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.
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.
Fire ratings for roof coverings are either ‘A’, ‘B’, ‘C’ or ‘nonrated’.
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.
Igniting an ‘A’ brand prior to placing on the roof test deck. Each face and edge of the brand is exposed to the gas flame for specified time intervals.
There are two ways for a roof to have a Class A fire rating:
This photograph shows an asphalt fiberglass composition ‘three tab’ shingle (‘asphalt comp’ shingles). It has a ‘stand alone’ Class A rating, and therefore doesn’t 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).
These are two photos taken at the beginning of the test (upper left) and at the end of the test (lower right). The standard test procedure calls for two joints in the sheathing. Flame and smoke can be seen coming through the horizontal joint at the bottom of the deck. The vertical joint can be seen above the stud in the upper portion of the lower right photograph.
An ‘A’ brand in burning on top of the roofing. Fire coming through the bottom of the deck indicates that, as built, this isn’t 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 doesn’t 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.
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.
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.
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’.
A pressure impregnated fire retardant treated wood shake roof has a ‘stand alone’ Class 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.
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.
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.
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.
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.
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.
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.