Wildfires are a risk throughout the U.S. and pose a significant threat to buildings and lives in wildfire-prone areas. As more and more businesses expand into the wildland-urban interface—or WUI, a term used to designate wildfire-prone areas where homes and businesses are also located—the wildfire risk to businesses will also grow. In this environment, business protection must take into account both the materials and design features of the building, as well as the selection, location and maintenance of landscape plants, including grasses, shrubs, bushes and trees.
Our Risk Solutions Partner, the Insurance Institute for Business & Home Safety (IBHS) provides an overview of wildfire hazards, mitigation strategies, useful resources and a step-by-step process for reducing wildfire risks.
Three sources of wildfire ignition
First, understanding how wildfires can threaten your property can help you make appropriate plans to protect your staff and property.
Most people associate wildfire damage with direct flame contact from the wildfire as it burns past the building. However, buildings also can be damaged or destroyed when they are exposed to burning embers and/or radiant heat. Building ignitions during wildfires occur when a component of a building is exposed to one or more of these three wildfire exposures.
1. Burning embers
Burning embers (also called firebrands), and wind-blown burning embers in particular, are the most frequent cause of building ignitions. These embers are generated by the burning wildfire itself, as well as by combustible items the wildfire has previously ignited, such as landscape plants, which includes grasses, shrubs, bushes and trees. Importantly, burning embers can travel for long distances before landing on or near a building.
Embers can ignite buildings in several ways:
- Embers can ignite combustible construction materials directly when accumulating on or immediately adjacent to them. Combustible construction materials are those that ignite and burn such as wood, plastic, and wood-plastic products used in decking and siding.
- Embers can ignite nearby plants and accumulated debris such as pine needles or other combustible materials such as a wood pile.
- Embers can enter a building through openings, such as an open window or attic vent, and ignite combustible items inside the building.
2. Direct flame contact
Direct flame contact refers to actual flames from the wildfire coming into contact with buildings or combustible items attached to or near the building.
3. Radiant heat
Fire generates radiant heat (the heat you feel when standing near an open flame). If it is high enough and the duration is long enough, radiant heat can ignite a combustible product (such as wood siding) or break the glass in a window. Additionally, exposures to lower levels of radiant heat can pre-heat materials, making them easier to ignite from a direct flame contact exposure.
Know your fire hazard severity zone
Fire Hazard Severity Zones (FHSZ) represent the wildfire hazard in a particular area based on an evaluation of the plants and landscaping, fire history, slope and other terrain features that may impact the growth and spread of fire. Zones are typically classified as “moderate,” “high,” or “extreme” (also referred to as “very high”).
Businesses can request the FHSZ rating and other relevant information from local building or fire officials in their area. Depending on the construction ordinances in a given community, certain requirements regarding materials and other construction details can be based on the FHSZ, which is also relevant to landscaping, maintenance, and other operational decisions.
Below are some of the most important considerations when readying your property for wildfire season.
- 1. Building setback
Buildings and structures should be set back a minimum of 30 ft from any property line adjacent to a national forest, state park, open space preserve, or other protected wildland.
- 2. Vegetation management plan (VMP)
Both naturally occurring plants and species introduced into an area influence the potential for fire to spread to buildings located on the property—with some plants, like junipers, being more likely to ignite than others. Because plants vary greatly from region to region, IBHS provides links to detailed, region-specific information at DisasterSafety.org/ibhs-risks-wildfire/ibhs-regional-wildfire-guides/.
A Vegetation management plan (VMP) may be required in certain wildfire-prone areas. A VMP provides important information about the land, such as:
- Topography (slope and aspect).
- Location of building(s) on the land.
- Proposed fuel treatment details (suggested actions such as thinning and prescribed burning to minimize wildfire risks).
- Environmental concerns (threatened and endangered species, state-listed sensitive species and wetlands, etc.).
Plants used around the building should have low combustibility characteristics such as high moisture content, low oil or resin content, deep roots with thick heavy leaves, and minimal production of dead vegetation.
The VMP also provides detailed information on how the three defensible space zones (see below) will be developed and maintained. When developing a VMP, consult a landscape professional such as a forester, range manager, or natural resource specialist.
- 3. Fire hydrants
Fire hydrants should be located no more than 250 feet from the primary building and connected to a reliable public or private water source.
- 4. Exterior wall cladding
Noncombustible siding materials such as concrete and brick will provide the greatest fire protection from flames, embers, and radiant heat.
The distance between the ground and the bottom of the siding on the exterior wall affects a building’s vulnerability, particularly when walls are made of materials that can ignite. The building code generally calls for 6 inches of clearance between the ground and the start of the siding.
- 5. Windows
Using dual-paned windows with tempered glass will reduce the vulnerability of windows.
Operational windows should have screens covering those sections that can open. Always close windows when wildfire threatens.
- 6. Vents
At a minimum, vents should be covered with 1/8-inch noncombustible mesh screening. This will minimize the size of embers that can enter into the attic or crawlspace area.
Vents that are perpendicular to the flow of wind, such as a gable end vent or those in under-eave blocking, are more vulnerable to ember entry. Ridge vents that are rated to resist the entry of wind-driven rain or vents in boxed-in under-eave construction are better options.
- 7. Roofs
Since the roof is a large, relatively horizontal surface covering the building, it is often considered the most vulnerable component, particularly from an ember exposure perspective.
Fire ratings for roof coverings are either Class A, B or C (or, in the case of a non-fire-retardant-treated wood shake covering, not rated). IBHS recommends a Class A covering. For low-slope roofs, a Class A–rated roof cover includes testing and rating of the entire roof system assembly, which includes the roof cover, insulation, vapor or air barriers, and the type of deck. Examples of roof systems considered to be Class A regardless of the deck type include built-up roofing and ballasted single-ply membranes.
There are a number of Class A–rated steep-slope roof coverings, including asphalt composition shingles, clay tile and steel. Some Class A tile designs provide for an opening at the eave end and ridge. These openings should be plugged with a noncombustible material to minimize entry of embers in the area under the roof covering.
- 8. Gutters and downspouts
Gutters and downspouts should be made of noncombustible materials such as aluminum. The gutter should incorporate an integral metal flashing at the roof edge, or a separate flashing should be used at the roof edge. Debris should be regularly removed from roof and gutters, since it can be ignited by wind-blown embers.
- 9. Signage
All signage should be made from noncombustible materials.
- 10. Combustible storage
Outdoor storage of large quantities of combustible and flammable liquids should be located in detached noncombustible buildings more than 50 ft away from other buildings.
- 11. Defensible space
Defensible space around the building should be planned and maintained.
What is defensible space?
Defensible space is the area between a building and an approaching wildfire, where vegetation and other combustible materials have been managed to reduce the wildfire threat and improve the likelihood of a building surviving without assistance from firefighters, as described in “Fire Adapted Communities: The Next Step in Wildfire Preparedness,” University of Nevada Cooperative Extension, Publication SP-10-10.
This space is organized into three zones.
To create defensible space around your business, it’s important to understand how plants and other materials should be selected, located and managed around the property. Proper management will reduce the opportunity for the fire (or any spot fires from ember ignitions) to burn to the building, and minimize the chance for radiant heat from ignited plants and other materials to pose a threat to the building.
As seen below, defensible space is usually discussed in terms of zones that extend outward from the building. Each zone has specific recommendations for types of plants, including how they should be grouped and maintained.
Defensible space zones
Three zones extend outward from the building, which is represented by the dark square in the center.
Zone 1 (0–5 ft from the building) (also called the near-building, noncombustible or low-combustible zone)
The objective of this zone is to reduce the chance that ignition will result in a direct flame contact exposure to the building. Woody vegetation (such as trees and shrubs) should not be used in this zone. Noncombustible mulches, such as rock mulch, are recommended. Because this zone is closest to the building, it requires the most careful selection and intensive management of plants, shrubs and trees, and other materials. Dead plant materials should be removed from plants.
Zone 2 (5–30 ft or to the property line)
The objective of vegetation management in this zone is to reduce the opportunity for fire to climb into the crown or upper portions of trees or shrubs, and to minimize the opportunity for fire to burn directly to the building. Trees and shrubs in this zone should be in well-spaced groupings and well maintained. Eliminate tall grasses, hanging tree branches and other ladder fuels (plants that allow fire to climb up trees), and create separation between plants or plant groupings.. While a parking lot can provide a good source of defensible space on your property, any vegetation in the parking lot should be well maintained.
Zone 3 (30–100 ft or to the property line)
The objective of this zone is to slow down and reduce the energy of a wildfire. Tree and brush spacing should force fire in tree crowns (or fire in shrub and brush) to drop to the ground. Dead trees and shrubs should be removed. The rate of fire spread and flame length is affected by slope—a steeper slope will result in a faster-moving fire with longer flame lengths.
For more information on protecting your building from natural hazards, see our Hanover Risk Solutions Property page.
This material is provided for informational purposes only and does not provide any coverage or guarantee loss prevention. The examples in this material are provided as hypothetical and for illustration purposes only. The Hanover Insurance Company and its affiliates and subsidiaries (“The Hanover”) specifically disclaim any warranty or representation that acceptance of any recommendations contained herein will make any premises, or operation safe or in compliance with any law or regulation. By providing this information to you, The Hanover does not assume (and specifically disclaims) any duty, undertaking or responsibility to you. The decision to accept or implement any recommendation(s) or advice contained in this material must be made by you.
LC 2021-362 and 363