WUI structureparcelcommunity fire hazard mitigation methodology

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In the last twenty years, wildland-urban interface (WUI) fires have been growing 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. Structural losses have been attributed to exposures from embers (firebrands) and fire (radiation and/or convection). As structural losses continue to increase, there is need for a comprehensive hazard assessment and mitigation methodology to harden appropriate structures and parcels effectively and efficiently against ember and fire exposures. The Hazard Mitigation Methodology (HMM) described in this report was developed explicitly to meet these needs. The science-based methodology uses the knowledge collected from post-fire field observations spanning over 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. The HMM is the result of a sixteen-month collaborative effort between NIST, the California Department of Forestry and Fire Protection (CAL FIRE), and the Insurance Institute for Business and Home Safety (IBHS). The HMM is a spatial hazard mitigation methodology that provides an implementable path forward by considering the spatial relationships of fuels and 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. The methodology further demonstrates where structure hardening in the absence of parcel hardening does not work by highlighting the relationships between exposure and hardening. Housing density, structure separation distance, and parcel layouts are also addressed. The methodology was explicitly designed to address the current building stock, i.e., to solve the 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 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.