Roof

Fire hazard – Roofs

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The roof of a house protects it from the elements and the weather. Its large surface area, however, makes it extremely vulnerable to embers during a fire. Specifically, debris can accumulate close to roof-wall intersections, edges, skylights, and vents, and ignite during a fire. Complex roofs, where the roof meets vertical walls (e.g., a roof with dormers, or a split-level house) present additional vulnerabilities that must be addressed. As a general rule, roofs will ignite at edges and intersections, so these areas need to be considered.

The more intersections and shapes included in the roof design, the more opportunity there will be for leaves, needles, and other vegetative debris to accumulate. If the siding is installed close to the roof, the siding could be ignited directly by embers (i.e., without initial ignition of vegetative debris).

How can you protect your roof?

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Because of the variety of roofs and designs, here we provide some guidelines based on the different roof components. However, the most important thing is to keep your roof free of debris and combustible materials.

Valley (where two roof plains intersect). Leaves and needles often accumulate in these areas. It is possible to increase the fire resistance of the valley by using metal flashing, but in this case a mineral-surfaced cap sheet must be incorporated into the assembly. Metal flashing can be avoided with Class A asphalt composition shingles by interweaving the shingles. Cut valleys can also be used.

For dormers and other exposed siding, an underlayment of fire-rated gypsum can increase its resistance to flame penetration. Type X is the product that many gypsum board manufacturers produce for this application. Corrosion-resistant metal flashing can be used at roof-to-wall intersections when combustible siding products are used to reduce the likelihood that embers can ignite the siding. Detailed installation is crucial to avoid moisture-related.

Skylights can provide an entry point for embers. If they can open, make sure they are shut during a wildfire. Skylights are vulnerable to fire depending on the slope of the roof, the location and proximity of nearby combustible vegetation, and the accumulation of leaves and other vegetative debris on and around them. Skylight domes can break with flame impingement, leaving the inside of the house completely exposed. Make sure that debris is kept away from the skylight. The branches of trees that overhang the skylight should be removed in order to prevent them from falling and breaking the glass or plastic components. 

The roof edge is vulnerable to wildfire exposures via 1) a debris-filled gutter (see gutters section for more information), 2) gaps between the roof covering and the roof sheathing, and 3) when a metal drip edge is not installed. 

The attic and crawl space vents are also possible entry points for embers. The vents section discusses risks and mitigations in more detail.

Solar panels on the roof can become vulnerable to wildfires if debris accumulates underneath them. Plastic components on the underside of solar panels would be vulnerable to flames, while the surface of the panels would burn only under very high heat fluxes (i.e. the entire house is on fire). As discussed before, the removal of debris from the roof should be an ongoing maintenance task.

California regulations

The roof covering should be built to prevent the intrusion of flames and embers. According to Chapter 7A of the California Building Code:

  • Roofs shall have a roofing assembly installed in accordance with its listing and the manufacturer’s installation instructions. 
  • Where the roof profile allows a space between the roof covering and roof decking, the spaces shall be constructed to prevent the intrusion of flames and embers, be firestopped with approved materials or have one layer of minimum 72 pound (32.4 kg) mineral-surfaced nonperforated cap sheet complying with ASTM D 3909 installed over the combustible decking.
  • Where valley flashing is installed, the flashing shall be not less than 0.019-inch (0.48 mm) No. 26 gage galvanized sheet corrosion-resistant metal installed over not less than one layer of minimum 72 pound (32.4 kg) mineral-surfaced nonperforated cap sheet complying with ASTM D 3909, at least 36-inch-wide (914 mm) running the full length of the valley.

Local additions to the code may exist in your community.  Your local authorities can provide you with more information.

Products and assemblies that have complied with Chapter 7A can be found in the WUI Product Handbook. Since “ignition resistant” and “non-combustible” are performance-based characteristics (i.e., there is a standard test to show compliance), these materials are included in the Handbook.

What does fire rating mean?

Fire ratings for roof coverings are based on a standard test method that evaluates three fire-related characteristics of a roof covering, including the ability to 1) resist the spread of fire into the attic (or cathedral ceiling) area, 2) resist flame spreading on the roof covering, and 3) resist generating burning embers. This basic standard test method is provided in the American Society for Testing and Materials (ASTM) Standard E-108, Underwriters Laboratory (UL) Standard 790, and National Fire Protection Association (NFPA) Standard 276. These standard test methods are similar, so there is no need to worry about which test was used to evaluate your roof covering.

Roofs with Class A ratings provide the highest protection and those with a Class C rating provide less protection. Non-fire retardant treated wood shakes are not rated (i.e., their rating doesn’t meet the requirements for Class C) and should not be used in wildfire prone areas. Class A materials include flat or barrel-shaped roof tiles, fiberglass asphalt composition shingles, and metal roofs (i.e., steel or copper).  Fire-retardant pressure-treated shakes and shingles are the most common Class B roofing material.  Class C fire rated roof coverings include recycled plastic and rubber products and roof aluminum coverings. Class B and C roof coverings can typically meet Class A requirements when additional underlayment materials are used.

Details matter: stand-alone or assembly-rated?

Some roof coverings rely on an underlying material, or special installation techniques, to improve their fire rating. Fire ratings for roof coverings that require an additional material are typically referred to as a "by-assembly," as opposed to a "stand-alone," fire rating. Examples of a “by assembly” fire rating include fire-retardant treated wood shakes, aluminum and recycled plastic and rubber roof coverings. Common underlying materials include a mineral surfaced cap sheet (formerly referred to as a Type 72 cap sheet) adequate to provide Class A membrane construction, and a panelized fiberglass gypsum board.

Examples of roof vulnerabilities

Class A asphalt composition shingles in a woven valley
Woven valley. Class A asphalt shingles were used for this roof covering. It is not necessary to use a mineral surfaced cap sheet material since the shingles can be interwoven.
Valley with metal flashing
Woven valley with metal flashing. This roof was covered with Class A asphalt shingles. The use of a metal flashing in the valley requires the use of a cap sheet material in the valley, underlying the metal flashing.
Complex roof design with wood shingle siding
Complex roof. This photo shows a complex roof. In this case, the wood shingle siding is far more vulnerable than the Class A asphalt shingle roof covering. The installation of metal flashing or non-combustible siding is recommended.
Roof with debris accumulation
Accumulation of leaves on complex roof. Note debris accumulation on the roof in the area adjacent to the shingle siding. This debris can be ignited by wind-blown embers. The resulting fire would cause flaming exposure to both roof and siding. Removing debris from these rooves should be a routine maintenance item and is critical to the home’s fire resilience.
Tempered glass skylights
Flat skylight (tempered glass). Skylights constructed of tempered glass are more resistant to radiant heat. However, they could still break when exposed for long times or if they get hit by flying debris. Pine needles and vegetation should be removed from the proximity of the skylight.
Domed skylight with debris accumulation
Skylights with leaves accumulation. In this case, the skylights would be the most vulnerable parts of the roof. This picture demonstrates the vulnerability of roofs to leaf accumulation. Maintenance should be performed on a regular basis.
Solar panels installed on a flat roof with accumulated debris
Solar panels on flat roof. Debris can accumulate under solar panels, especially on flat roofs. Any accumulated debris should be removed from under solar panel because their ignition could result in a flaming exposure to a portion of the panel. Installation of solar panels on a flat roof can be accomplished with a weighted mounting system (known as a ballast system) rather than a bolted connection.
Solar panels installed on a sloped roof
Solar panels on sloped roof. Solar panels installed on a sloped roof will have a mechanical fastening system (e.g., bolted through the roof covering to the underlying roof sheathing). It is important to keep the area clean from debris.