University of California
Fire in California

Roof

DISCLAIMER: Information in this section was compiled as part of the Building in Wildfire Prone Areas project in 2009. This information may or may not be up to date. More recent information may be available from the Insurance Institute for Business and Home Safety, the California Wildland-Urban Interface Code Information, and the CAL FIRE Wildland Hazard/Building Codes site.

Fire Rating Classification and California Code Requirements

Fire ratings for roof coverings is based on test methods developed by standards writing organizations. These include 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 standards evaluate three fire-related characteristics of a roof covering, including 1) the ability to 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.

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This is a diagram of the test apparatus used in the ASTM E-108 test. It is a small wind tunnel. The view shown here is for the penetration test, consisting of a 3 ft by 4 ft test deck at the (left) end. The distance between the end of the tunnel and the test deck is specified, as is the location of the burning brand on the test deck and the air flow velocity.

The test deck for the spread of flame test is a different size.

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Fire ratings for roof coverings are either A, B, C or nonrated.

  • A - brand: 12x12 three layer sandwich made from square Douglas-fir sticks
  • B - brand: 6x6 three layer sandwich made from square Douglas-fir sticks
  • C - brand: Small piece of Douglas-fir, about the size of an ice cube

In the brand portion of the ASTM E-108 test one A or one to two B brands will be ignited and placed on the sample roof deck during the test. Multiple C brands are placed on the roof deck during the course of the test.

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  • From the covering alone (sometimes called a stand alone Class A roof)
  • From the covering and underlying materials (sometimes called Class A by assembly)

This photograph shows an asphalt fiberglass composition three tab shingle (asphalt comp shingles). It has a stand alone Class A rating, and therefore doesnt depend on underlying materials for its fire rating. An asphalt composition shingle with organic (wood) fiber has a Class C fire rating. Asphalt comp shingles in California contain fiberglass.

An assembly rated Class A roof will typically incorporate additional fire resistant materials. The added material can itself be a stand alone Class A, but not necessarily so. The main requirement is that it provide sufficient fire resistance to allow the roof covering to pass the tests associated with ASTM E-108. The underlying material really only help with the fire penetration part of the test (and not the flame spread and ember generation sections).

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An A brand in burning on top of the roofing. Fire coming through the bottom of the deck indicates that, as built, this isnt a Class A roof covering.

This cement-based (noncombustible) roofing product is flush with roofing paper and sheathing, and shows the importance of the performance of the entire roofing assembly. As a noncombustible material, it doesnt technically have to be tested comply since it is noncombustible, but in this case the performance was dependent on the type of sheathing used. Oriented strand board (OSB) was used as the sheathing material. If plywood were used instead, the assembly would have passed. Heat transfer (by conduction) through this product resulted in sufficient temperature rise in the OSB sheathing to result in flaming combustion.

Although this product is no longer commercially available, installation instructions clearly stated that it should be installed over plywood sheathing.

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Chapter 15 in the California Building Code (and International Building Code) specify that certain roofing materials are Class A. These materials include slate, clay, concrete roof tile, an exposed concrete roof deck, and ferrous (i.e., steel) and copper shingles. These products won't have to test to ASTM E-108.

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Chapter 15 states that ferrous and copper shingles are Class A (stand alone). This means that non-ferrous, non-copper metal roof coverings, such as aluminum will have to test to ASTM E-108. As it turns out, it has a Class A assembly rating. Even though an aluminum-based roof covering is noncombustible, it has a low melting point and therefore can only obtain a Class A rating by adding an additional material that enhances fire performance.

In this photograph, the burning A-brand melted the aluminum roofing material, and is sitting on top of the cap sheet material. The fire performance of the metal roofing is dependent on the performance of the cap sheet material (or other material that is used to enhance fire performance). The burning A-brand melts the aluminum roofing material, and sits on top of the cap sheet material for most of the test.

For assembly rated roof coverings, it is important to carefully follow manufacturers instructions regarding both installation and materials used in the assembly. The fire rating was obtained using a specific set of materials deviating from the specified materials list could compromise the fire rating of the roof covering.

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A Type 72 cap sheet material sold in roofing supply stores is commonly used as a commodity item. If enhanced fire performance is required in a given application, you should look for a label on the package similar to that shown here. This material is tested to an external fire exposure. These materials typically do not offer Class A performance by themselves, but are commonly used in Class A assemblies.

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A pressure impregnated fire retardant treated wood shake roof has a stand aloneClass B rating. With an underlying fire-barrier material, it can attain a Class A rating based on the assembly [the Class B shake, plus the underlying material to enhance fire performance]. Fire retardant treated wood shakes must successfully complete a natural weathering exposure, and subsequently pass ASTM E-108 fire tests, before being sold in California. Successfully completing the natural weathering cycle gives the treated shake an exterior use rating.

The white sheet material shown here is this photograph is a proprietary gypsum / fiberglass product called DensDeck. This product has a stand-alone Class A rating. With assembly rated roof coverings, it is important to build the roofing assembly using the same materials and details that were used when testing for compliance. This information will be included in the installation instructions.

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Chapter 7A works in conjunction with Chapter 15 of the California Building Code. Chapter 15 requires a Class A roof covering in Very High Fire Hazard Severity Zones (VHFHSZ), Class B in High FHSZs and Class C in Moderate FHSZs. Chapter 7A doesn't change this requirement.

Chapter 15 addresses roof field issues. Chapter 7A addresses roof edge issues.

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Large gaps between the roof covering and roof deck (sheathing) have to be plugged. The most common example of this kind of gap is in a clay-barrel tile roof. Plugging is often called "bird stopping". With non-bird stopped roofs the protection offered by a noncombustible / Class A roof covering can be bypassed. In this case, a bird can access the space between the roof covering and the sheathing, and build a nest. During a wildfire, embers can penetrate the roof edge and ignite the nest or other debris in that cavity. Gaps at the ridge must also be filled.

A mortar type compound is used to plug the gaps on this roof.

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This tile roof used a manufacturer provided end-piece for a bird-stop. Note the missing stop. Although Chapter 7A can't enforce conducting routine maintenance, it is a critical component to maintaining a fire safe home.

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Gaps don't typically occur at the roof edge when flat tiles are used, but as seen in this photograph, can occur at the ridge. These gaps should also be plugged.

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Another roof edge detail that is address in Chapter 7A is the valley. The provision applies to all roof covering materials. If metal valley flashing is used, an underlying Type 72 cap sheet material must be incorporated into the assembly. With Class A asphalt composition shingles, use of metal flashing can be avoided by interweaving the shingles. A cut valley would also be ok.

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Class A asphalt composition shingles, the upper left with a woven valley and lower right with a cut valley. Type 72 cap sheet material isn't required in either case.

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Complex Roof Issues


Roofs that have locations where roof meets wall surfaces are referred to as 'complex roofs'. These intersections are locations where debris can accumulate, and also embers. The wall surfaces may not provide the same resistance to fire as the roof covering.

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The shingle siding shown in this photograph is far more vulnerable than the Class A asphalt composition roof covering. Note debris accumulation on the roof in area adjacent to the shingle siding. If the debris is ignited by embers, the resulting fire will expose both roof and siding. This is another way that the fire protection provided by a Class A roof can be by-passed.

In this case, removal of debris on a regular basis is a critical component to maintaining a fire safe building.

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This is an example of a complex vegetative (green) roof. The front of the dormer has a window in it. If the roof is ignited by embers, the fire won't likely penetrate through soil layer, but it could spread laterally up the roof, potentially resulting in a flame impingement exposure on adjacent material. In this case, a flame impingement exposure on the window could result in glass breakage, and ignition of interior components.

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Another example of a complex roof. In this if embers ignited the roof debris, flame would impinge on the siding, and flame and embers could enter the attic through the gable end vent. Again, removing debris from the roof should be a routine maintenance item.

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Skylights

Besides the glass or plastic component of a skylight, what other factors would make it vulnerable to wildfire exposures?

Take a careful look at and around your skylights, and observe where debris accumulates. If that debris is ignited, what portion of the skylight would the flame impinge on? The best solution would be to keep debris cleared away from the skylight. Normally debris doesn't stay on the domed skylights, but can stay on the flat ones and on the uphill side of the skylight.

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The operable skylight shown in this photograph has a domed acrylic shell. It was installed in the late 1980's and does not have an inner layer of flat glass, but since it can be opened, has a screen at the top of the skylight frame. Similar to windows, an open skylight would be vulnerable to the entry of embers. They should be closed when wildfires are burning nearby.

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Skylights that contain a layer of tempered glass over a layer of labeled fire resistant polished wire glass (so, two different layers of glass) would be a good option to improve their wildfire resistance. Skylights with this construction are commercially available.

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Besides the glass or plastic component of a skylight, what other factors would make it vulnerable to wildfire exposures?

Take a careful look at and around your skylights, and observe where debris accumulates. If that debris is ignited, what portion of the skylight would the flame impinge on? The best solution would be to keep debris cleared away from the skylight. Normally debris doesn't stay on the domed skylights, but can stay on the flat ones and on the uphill side of the skylight.

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Another photograph showing the roof of a home in a forested areas and showing the accumulation of vegetative debris on the roof and around the skylights.

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Does the roof have a steep slope? If the answer is 'yes', what kind of radiant exposure could the skylight 'see' (potentially from vegetation, or other nearby by buildings)? The potential severity of the radiant exposure would indicate how likely the glass would be to break.

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As is always the case, vegetation management should be part of any solution. If tree branches overhang the roof, and specifically the skylight, they should be removed (even without considering a fire exposure, if an overhanging branch should break, it could break through the glass in the skylight). In this photograph, overhanging vegetation resulted in debris accumulation on top of these flat skylights.

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This is a photograph of a skylight in an untreated wood shake roof. The wood shake roof is by far the more vulnerable component on this home.

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