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Engineering Laboratory / Fire Research Division

Wildland-Urban Interface Fire Group

Surrounding Parcel Hazard Mitigation

The potential exposure from neighboring structures
should be assessed and mitigated if appropriate.

Depending on the spacing of existing structures on neighboring parcels, mitigating actions may provide resistance to structure-to-structure fire spread. This concept is illustrated in the figure below with three scenarios.

If the minimum SSD is too close and fuels cannot be removed or relocated (e.g. homes and primary structures), then targeted home protection actions can help in certain cases.

Scenario 1 - Homes have space across property boundary. No hardening for flames is needed, as SSD is large enough (< 50 ft) and the structure will not experience significant fire (radiant and convective) exposure from the neighboring structure.

Scenario 2 - Neighboring homes are close enough where one may ignite the other if they catch fire. Targeted, directional structure hardening (indicated by the blue walls) can increase ignition resistance from the expected level of flame exposures

Scenario 3 - Structures are close together and structure hardening is not likely going to increase structure survivability. Exposures will be too high and ignition is likely even if efforts have been taken to harden the exteriors. Both homes must be prevented from ignition because ignition of one will lead to ignition of both. Hardening for embers can enhance ignition resistance of each. Fuel removal and reduction is essential. In a high fuel density scenario as such, a structure could be a residential structure or an auxiliary structure for e.g., storage shed or auxiliary dwelling unit.

Created July 6, 2023, Updated August 22, 2023