Although the severity and timing of fire seasons vary widely from region to region, wildfires often pose a threat to lives, property, and resources. During an average fire season, hundreds of homes are damaged or destroyed by wildfire, and in extreme fire conditions, thousands of homes can be damaged or destroyed. Severe fire weather in areas with significant amounts of wildland fuels can lead to extreme fire behavior. This article describes the various responses that building construction can adopt in response to wildfire.
Wildfires can damage buildings through direct flame contact, convection (heat that rises from a fire and creates a smoke column), conduction (heat that transfers through material such as metal roofs and railings), and radiation (heat from a fire next to the building). Wildfires can also create burning embers that rise in the smoke column and fall on buildings. Firebrands (large pieces of wind-driven, burning material) can be blown through windows or lodged against a building and lead to ignition of the building. Traditional efforts to protect buildings from wildfires have focused on fighting the fire before it reaches the buildings. With the expansion of residential construction into previously undeveloped forests and wildlands, more buildings are now at risk from wildfires. Fighting or suppressing the fire is often difficult or impractical; buildings must therefore also be constructed to be fire-resistant. No building can be completely fireproof, but implementing certain recommendations cangreatly reduce the potential for damage to a building and greatly increase its chance of survival.
During a wildfire, combustible exterior building components such as roof coverings, siding, and decks can ignite, leading to severe damage to or total loss of the building. Therefore, the use of noncombustible or fire-resistant materials should be considered for exterior components. Figure 2 shows the components of the building envelope. Also critical is the configuration of the noncombustible or fire-resistant materials. Unless construction measures that provide protection from a wildfire are implemented, heat and embers can penetrate the building envelope at vents, unsealed mechanical or electrical openings, and through windows broken by heat or wind-blown firebrands. When these openings are penetrated, the building can burn from the inside out. If the envelope has been designed and constructed to be fire-resistant, both the exterior and interior of the building will be more capable of resisting a wildfire long enough for the danger to pass or for firefighters to arrive.
Roof assemblies are the most vulnerable component of the building envelope in a wildfire because of their horizontal orientation and size. Embers and firebrands can ignite the roof covering, other roof components, and debris on the roof. Once the roof has ignited, the fire commonly propagates into the interior of the building, resulting in substantial damage to or total loss of the building. The probability that a home will survive a wildfire is greatly influenced by the components of the roof assembly.
The type and arrangement of the components govern their potential for ignition and their propensity to transfer heat into the interior of the building. The complexity of the roof’s shape also influences the potential for ignition. A roof with valleys and roof/wall intersections where combustible debris such as leaves and needles can collect has more potential for ignition than a roof without them.
Fire-rated Roof Assemblies
The resistance of roof assemblies to external fire is rated by the American Society of Testing and Materials (ASTM), using test method E108. The method includes measurements of the surface spread of flame, the ability of the roof assembly to resist fire penetration from the exterior of the building to the underside of the roof deck, and the potential for the roof covering to develop flying brands of burning material. Roof assemblies are rated Class A (highest rating), B, or C. Assemblies that fail the test (do not meet the Class A, B, or C criteria) are unrated. Class A provides the greatest degree of fire resistance, but there is a range of protection within the Class A rating. For example, some Class A rated assemblies have noncombustible roof.
Roof Construction: Guidance for New Buildings
Many types of roof assemblies are rated Class A. Recommendations for various components of roof assemblies that are rated Class A are provided below.
Steep-slope Roof Covering
A steep-slope roof is a roof with a slope greater than 3:12. The following design and installation practices are recommended:
2. Metal shingles and panels
3. Fiberglass-reinforced asphalt shingles
Clay and concrete tile are noncombustible and because of their relatively large thermal mass, retard the transfer of heat. Lightweight tile products are available, but normal-weight tiles are recommended because of their greater mass. If tiles are installed over wood battens, embers may be blown under the tiles and ignite the battens. Fire-retardant-treated battens are therefore recommended. If tile is used, the following are also recommended:
• Eaves, hips, and ridges: Embers can be blown under tiles at the eaves, hips, and ridges. Birds can build nests in the space between the underlayment and the bottom of the tiles if the space is accessible, providing combustible debris that can be ignited by embers. Installing birdstops at eaves and fully mortaring hips and ridges are both recommended to avoid the accumulation of debris under tiles and to keep embers out.
• Valleys: Unless special metal flashing is installed, combustible debris can accumulate in valleys and then under the tiles. Flat and plain tiles should be specified to be tightly butted to form a closed valley, and pieces of metal flashing should be installed under each tile course along the valley centerline. For profiled tile, lead or flexible flashing should be used, as recommended by the tile manufacturer.
Metal shingles and panels
Metal shingles and panels are noncombustible, but they readily transfer heat. If they are installed over wood battens, fire-retardant-treated battens should be specified and installed. If shingles or panels are installed over wood decking, 5/8-inch gypsum roof board complying with ASTM C 11771 should be installed over the decking.
Fiberglass-reinforced asphalt shingles
Although a roof assembly that has fiberglass-reinforced asphalt shingles can be rated as Class A, these shingles contain combustible material (e.g., asphalt). If this type of shingle is used, a 5/8-inch gypsum roof board that complies with ASTM C 1177 over the wood decking should be installed for enhanced protection of the decking. Care needs to be taken when the shingles are nailed that the nails are not overdriven.
Steep-slope Roof Underlayment
Embers can be blown under some types of steep-slope coverings such as tile, slate, and metal shingles and panels. If tiles become dislodged or cracked, embers can land on the underlayment below. Installing an underlayment that has enhanced fire resistance is recommended to provide protection from embers that reach the underlayment. An example of an enhanced underlayment is a mineral-surface cap sheet that is rated for use in a Class A rated assembly. If a mineral-surface cap sheet is used under metal panels or shingles, measures should be taken to prevent the metal from bearing directly on the cap sheet and the cap sheet from abrading the metal (thereby making the metal susceptible to corrosion).
Roof Assemblies: Guidance for Existing Buildings
For homeowners with roof assemblies that are not Class A rated, the only long-term, reliable way to reduce roof vulnerability to wildfire is to reroof. Reroofing normally involves removing the materials above the roof deck and replacing them with new materials. The recommendations that are listed above for new buildings are applicable to reroofing design and installation. Roof assemblies that have wood shingles or shakes and are not rated Class A and roofs with organic-reinforced asphalt shingles are vulnerable to a wildfire. These types of roofs should be replaced as soon as possible.
Roof Assembly Considerations
The homeowner should have roof debris removed from the roof surface and gutters regularly. Aging does not affect the ignition potential of tile. However, some roof coverings, such as wood or fiberglass-reinforced asphalt shingles, become more susceptible to ignition as they age. The roof covering should be replaced before deterioration of the covering significantly degrades resistance to ignition. Birdstops should be inspected annually to ensure that they have not fallen out of place.
Eaves, Overhangs, and Soffits
Windborne embers, convective heat, and radiant heat can be trapped under overhangs and in the upper portion of exterior walls. Overhangs and walls can ignite if not constructed of noncombustible or fire-resistant materials. Typical construction materials for eaves, overhangs, and soffits are not fire-resistant and are therefore susceptible to ignition by embers and hot gases. Once an eave, overhang, or soffit has ignited, fire can spread onto the roof, into the attic, or onto and through the exterior wall. Soffits normally have vents as part of the attic ventilation system. Unprotected vents can allow embers and hot gases to enter the attic .
Eaves: Typical Design and Construction
Eaves are located at the down-slope edge of a sloped roof and serve as the transition between the roof and fascia/wall. An eave typically has a metal edge flashing and gutter that are attached to a wood fascia trim board. Overhangs are extensions of the roof beyond the exterior wall (i.e., the joists, rafters, or trusses and the decking they support cantilever past the wall). An overhang protects the upper portion of the wall that it is attached to from rainfall, and it also shades the windows under it from the sun. Overhangs can be open, in which the trusses/rafters and decking are exposed, or enclosed by a soffit. A soffit encloses the underside of sloped- or flat-roof overhangs. Soffits are commonly constructed from fiber-cement panels, metal panels, stucco, vinyl panels, or wood sheathing. Metal panels, untreated wood panels, and vinyl panels are vulnerable to damage from wildfires. Metal panels conduct heat and can distort and allow passage of embers and hot gases. Untreated wood panels can ignite, and vinyl panels can melt and fall away.
Eaves, Overhangs, and Soffits: Guidance for New Buildings
Consider designing the building without overhangs to avoid the fire-related problems associated with soffits or minimize as much as possible the extent of the overhang to reduce the potential for entrapment of embers and hot gases. If no overhangs or short overhangs are unacceptable because of aesthetics or a desire to protect the walls from rainfall or windows from the sun, enclose the overhangs with soffits that have a minimum 1-hour fire-resistance rating to prevent embers and hot gases from making contact with the joists, rafters or trusses, or the underside of the roof decking. Use flat, horizontal soffits instead of attaching the soffits to the sloped joists, which creates sloped soffits. A flat soffit reduces the potential for entrapment of embers and hot gases. For the fascia, use noncombustible or fire-resistant materials (e.g., fire-retardant-treated lumber, fiber-cement board).
Vent Systems and Vent Openings
Vents can be divided into those for attics; ventilated cathedral ceilings; crawlspaces; and heating, ventilation, and air conditioning (HVAC) systems.
Venting: Key Issues
Embers and hot gases can be blown or pulled into vent openings and enter attic spaces, crawlspaces, and ductwork, leading to ignition of the interior of the building. Debris can accumulate at vent openings and ignite during a wildfire.
Vent Systems and Vent Openings: Guidance for New Buildings
Specify and install noncombustible material for all vents. Metal products are recommended for vents and vent flashing. Specify and install corrosive-resistant, metal mesh screens with a maximum opening of 1/4 inch at all vent openings. Place all vent openings at least 10 feet from other buildings or property lines to avoid ignition from embers and hot gases from an adjacent building that has ignited.
Protecting attic spaces from wildfires is a challenge because air is naturally drawn into attics through vent openings. Although insect screens can prevent the entry of many embers, vent screens and louvers do not prevent the entry of hot gases. Vents that allow air to flow into and out under normal conditions and also avoid the entry of embers and hot gases in a wildfire can be provided in the following ways:
Instead of using ridge vents, one can install gable-end vents with specially designed metal shutters. When a wildfire threatens, the shutters can be placed over the gable-end vent. A hinged shutter that can be latched in an open or closed position is recommended. A detachable shutter design can be used, but when the shutters are needed, the homeowner must remember quickly where they are stored. Shutters should have a gasket that provides a tight seal between the shutter and gable-end vent. For a more conservative shutter, a shutter with an insulated core encapsulated by metal (similar to a refrigerator door) can be used. When gable-end vents are combined with soffit vents, effective attic ventilation can be achieved when the attic space is simple and relatively small, such as a small, gable-roofed house. If the house has a complex roof area or the attic is too large to be effectively ventilated by gable-end vents, ridge vents or through-roof vents should be used.
The serious downside of gable-end venting is that it is not a passive system. Rather, generally it relies on the homeowner to take action to implement the defensive measures. The homeowner, hard pressed in the case of an emergency situation, might want to abstain from having to take active measures to close down their attic venting.
Continuous ridge vents can provide effective ventilation, but typical ridge vents are not highly fire-resistant. Although air is normally exhausted through ridge vents, hot gases can enter the ridge vent and flow into the attic. Embers can also enter through the ridge vent if the vent does not have internal baffle media. Typical ridge vents are therefore not recommended unless the attic configuration or size requires ridge vents to ventilate the attic effectively. If ridge vents are used, metal vents with internal baffle media are recommended, but during a wildfire, the internal baffle material may melt and become ineffective in
at preventing embers from entering.
As with ridge vents, typical soffit vents are not highly fire-resistant, but because they are a necessary element of a vented attic, they cannot be eliminated. To avoid embers and hot gases from entering the vents, specially designed metal shutters over the soffit vent openings should be specified and installed. See the shutter recommendations for gable-end vents above.
Through-roof vents are not recommended for attic ventilation because there is no known effective strategy for avoiding embers and hot gases from entering, other than blocking the vent. A 1/4–inch screen is effective at keeping out firebrands and embers that are larger than 1/4-inch but will not keep out smaller embers or hot air.
The Un-ventilated Attic
The most conservative approach to preventing embers and hot gases from entering the attic is to eliminate attic ventilation, but unventilated attics are controversial. Although allowed by the International Residential Code, provided the Code’s criteria are met, unventilated attics may not comply with local building codes. However, when unventilated attics are allowed by the building code or code compliance is not an issue, and when climatic and interior humidity conditions (e.g., no indoor swimming pools) are conducive to an unventilated design, an unventilated attic is a reliable way to prevent embers and hot gases from entering the attic.
Specify and install specially designed metal shutters over crawlspace vent openings. A 1/4-inch mesh screen should also be installed over the vent opening.
Wall-louvered HVAC System Vents
Specify and install specially designed metal shutters over wall louvers or specify and install wall louvers that have adjustable tight-fitting blades that can be closed when a wildfire threatens. As an additional conservative measure with either shutters or adjustable blades, specify and install fire dampers within the ducts immediately behind the wall louvers. If sufficiently high heat penetrates beyond the louver, the fire damper will automatically close and prevent high heat from penetrating farther.
As discussed in attic ventilation, through-roof vents are not recommended. Rather than running ductwork through the roof, extend the ductwork to an exterior wall where it can be fitted with a wall louver and shutter. If it is necessary to penetrate the roof, however, install a fire damper in the duct at the plane of the roof assembly.
Vent Systems and Vent Openings: Guidance for Existing Buildings
If the home has ridge vents or wall louvers and they are not metal, replace them with metal •
vents according to the guidance provided above. If existing vent openings do not have screens or if the screen openings are larger than 1/4-inch, install metal screens according to the guidance provided above. Install shutters over gable-end vents, soffit vents, crawlspace vents, and wall louvers. If the existing wall or soffit is combustible, shutters may not be effective. In this case, installing shutters is probably cost-effective only if done in conjunction with upgrades to the wall or soffit.
Vent Systems and Openings Considerations
Low-profile, through-roof vents have been used in place of soffit/eave vents with great success. The homeowner should periodically have a professional remove debris that has accumulated near or on vent openings, vent screens, and louver blades. The amount of vegetation near vent openings should be limited. To minimize the possibility that embers and hot gas will be pulled into the home, the HVAC system, including exhaust fans, should be turned off when a wildfire threatens. Attic exhaust fans should also be shut down. Attic exhaust fans that are controlled by a thermostat may need to be deactivated by tripping the circuit breaker.
Windows and Skylights
Glazing in homes is typically vulnerable to wildfire. Flame impingement and radiant heat can be severe enough to melt or break many types of glazing. A single pane of typical residential glass can fail within 5 minutes of exposure to a wildfire. Windborne firebrands can have sufficient momentum to break many types of glazing. The frames for windows, sliding glass doors, and •
skylights are constructed of metal, plastic, wood, or a combination of these materials. Plastic and wooden frames are susceptible to failure from burning or melting. If the frame or sash fails, the entire glazing may fall out.
Windows and Skylights: Guidance on New Buildings and Recommendations
A variety of products are available for glazing in windows, sliding glass doors, door vision panels, and skylights. Glazing can be in a single- or multi-paned configuration. The recommended glazing products for homes in wildfire zones are laminated glass, tempered glass, glass with a low-emissivity, fiberglass-reinforced translucent glazing, and insulated glazing units (IGUs). Glazing products that are not recommended are annealed glass, ceramic glass, and plastic glazing.
Tempered glass: Tempered glass is more resistant to heat and flames than laminated glass or annealed glass. The resistance of tempered glass can be enhanced with a low-e coating or a proprietary reflective coating, as discussed. Firebrands with sufficient momentum can break tempered glass. To avoid breakage, the glass can be protected by shutters, as discussed below. Another alternative is to specify and install an IGU with a laminated glass inner pane.
Glazing: Low-emissivity (low-e) coating
Glass with a low-e coating provides a higher level of resistance to radiant heat than other types of glazing because the coating reflects radiant heat, reducing the probability that the heat will be able to enter the building. The coating should be on the inner surface of the exterior pane.
Exterior doors are subject to the same types of exposure as exterior walls in a wildfire. However, exterior doors are typically much thinner and less fire-resistant than exterior walls and can therefore burn through much faster. Flames and hot gases can ignite •
combustible materials in a door and door frame. Flames and hot gases can penetrate openings between the door and frame and between the door and threshold (or floor if no threshold exists). Embers can become lodged in openings between the door and frame and between the door and threshold (or floor if no threshold exists). Embers can also be blown through the openings into the interior of the building. Flames, convective or radiant heat, and airborne firebrands can break glass in a door.
Exterior Door Characteristics and Ratings
Types of exterior doors include solid entrance doors, entrance doors with glass vision panels, sliding glass doors, storm doors, screen doors, garage doors, and cellar doors. Solid exterior doors are typically made of wood or metal. Doors with a solid, noncombustible mineral core are classified as fire-rated doors and are rated by Underwriters Laboratories (UL) according to the length of time they can resist fire (UL Standard 10C) (UL, 1998). UL classifications for interior and exterior fire-rated doors and their frames range from 3-hour to 20-minute ratings. Exterior fire-rated doors are available with a rating of 1½ hour or 3/4 hour. The fire rating for doors is intended to equal three-fourths of the fire rating of the surrounding wall. For example, a door with a 1½-hour rating is intended to be used in a wall with 2-hour rating, and a door with a 3/4-hour rating is intended to be used in a wall with a 1-hour rating. However, a door with a higher fire rating may be used.
Doors: Guidance for New Buildings
If a fire-rated exterior wall is specified, specify and install a fire-rated door and frame. As explained above, the rating of the door and frame should be at least three-fourths of the rating of the wall. In addition, specify and install fire-rated hardware. To avoid embers and hot gases penetrating the interior of the building between the door and the door frame, install adjustable weatherstripping on the interior side of the door frame and specify and install an automatic door bottom or threshold weatherstripping. The weatherstripping and door bottom should be tested in accordance with UL Standard 10C. Weatherstripping is relatively inexpensive.
Garage doors are typically made of wood, aluminum, or steel and are insulated or non-insulated. Unlike standard egress/ingress doors, garage doors are not normally tested for fire resistance. To protect the garage door and entire building, specify and install weatherstripping that has been tested in accordance with UL Standard 10C around the entire garage door. For exterior trim that covers the opening between the door frame and exterior wall, specify and install noncombustible or fire-resistant material such as fire-retardant-treated wood or fiber-cement board. Metal garage doors and metal-clad door frames can transmit heat during a fire, and the heat can ignite the surrounding exterior wall if the wall is not constructed to fire-resistant standards
Doors: Guidance for Existing Buildings
Add weatherstripping to doors, as described above. Replace vision panels in doors, if necessary. Replace sliding glass doors and/or protect with shutters. Replace wooden garage doors, particularly if they do not have a solid core. Replace wooden egress/ingress doors without a solid core, although egress/ingress doors often relatively fire-resistant compared to other components of the building and therefore not normally a high priority for remediation.
For Further Reading:
- The NFPA (National Fire Protection Agency) has published recommended strategies for existing construction. The link is: https://www.nfpa.org/Public-Education/Fire-causes-and-risks/Wildfire/Wildfire-safety-tips
- Headwaters Economics has published an article describing cost/benefits at: https://headwaterseconomics.org/wp-content/uploads/building-costs-codes-report.pdf
- The University of California has published an article on constructing a safe home in wildfire-susceptable locations. The link is: https://ucanr.edu/sites/postfire/Homeowners_Guide_to_Recovering_from_Wildfire/Constructing_Fire_Safe_Homes/