Researchers assess how fences can spread fires and ignite structures

Examples of very high risk fences and mulches: A) parallel privacy fences, B) double
mesh fences, C) #1 wood-plastic composite fence, D) good neighbor fence, E) rubber mulch. (Based on NIST research)

Three weeks ago, a Dallas television station, FOX 4, broadcast live video as a grass fire spread through a field and reached densely populated homes, eventually destroying 9 and damaging 17 more. . There were several reasons why the structures were so vulnerable, including moderately dense vegetation in the field adjacent to the property lines, very little impact from the fire department’s efforts to stop the spread, no overhead support and wooden fences that connected all the properties at the back and sides of the houses.

fences burn houses destroyed in texas
Parallel fences burn as the grass fire spreads through the neighborhood of Balch Springs, Texas on July 25, 2022. Image from FOX 4 Dallas-Fort Worth video.

On August 10, the National Institute of Standards and Technology (NIST) released the results of 187 experiments that examined how fire spreading toward a structure is affected by combustible fencing and mulch under conditions that may be encountered in a fire at the forest-urban interface (WUI). They looked at mulch only, fence only, fence plus mulch, parallel fences, and long range embers. The materials studied were western red cedar, California redwood, pine, vinyl, and wood-plastic composites. Fence styles included privacy, lattice, and good neighbor (boarding).

A small structure was located between zero and up to six feet downwind of the fence or mulch bed as a target for the flames and burning sparks. A target mulch bed at the base of the structure tested the ability of firebrands produced by the burning fence and mulch bed to ignite spot fires that threatened the structure.

Although few homeowners place store-bought mulch under their fences, it can be common for debris to accumulate along the base on one or both sides. The amount and flammability can be extremely variable, but the researchers’ use of mulch allowed them to control the flammable material that could be duplicated in various situations.

The study found that sparks capable of starting downwind spot fires were generated by almost all of the fence and mulch combinations tested in this study. All wooden fences with mulch at the base caused localized fires in the targeted mulch bed. Spot fires were often started within minutes of igniting the mulch and fence. The shredded hardwood mulch and pine bark mulch burned and emitted firebrands for over an hour. Ignition of point fires has also been demonstrated from burning fires carried by the wind for distances of up to 156 feet from the burning element in high wind conditions and on a paved surface.

The research generated a number of recommendations:

  • Avoid parallel fences to reduce exposure to large flames. Parallel fences can create very dangerous fuel accumulation corridors that are difficult to access and maintain. A spacing of 3 feet between fences is not sufficient.
  • Avoid proximity to other combustibles to reduce fire intensity and limit fire spread. This includes fuels over the fence and fuels across parcel boundaries. Avoid mulch at the base of the fence.
  • Avoid proximity of combustible fences to residence, including neighboring residence, to avoid direct ignition.
  • The spread of fire is more likely with wood and wood-plastic composite fences than with fences made of vinyl or non-combustible materials such as stone, brick or steel.
  • Keep the fence and yard clear of debris to reduce fuel and potential fire pathways.
  • Reinforce structures against firebrands to prevent ignition of the structure by embers produced by fences or other combustible sources.

Video of the July 25 fire in Texas does not clearly show fences between residences, but it is likely that they are directly connected to structures. An example is seen in the photo below of two homes in Superior, Colorado, one of the communities devastated by the Marshall Fire which destroyed 991 structures south of Boulder on December 30, 2021.

Fence connecting houses risk of fire spreading
File photo of a fence connecting homes in Superior, Colorado. Google image from July 2012.

The images below are from the FOX 4 Dallas-Fort Worth video shot on July 25, 2022.

fences burn houses destroyed in texas
Fences burn as the grass fire spreads through the neighborhood of Balch Springs, Texas on July 25, 2022. Image from FOX 4 Dallas-Fort Worth video.
fences burn houses destroyed in texas
Fences burn as the grass fire spreads through the neighborhood of Balch Springs, Texas on July 25, 2022. Image from FOX 4 Dallas-Fort Worth video.
fences burn houses destroyed in texas
Fences burn as the grass fire spreads through the neighborhood of Balch Springs, Texas on July 25, 2022. Image from FOX 4 Dallas-Fort Worth video.

Most structures that burn in a wildfire are not ignited by the direct impact of the flames, but by burning embers being lifted and carried downwind in front of the fire. At Wildfire Today, we first covered the role of embers in igniting structures in 2010, a concept brought into public consciousness by Missoula Fire Science Lab researcher Jack Cohen. To reduce the chances of a home burning down in a wildfire, the best value for money is to focus on the household ignition area. Flammable material near the structure should be modified, reduced, or eliminated to the point where multiple hot embers landing in the area will not spread the fire and spread to the structure.


More information: Six things that must be done to protect fire-prone communities

Thanks and hats off to Gerald.

Author: Bill Gabbert

After working full time in wildfires for 33 years, he continues to learn and strives to be a student of fire. View all posts by Bill Gabbert

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