Researchers evaluate how fences can spread fires and ignite structures

Tests fire hazard fences
Examples of very high hazard fences and mulch: A) parallel privacy fences, B) double
lattice fences, C) wood-plastic composite #1 fence, D) good neighbor fence, E) rubber mulch. (From the NIST research)

Three weeks ago a television station in Dallas, FOX 4, streamed live video as a grass fire spread across a field and reached densely-packed houses, eventually destroying 9 and damaging another 17. There were several reasons why the structures were so vulnerable, including medium-heavy vegetation in the field adjacent to the property lines, very little impact from efforts of the fire department to stop the spread, no air support, and wood fences that connected all of the properties on the rear and sides of the homes.

fences burn fire Texas homes destroyed
Parallel fences burn as grass fire spreads into neighborhood in Balch Springs, Texas, July 25, 2022. Image from video by FOX 4 Dallas-Fort Worth.

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

A small structure was located between zero and up to six feet downwind of the fence or mulch bed as a target for flames and firebrands. 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.

While not many homeowners place store-bought mulch below their fences, it can be common for debris to build up along the base on one or both sides. The amount and flammability can be extremely variable, but the researchers’ use of mulch gave them control over the flammable material which could be consistently duplicated in their experiments.

The study found that firebrands capable of igniting spot fires downwind were generated by nearly all combinations of fence and mulch tested in this study. All wood fences with mulch at the base caused spot fires in the target mulch bed. Spot fires were often ignited within a few minutes of mulch and fence ignition. Shredded hardwood mulch and pine bark mulch burned and emitted firebrands for longer than an hour. Ignition of spot fires was also demonstrated from firebrands transported by the wind over distances as far as 156 feet from the burning item under high wind conditions and over a paved surface.

The research generated a number of recommendations:

  • Avoid parallel fences, to reduce exposure to large flames. Parallel fences can result in highly hazardous fuel accumulation corridors that are difficult to access and maintain. Spacing of 3 feet between fences is not sufficient.
  • Avoid proximity to other combustible fuels, to reduce fire intensity and limit fire spread. This includes fuels above the fence and fuels across parcel boundaries. Avoid mulch at base of fence.
  • Avoid proximity of combustible fences to residence, including neighboring residence, to prevent direct ignition.
  • Fire spread is more likely with wood and wood-plastic composite fences than with fences made of vinyl or noncombustible materials such as stone, brick, or steel.
  • Keep fence and yard clear of debris, to reduce the amount of fuel and potential pathways for fire.
  • Harden structures against firebrands to prevent structure ignition from embers produced by fences or other combustible sources.

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

Fence connecting houses fire spread hazard
File photo of fence connecting houses in Superior, Colorado. July, 2012 Google image.

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

fences burn fire Texas homes destroyed
Fences burn as grass fire spreads into neighborhood in Balch Springs, Texas, July 25, 2022. Image from video by FOX 4 Dallas-Fort Worth.
fences burn fire Texas homes destroyed
Fences burn as grass fire spreads into neighborhood in Balch Springs, Texas, July 25, 2022. Image from video by FOX 4 Dallas-Fort Worth.
fences burn fire Texas homes destroyed
Fences burn as grass fire spreads into neighborhood in Balch Springs, Texas, July 25, 2022. Image from video by FOX 4 Dallas-Fort Worth.

Most structures that burn in a wildfire are not ignited by direct flame impingement, but by burning embers that are lofted and carried downwind ahead of the fire.  At Wildfire Today we first covered the role of embers in igniting structures in 2010, a concept brought into the public consciousness by Jack Cohen, a researcher at the Missoula Fire Science Lab. To reduce the chances of a home burning in a wildfire, the most bang for the buck is to concentrate on the Home Ignition Zone. The flammable material near the structure needs to be modified, reduced, or eliminated to the point where multiple burning embers landing in the zone will not propagate the fire and spread to the structure.

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

Thanks and a tip of the hat go out to Gerald.

Save communities by thinning forests or hardening structures?

Moose Fire August 2, 2022 in Montana
The result of aerial ignition on the Moose Fire August 2, 2022 in Idaho. By Mike McMillan for the USFS.

Bloomberg Law has an interesting article by Bobby Magill about efforts to reduce the wildfire threat to homes. It discusses and compares forest thinning vs. hardening structures. Here are excerpts, but read the entire article.

Congress is spending billions to save communities from Western megafires by thinning large swaths of forests even as scientists say climate change-driven drought and heat are too extreme for it to work.

The money would be better spent thinning woods closest to homes and shoring up houses against embers raining down from firestorms, according to academics, former agency officials, and others who study wildfires.

“If our goal is to keep homes and communities from burning, the experts are telling us to focus from the home outwards. First, harden the home so it is less likely to ignite,” said Beverly Law, an emeritus professor of forestry at Oregon State University.

Megafires are sustained by drought and heat, and “no amount of thinning treatment will prevent such fires from occurring,” she said.

[…]

No Scientific Consensus
As the federal government focuses on forest thinning, no scientific consensus exists that removing vegetation, especially at a landscape-scale, will save communities in the paths of firestorms amid the West’s historic 23-year drought.

The science is clear that “there isn’t a great connection between home loss and these fuel treatments,” though they sometimes help firefighters gain a foothold on some fires, Cheng said.

Randy Moore, the Forest Service chief, said the agency is confident that as homes are built deeper and deeper into the woods, its research shows that removing “overstocked” trees is the best way to protect them.

“We know where we do nothing, or where we do a little, we’re seeing the evidence out on the landscape,” Moore said, referring to recent megafires. “We feel compelled to do something.”

In a wildfire burning embers transported downwind are what cause most structures to burn

Sheltering from the Creek Fire at the Mammoth Pool Reservoir
Sheltering from burning embers and the Creek Fire at the Mammoth Pool Reservoir Boat Launch, Sept. 5, 2020. Photo by Cameron Colombero, via Mike Ikahihifo.

Most structures that burn in a wildfire are not ignited by direct flame impingement, but by burning embers that are lofted and carried downwind ahead of the fire.  At Wildfire Today we first covered the role of embers in igniting structures in 2010, a concept brought into the public consciousness by Jack Cohen, a researcher at the Missoula Fire Science Lab. To reduce the chances of a home burning in a wildfire, the most bang for the buck is to concentrate on the Home Ignition Zone. The flammable material near the structure needs to be modified, reduced, or eliminated to the point where multiple burning embers landing in the zone will not propagate the fire and spread to the structure.

The video below produced by the New Jersey Forest Fire Service elaborates on this concept. It is queued up to 1:42 where the issue is addressed.

More information is in our articles tagged “Home Ignition Zone.”

Data shows building codes can reduce vulnerability of homes in wildfires

Slave lake burned homes
Burned homes in Slave Lake. May 16, 2011. Alberta, Canada. CTV.

Researchers have found that building codes based on lessons learned during the deadly 1991 Tunnel Fire in the Oakland Hills of California can reduce the vulnerability of homes to wildfires.

In a paper titled Mandatory vs. voluntary adaptation to natural disasters: the case of U.S. Wildfires, authors Patrick W. Baylis and Judson Boomhower describe how they scoured property and wildfire records to identify which homes were constructed under building codes requiring enhanced resistance to wildfires.

Chapter 7A of the California Building Code which went into effect in 2008 requires certain fire resistance measures, including exterior construction materials used for roof coverings, vents, exterior walls, and decks. It applies to new construction of residential and commercial buildings in designated fire hazard severity zones.

The researchers discovered that a 2008 or newer home is about 16 percentage points (40%) less likely to be destroyed than a 1990 home experiencing an identical wildfire exposure. There is strong evidence, they concluded, that these effects are due to state and local building code changes – first after the deadly 1991 Oakland Firestorm, and again with the strengthening of  wildfire codes in 2008. The observed vintage effects are highly nonlinear, appearing immediately for homes built after building code changes. There are no similar effects in areas of California not subject to these codes or in other states that lack wildfire codes.

Their findings are similar to those in a paper published October 4, 2021 in which researchers analyzed the structures that were destroyed and those that survived the Camp Fire that ran through the city of Paradise, California in 2018. They considered at least four primary characteristics of structures:

  • Were they built before or after the adoption in 2008 of Chapter 7A of the California Building Code.
  • Distance to nearest destroyed structure.
  • Number of structures destroyed within 100 meters.
  • Pre-fire overstory tree canopy within 100 meters

They found that the last three criteria were the strongest predictors of survival. Homes more than 18 meters (59 feet) from a destroyed structure and with less than 53 percent pre-fire overstory canopy within 30 to 100 meters (98 to 328 feet) survived at a substantially higher rate than homes in closer proximity to a destroyed structure or in areas with higher pre-fire overstory canopy. Most fire damage to surviving homes appeared to result from radiant heat from nearby burning structures or flame impingement from the ignition of near-home combustible materials. The researchers concluded that building and vegetation modifications are possible that would substantially improve outcomes. Among those include improvements to windows and siding in closest proximity to neighboring structures, treatment of wildland fuels, and eliminating near-home combustibles, especially within 1.5 meters of the structure.

(The video below was shot December 31, 2021, the day after the Marshall Fire destroyed more than 1,000 homes in Boulder County, Colorado. Notice that most of the surviving homes seen in the video had fewer homes in close proximity.)

The authors noted that while Chapter 7a includes requirements not found in many building codes, a few other codes are more complete incorporating multiple construction classes based on anticipated radiant heat, flame, and ember exposure levels. For example Chapter 7A does not consider the interaction between components such as siding, window, and the under-eave area on an exterior wall.

California is embarking on a pilot project in which owners of vulnerable homes in lower income neighborhoods will be given grants up to $40,000 to retrofit the structures, making them more resistant to wildfires.

Home Ignition Zone

This data helps to illustrate that the condition and characteristics of the Home Ignition Zone should be an extremely high priority in preventing structures from burning as a wildfire approaches.

There is an opportunity for much needed improvement in both current building codes and how we live in wildfire prone WUI areas.

Thanks and a tip of the hat go out to Gerald.