By Gregory Havel
A recent review of the Web sites and the printed advertising of several “big box” home improvement stores showed roofing materials promoted as “Class A and Class B Fireproof.” This kind of advertising is inaccurate and can lead to roofing materials of the wrong type being installed in the wrong application.
Underwriters Laboratories (UL) 790 and American Society for Testing and Materials (ASTM) E108 use the same test methods and procedures for testing the resistance of roof coverings to fire sources outside the building on which the roof coverings are installed. These tests apply to roof coverings on either combustible or noncombustible decks when the roof coverings are installed as designed. These tests are the ones most frequently referenced in building and fire codes in North America.
Roof coverings are classified as Class A, Class B, Class C, or “unrated” if the roof covering does not meet the requirements for any of the other classes.
Class A is the best rating, offering the greatest resistance to fire, and “unrated” offers the least resistance to fire. Class A roof coverings include slate, concrete or clay tiles; fiberglass asphalt composition shingles; and some metal roofs. Untreated wood shakes and shingles are often “unrated,” although impregnation with fire retardant chemicals can give them a Class B fire resistance. Including additional fire-resistive materials in the roof assembly can give it a Class A rating.
UL 790 and ASTM E108 classify the roof’s performance by evaluating the following:
- Flame penetration through the roof covering and deck into the attic space.
- Flame spread on the surface of the roof covering.
- The likelihood for the roof covering to generate embers.
Class A roof coverings protect against severe fire test exposures; they provide good protection to the roof deck, do not slip from position, and are unlikely to produce flying brands.
Class B roof coverings protect against moderate fire test exposures; they provide moderate protection to the roof deck, do not slip from position, and are unlikely to produce flying brands.
Class C roof coverings protect against light fire test exposures; they provide a smaller degree of protection to the roof deck, do not slip from position, and are unlikely to produce flying brands.
“Unrated” roof coverings are those that have not been tested or have failed testing for even a Class C rating.
These tests demonstrate the performance of roof coverings during the types and duration of fire exposures involved in the testing. They are not intended to prove that the roof coverings will be undamaged during the tests since even a Class A roofing material can be damaged by fire. They are not intended to compare the expected performance of the roof coverings under all types of actual fire conditions.
The most common type of fire source outside a building is the “flying brand.” The brand in photo 1 was generated during the burn-down phase of live-fire training in an acquired structure according to National Fire Protection Association 1403, Standard on Live Fire Training Evolutions, and which landed on asphalt pavement about 50 yards from the burning building. It measured three inches (7.65 cm) from lower left to upper right, five inches (12.7 cm) from lower right to upper left, and about 1¼ (3.15 cm) inches thick.
(1)
Photo 2 shows the comparative sizes of the brands used in testing roofing materials before ignition.
(2)
- Class A brands measure 12 x 12 x 2¼ inches (30.5 x 30.5 x 5.7 cm), weigh 4.4 pounds (2,000 grams), and are made of three layers of 12 wood strips stapled together perpendicular to one another.
- Class B brands measure 6 x 6 x 2¼ inches (15.25 x 15.25 x 5.7 cm), weigh 1.1 pounds (500 grams), and are made of three layers of six wood strips stapled together perpendicular to one another.
- Class C brands measure 1.5 x 1.5 x 0.781 inches (38 x 38 x 19.8 mm), weigh 3.26 ounces (9.25 grams), and are made of solid wood with saw kerfs in opposite faces cut perpendicular to each other.
The sizes of the Class A and Class B brands are larger than those commonly carried by wildfires.
For the test, the test brand is placed on the surface of the roof covering that is to be tested on a combustible or noncombustible roof deck as determined by its design, with the bottom edge of the brand 12 inches (30.5 cm) above the bottom of the roof’s slope. A ducted fan is set up blowing air upslope at a speed of 12 mph (19.2 km/h). The burning brand sample is monitored, photographed, and videoed for up to 1.5 hours, until the brand burns out, or until the roof assembly fails.
The distance that the fire spreads on the roof covering from the edges of the brand is also measured and recorded. Class B and C roof coverings are permitted larger flame spread distances than Class A.
Failure of a roof covering on a combustible roof deck is the point at which the roof deck ignites. Failure of a roof covering on a noncombustible roof deck is the point at which fire penetrates through the roof deck. Failure of a roof covering on any type of deck can also result from flame spread on the roof covering that is too great for the classification being tested.
Roof coverings on combustible roof decks may be required to be tested three times since combustible construction materials add variables that are not present on noncombustible roof structures.
Roof coverings have no labels on individual shingles or shakes. The only indication of the fire test classification of the roof covering is the label on the bundle (photo 3) or roll. Similar information may be included on the invoice from the building materials supplier. Building owners should be encouraged to keep copies of these documents for future reference by building officials and insurance agents.
(3)
Code enforcement officials should work toward a requirement for at least Class B (and preferably Class A) roofing materials in neighborhoods where buildings are close together and are prone to fire spread by brands as well as in areas where wildfires are a threat. In some areas of North America, this has already been accomplished; in other areas, it is still a work in progress.
Firefighters must keep in mind that a Class A roof is no stronger than the roof deck and the rafters or trusses that support it; the roof’s Class A rating has no meaning during a fire in the attic. These roof classifications are a rating system for exposure to fire from the exterior of the roof. They do not reflect its resistance to a fire inside the building or its structural strength for vertical ventilation.
The author would like to thank Stephen Kerber P.E., director of the Firefighter Safety Research Institute and UL, for his assistance with the research for this article.
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Gregory Havel is a member of the Town of Burlington (WI) Fire Department; retired deputy chief and training officer; and a 35-year veteran of the fire service. He is a Wisconsin-certified fire instructor II, fire officer II, and fire inspector; an adjunct instructor in fire service programs at Gateway Technical College; and safety director for Scherrer Construction Co., Inc. Havel has a bachelor’s degree from St. Norbert College; has more than 35 years of experience in facilities management and building construction; and has presented classes at FDIC.
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