) embarked on a sixteen-month collaborative effort, culminating in this Hazard Mitigation Methodology (HMM).
The HMM provides an implementable path forward by considering the spatial relationships between fuels, exposures, and hardening at the structure and parcel levels. The HMM demonstrates how complex structure hardening is, and how and why hazards associated with both fire and ember exposures need to be mitigated. By describing the relationships between exposure and hardening within the methodology, HMM highlights situations where structure hardening does not provide sufficient protection in the absence of parcel hardening. The HMM also addresses housing density, structure separation distance, and parcel layouts. The methodology was explicitly designed to address the current building stock, i.e., to solve retrofit challenges, and efforts were made to limit retrofit expenses. While the methodology was developed primarily for retrofits, the presented strategy can also be applied to new construction.
This science-based methodology uses the knowledge collected from post-fire field observations spanning a dozen years and tens of thousands of hours of field data integration and analysis. Additionally, the HMM utilizes the latest technical knowledge gained from laboratory and large-scale research in fire propagation and hazard mitigation in the WUI.
This report documents the methodology and addresses the critical issues of mitigation effectiveness at the parcel and community levels. The impacts of partial mitigation at the parcel and community level were addressed for different types of WUI communities.