Wind-Driven Fire Spread to a Structure from Composite Fences

Published

November 26, 2025

Author(s)

Kathryn Butler, Erik Johnsson, Marco Fernandez, Shonali Nazare, Alexander Maranghides

Abstract

Field observations show that combustible landscaping features are a common pathway leading fire spread directly to homes during wildland-urban interface (WUI) fires. NIST is studying how these features near a home burn to better understand their levels of hazard and potential roles in spreading WUI fires. Following up on an earlier NIST study involving a variety of fences and mulch, full-scale fire experiments and cone calorimeter tests were conducted on two types of wood-plastic composite (WPC) fences and one steel-plastic composite (SPC) fence available to consumers. Full-scale experiments examined the effects on fire spread toward a structure for each type of composite fence, with and without shredded hardwood mulch placed beneath the fence. A wind machine provided a nominal mean wind speed of 6 m/s (13 mi/h). The fence and/or mulch bed were ignited by a propane burner on the ground at the end farthest from the structure. A small structure was located 1.83 m (6 ft) downwind of the fence as a target for flames and firebrands. A target mulch bed at the base of the structure tested the ability of firebrands or flames produced by the burning fence and mulch bed to ignite spot fires that threatened the structure. Cone calorimeter tests were performed on samples from all three composite fences and compared to previous results from western red cedar and vinyl privacy fences. Ignitability and flammability measurements were obtained in horizontal and vertical orientations according to standard cone calorimeter protocols described in ASTM E-1354. Full-scale experiments showed severe fire behavior from two of the three composite fences, with flames extending above and well downwind of the fence even in the absence of mulch beneath. For one WPC fence, the upper frame distorted in the fire, allowing burning vertical boards to fall to each side of the fence and creating a burning zone along the fence path whose width was twice the fence height. The SPC fence melted and dripped, resulting in large flames and hanging strands of plastic blown by the wind field. The fire behavior of the second WPC fence was less severe, attributed primarily to its horizontal board design. As the fire near the ground consumed each horizontal board in turn, the boards above it slipped downward within the frame. As a result, the flame height stayed below the halfway point on the fence, the burning boards remained close to the centerline of the fence as they fell to the ground, and the fire diminished on its own as it ran low on fuel. In the absence of mulch, this fence burned slowly in a hole near the ignition site, with glowing combustion and occasional small flames. Findings from the cone calorimeter in a vertical configuration complemented full-scale experiments. Melting and dripping or slipping of the sample surface contributed to large peak heat release rates for the two composite fences with intense full-scale fire behavior. For all three composite fence samples, the effective heat of combustion was much higher than for wood or vinyl. Recommendations include developing fire test(s) for evaluating fences and fence materials that represent the actual fire hazard and avoiding very high hazard composite fences. This study of fire spread over composite fences is part of a series designed to better inform standards and codes regarding placement of landscape features around homes that are at risk of exposure to wildland-urban interface fires.

Citation

Technical Note (NIST TN) - 2357

Report Number

2357

NIST Pub Series

Technical Note (NIST TN)

Pub Type

NIST Pubs

Download Paper

https://doi.org/10.6028/NIST.TN.2357

Local Download

Keywords

Composite fences, cone calorimeter, fence fires, fences, fence flammability, fire spread, structural ignition, structure vulnerability, wildland-urban interface fires, wind-driven fires, WUI fires

Citation

Butler, K. , Johnsson, E. , Fernandez, M. , Nazare, S. and Maranghides, A. (2025), Wind-Driven Fire Spread to a Structure from Composite Fences, Technical Note (NIST TN), National Institute of Standards and Technology, Gaithersburg, MD, [online], https://doi.org/10.6028/NIST.TN.2357, https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=960430 (Accessed November 27, 2025)