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Wildland Urban Interface (WUI) is the most common designation for an area that experiences wildfires. The term is defined as the zone of transition between unoccupied land and human development; the area or zone where structures and other human development meet or intermingle with underdeveloped wildland or vegetative fuels.

While the West is the first area that comes to mind when thinking of wildfires, WUI zones are present across the country, with varied wildland fuels. In the Pacific Northwest and the states bordering the Great Lakes, forests are the primary wildland fuel, while in Southern California, chaparral brush is the primary fuel. According to the Federal Emergency Management Agency (FEMA), the states with the greatest number of homes in WUI areas are California, Texas, Florida, North Carolina, and Pennsylvania. The states with the highest percentage of homes in WUI areas relative to total homes in the state are Montana, Wyoming, New Mexico, West Virginia, South Carolina, Maine, New Hampshire, and Vermont. FEMA says wildfires can spread primarily by ember generation and deposition onto combustible materials, radiant heat, flame impingement exposure from contact with combustible materials, and the transfer of heat from fuels to combustible materials

WUI concerns should not be geographically isolated for home builders, as these areas continue to grow by approximately 2 million acres per year, placing an emphasis on building ignition-resistant homes to prevent the spread and damage of wildfires should they occur. There are practical steps builders can take to make homes in WUI zones resilient against wildfires and several key areas that should be addressed during the planning and construction phases.

Site Planning and Considerations

For ignition-resistant construction, Christy Riggs, owner and architect for Colorado Springs–based firm 308 LLC, says the first step is site planning. Chief concerns for builders should be identifying if the building site is located on a steep hill, a densely vegetated area, or an area with an abundance of existing fuels. Considering fire paths and outside risks to the structure should be just as important as the typical site planning considerations, such as views, access, and available sunlight, Riggs says.

“Once a site has been selected, it is important to identify what the topography is like and what the vegetation is,” she explains. “Knowing how steep a lot is can give clues to how a wildfire may act—fires typically move uphill unless there are winds that can drive them back down a slope.”

Dense and dry vegetation can easily spread fires, so putting a home in a densely treed area without mitigation to remove that vegetation is unwise, Riggs says. “After site planning, vegetation is very important when planning to make sure the home is defensible and to give it space that if a fire does happen, there aren’t fuels right up against the home,” she adds. Many states have ignition-resistant construction design manuals that provide information on how to properly mitigate vegetation around homes in at-risk areas.

FEMA says creating a defensible space—an area where combustible material such as vegetation has been treated, cleared, or modified to slow the rate and intensity of an advancing wildfire—can improve the probability that a home will survive a wildfire. Riggs says the acceptable safe level of vegetation around a home will vary by the type of site. In less densely treed areas, vegetation is typically allowed to be right next to the home, since there are not large quantities that could propel the spread of fire. Urban areas require additional consideration, as there are often strict setbacks for property lines to maintain space between structures to mitigate fire spread.

“In areas where there is dense vegetation, such as in forests or grassland areas, it is recommended to keep the first 30 feet around a home trimmed and maintained, but not necessarily clear cut,” Riggs says. “Separating vegetation areas in this 30-foot area allows for larger and more dense plantings to occur, but then have spaces between them so that it’s not all one large vegetative fuels area.”

If a home is in a designated WUI zone, the adopted codes and guidelines in place often provide the minimum requirements to construct a home in such an area, says Riggs. While installing safety measures like fire sprinklers may help reduce the spread of some types of fires, the most important considerations for wildland fire prevention are hardened structures and maintaining a fire-safe zone around the home. Conversations with insurance agents may provide additional clarity on material choices and what level of coverage is available in instances of fires.

“In general, it’s best not to use natural wood products in a higher fire-risk area that are thin in dimension, like siding and shakes,” Riggs explains. “Larger timbers, such as deck supports and roof rafters, have better fire resistance, so these are typically allowed even in designated WUI areas.”

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Building Envelope and Material Concerns

Beyond planning, Riggs says ignition resistant construction is contingent on material choices for windows, doors, siding, decking, and roofing. Class A is the highest level fire-resistance rating an exterior material can achieve as tested under the UL 790 standards. Examples of Class A-rated materials include clay tile, stucco, metal roofing, and certain asphalt shingle roofs.

“The best way to ensure that ignition-resistant construction is achieved is to identify and document the materials for the project early on,” Riggs says. “If the siding isn’t decided until a week before it needs to go on, most likely you will be limited to what is available at the closest hardware store, and they may not have the Class A materials in stock that you were wanting to use.”

The roof ceiling and edge are the most vulnerable parts of a home, having the most severe exposure to the elements. Roofing materials such as cedar shake and shingles can ignite, sending flaming embers into the air, presenting the risk of structure-to-structure fire spread. Slate, clay, concrete, and asphalt roofing tile are popular noncombustible options, while metal roofing combines noncombustion, Class A protection, and low maintenance.

“The fire-resistive class of the material is a great starting point to understanding the level of exposure a material has,” Riggs says. “If there is no rating, then it’s going to be considered a flammable material, and the more holes, cracks, and areas for embers to catch in a material, the more likely it is to be affected by the spread of fire.”

For siding, the thinner and less material a product has, the less warranty it will typically carry, according to Riggs. Stone and brick siding typically have great fire resistance while vinyl siding and natural wood products are either direct fuel sources or will not hold up under fire. While stone and brick siding can run more expensive, lower-cost Class A exterior alternatives, such as stucco, manufactured stone, wood-look metal siding, composite siding, and fiber cement board, are also on the market.

Combustible exterior building components, including roof coverings, siding, and decks, can ignite and contribute to the damage and spread of wildfires. Careful consideration should be given to building materials that are used in WUI areas as well as construction measures to prevent the penetration of heat and embers at vents, mechanical or electrical openings, and windows.

Even with properly treated landscaping, burning vegetation can still present a risk to a home if eaves and gutters are vulnerable. Builders can address the spread of fire further by constructing eaves with noncombustible materials such as fiber cement or stucco. Crawlspace vents and attics also present a risk of ignition should windblown embers get sucked into the vents and ignite the attic. Builders can guard against this risk by covering vents with galvanized steel or stainless steel mesh, preventing embers from crossing the threshold into the vent or attic.

For windows, tempered glass has proved to be resistant to breakage from radiant heat, and double-pane windows are preferred to single-pane windows for performance against fires. The base of walls will often have the edge of the sheathing exposed on the underside; in a ground fire, embers can be pushed up against a home and get caught where the exposed sheathing is. Builders can minimize this risk by covering the exposures with materials such as concrete, metal mesh, or stucco.

Additional Resources

In addition to building codes and ignition-resistant construction design manuals, the UL Product iQ and International Code Council for Building Codes are additional resources for builders. The Product iQ provides information on thousands of tested assemblies that most building departments recognize as automatically meeting the requirements for fire ratings. An example commonly used in the residential construction industry is the UL U305 wall assembly, a one-hour fire-rated bearing wall using minimum 2x4 wood studs that are firestopped. The UL Product iQ lists drywall manufacturers that can be used, how to treat nail heads, types of insulation that can be used, and optional items such as furring channels that can be added as needed. Additionally, most manufacturers typically provide information on product fire ratings.

Beyond becoming familiar with codes, ratings, and approved systems, working with contractors and subcontractors that understand the requirements of ignition-resistant construction can be helpful if the area is unfamiliar. The knowledge of the specialty trades ensures that the assembly specified in the planning stage is up to the standards required for ignition-resistant construction.

“Being familiar with the plans is crucial, but going through and highlighting unique areas is [also] a good first step. This will allow you to hold a pre-construction meeting with the subs to relay these unique areas to them and point out where they need to make sure the plans are directly followed,” Riggs says. “During construction, having someone who is on-site to guide all trades and ensure they are coming together correctly is also important.”