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Structure Vulnerability to Firebrands from Fences and Mulch



Kathryn M. Butler, Erik L. Johnsson, Wei Tang


Fences and mulch contribute to the spread of WUI fires. They act as both ignition targets and as sources that may themselves ignite nearby objects through direct flame contact and firebrand generation. The linear nature of fences gives them the capability of spreading fire over long distances. This paper presents the findings from outdoor experiments that investigated the spread of fire through firebrand spotting from fences and mulch beds near a structure in a wind field. In these experiments, a fence section, with or without a mulch bed beneath, was arranged perpendicular to the wall of a small structure, at a distance between 0 m and 1.8 m from the wall. Beyond the fence was a large fan that generated a wind field of 6 m/s to 14 m/s directed toward the structure. Along the base of the structure was a second mulch bed that acted as a target for firebrands generated by the fence and its mulch bed. A propane burner was used to ignite the fence at its base at the windward end farthest from the structure. The time for a firebrand to ignite a spot fire in the target mulch bed that eventually reached the wall was recorded, along with the time for the spot fire to reach the wall. Data were also collected on the flame spread rates over the fence and mulch bed. A variety of fence and mulch types and materials were tested. Fence type, proximity to the structure, wind speed, and type of ground cover beneath the fence all affect firebrand spotting. Firebrand spotting is random in nature, but some trends are apparent from the data. In most cases with fences combined with mulch beds, spotting occurs within the first seven minutes after ignition. Spotting tends to be delayed for the slowest (6 m/s) wind speed. Compared to the cases with the mulch bed only, the presence of a fence tends to decrease the time to spotting. A bed of pine straw mulch tends to burn quickly and intensely but gives off small firebrands that do not generally result in spot fires.
Proceedings Title
The Fire Continuum Conference
Conference Dates
May 21-24, 2018
Conference Location
Missoula, MT


firebrands, embers, wildland-urban interface, WUI fires, fences, mulch, structure vulnerability


Butler, K. , Johnsson, E. and Tang, W. (2020), Structure Vulnerability to Firebrands from Fences and Mulch, The Fire Continuum Conference, Missoula, MT, [online], (Accessed June 21, 2024)


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Created July 22, 2020, Updated August 25, 2020