Roofing Materials Fire Ratings Explained: Class A, B, C

What the Fire Rating Classes Mean

Roofing fire ratings are determined by three standardized tests defined in ASTM E108 and UL 790. Each test measures a different aspect of fire resistance, and the material must pass all three tests at the appropriate severity level to earn its class rating.

Burning brand test: A burning ember of specified size is placed on the roof surface. Class A uses a 12-inch by 12-inch brand (simulating a large piece of flaming debris landing on the roof), Class B uses a 6-inch by 6-inch brand, and Class C uses a 1.5-inch by 1.5-inch brand. The test determines whether the brand ignites the roofing material and whether fire penetrates through to the deck below.

Spread of flame test: A gas burner is applied to the edge of the roof assembly on a slope, and the distance that flames spread across the surface is measured. Class A allows minimal flame spread, Class B allows moderate spread, and Class C allows greater spread before the test concludes.

Intermittent flame test: A gas flame is applied to the roof surface and then removed in repeated cycles. This simulates the intermittent exposure that occurs during a wildfire as wind gusts push flames across a neighborhood. The test measures whether the material reignites after the flame source is removed.

The critical point is that these are assembly tests, not material tests. The fire rating applies to the entire roof assembly including the roofing material, underlayment, and deck as they are installed together. A material that earns Class A when installed over a noncombustible deck may earn only Class B when installed over a plywood deck, depending on the specific assembly configuration.

Fire Ratings by Material

Metal roofing (all types): Class A. All metal roofing products are noncombustible by nature. Steel, aluminum, copper, and zinc cannot ignite, burn, or contribute to flame spread under any conditions. Metal automatically earns Class A in any assembly configuration. This inherent noncombustibility makes metal the safest roofing choice in fire-prone environments.

Natural slate: Class A. Slate is metamorphic rock that has already survived geological temperatures and pressures far exceeding any fire exposure. It is completely noncombustible and earns Class A in all configurations.

Clay and concrete tile: Class A. Both materials are fired or cured at high temperatures during manufacturing and are noncombustible in their finished form. Clay tile is fired at 2,000+ degrees, making it impervious to any residential fire temperature.

Asphalt shingles: Class A (most products). Modern fiberglass-based asphalt shingles achieve Class A ratings through the combination of the fiberglass mat (which is noncombustible) and the ceramic granule surface (which protects the asphalt from direct flame exposure). Nearly all asphalt shingles sold today carry Class A ratings. The one exception is some organic-mat asphalt shingles (made with cellulose rather than fiberglass), which achieve only Class C. Organic-mat shingles have been largely discontinued in the U.S. market.

Composite and synthetic shingles: Class A. Composite products achieve Class A through fire-retardant additives in the polymer matrix. The fire retardants prevent ignition and flame spread during standard testing conditions. While the base polymer could theoretically burn under extreme sustained heat, the fire-retardant formulation meets Class A performance requirements.

Wood shake (treated): Class A or Class B. Pressure-impregnated fire-retardant treated wood shake can achieve Class A or Class B ratings depending on the specific treatment process and the assembly configuration. The treatment introduces fire-retardant chemicals (typically phosphate or borate compounds) deep into the wood fibers, reducing the material's ability to ignite and sustain combustion. Treated wood shake costs $1.00 to $2.00 more per square foot than untreated, and the treatment must be reapplied if the shingles are refinished.

Wood shake (untreated): Class C or unrated. Untreated natural wood is combustible. Raw cedar shake can achieve Class C in some assembly configurations (particularly when installed over a fire-resistant underlayment on a noncombustible deck), but in other configurations it fails to achieve any fire class. Many building codes in fire-prone areas do not accept Class C roofing, effectively banning untreated wood.

Building Code Requirements

The International Building Code (IBC) and International Residential Code (IRC) require Class A, B, or C roofing on all residential buildings, with specific requirements varying by location and fire risk zone. Many jurisdictions that have experienced significant wildfire activity have adopted stricter requirements.

California's Chapter 7A of the Building Code requires Class A roofing in all Wildland-Urban Interface (WUI) zones, which covers most of the mountainous and foothill areas of the state. Colorado, Oregon, Washington, Nevada, Utah, Arizona, and Montana have similar WUI requirements in designated fire hazard zones. Some municipalities have adopted Class A requirements across their entire jurisdiction regardless of specific wildfire zone designation.

Before selecting a roofing material, verify the fire rating requirements for your specific property. Your local building department can confirm the applicable fire zone designation and the required fire class. This is especially important in Western states where fire zone maps are updated periodically as wildfire risk assessments change.

Insurance Implications

Homeowners insurance companies in fire-prone regions increasingly tie coverage availability and pricing to roof fire ratings. Some insurers in California, Colorado, and other high-risk states will not write new policies on homes with Class C or unrated roofing. Others offer premium discounts of 5 to 15 percent for Class A roofing, particularly metal or tile, which provide the highest fire resistance.

The insurance market for wildfire-prone properties has tightened considerably since 2020, with several major insurers withdrawing from high-risk California ZIP codes entirely. For homeowners in these areas, a Class A roof is not just a safety improvement but a prerequisite for maintaining affordable insurance coverage. Upgrading from a wood shake or lower-rated roof to a Class A metal or tile roof can be the factor that determines whether an insurer will renew your policy.

Practical Fire Safety Beyond the Rating

The fire rating of the roofing material is important but does not tell the complete fire safety story. The surrounding conditions matter as much as the roof itself. Embers from a wildfire travel on wind currents and settle in gutters, roof valleys, and against vertical surfaces. A Class A roof covered in dried pine needles and leaves is still vulnerable to ignition at the debris layer, not the roof surface.

Maintain defensible space around your home by clearing vegetation, keeping gutters clean of organic debris, and ensuring that the area where the roof meets walls and other vertical surfaces is free of combustible accumulation. Screening gutter guards, mesh over attic vents (1/8-inch or smaller openings), and enclosed eave soffits prevent ember intrusion into the roof and attic space.

For homes in the highest wildfire risk zones, the combination of a Class A metal or tile roof, ember-resistant vents, noncombustible gutters, and maintained defensible space creates a comprehensive fire-resistant envelope that dramatically improves the home's survival probability during a wildfire event.