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The ABCs of Roof Fire Ratings
Roofing systems must meet or exceed building codes and insurance requirements. Fire ratings may pertain to resistance to fire from above the roof system (the familiar Class A, B, or C ratings), or fire exposure from the building interior (the underside of the roof deck).
How Are Roofs Rated?
ASTM E108 defines fire test methods for roof coverings. These tests may be conducted at UL Inc., FM Global, or any other certified testing laboratory. E108 defines the following conditions:
Over non-combustible roof decks such as steel, poured gypsum, or concrete, only the spread of flame on the top surface of the roof system is evaluated. The maximum flame spread is 6 feet for a Class A rated roof, 8 feet for Class B, and 13 feet for Class C. The slope of the test specimen is preselected, and since steeper slopes are more of a challenge due to melting material feeding the fire, the rating applies to the maximum slope passed.
Gravel-surfaced built-up roofing and ballasted single-ply systems usually meet Class A, while mineral surfaced roofings may be Class B. Some unsurfaced systems, such as asphalt glazing, may be Class C or even unrated.
If the roof deck is combustible, such as wood, plywood, or OSB, two additional tests must be conducted. Both are burn-through tests, and the ultimate roof systems rating is the lowest of the three tests.
- The burning brand: The brands consist of an ignited wood lattice placed directly on top of the test specimen. The Class A brand weighs 2,000 grams, Class B 500 grams, and Class C 9-1/4 grams. Failure is defined as the point where the roof deck ignites.
- The intermittent flame test: A gas flame is cycled on and off. To meet Class A, the test specimen must resist 15 cycles, Class B eight cycles, and Class C three cycles.
Fire resistance ratings (time-temperature) include structural elements and everything above, including the roof membrane and its surfacing. A one-hour rating would mean the structural elements have not yet reached their yield point when exposed to a under-deck heat load defined in ASTM E119 for steel structural members as reaching 1,070 degrees F.
Interior fire hazard is evaluated by FM Global and/or UL using differing test procedures. Both relate to a major interior building fire in Livonia, MI, back in 1953. In that case, an insulated steel building that might be expected to be highly fire-resistant failed catastrophically. Heat from an under-deck fire melted and vaporized the asphalt used to adhere the thermal insulation to a steel deck, feeding the fire. Some 30 acres of building burned to the ground.
PageBreakBetter Standards Rise from Livonia’s Ashes
To understand better what happened in that fire, a test building was constructed, replicating the Livonia structure. The building, known as the “White House” (from where the “White House Test” is derived) was 100 feet long and utilized purlins, decking, and other parts that exactly matched the Livonia building. An under-deck fire was ignited at one end of the building, and the progression of the fire closely observed. The fire moved from end to end in 10 minutes. A new deck providing more limited fuel burned end to end in 12 minutes, and a roofing and deck system with no vapor retarder and only narrow ribbons of asphalt used to adhere the thermal insulation to the deck burned in 13 minutes.
The combustibility of all of these assemblies was considered unacceptable. When mechanical fasteners were used to attach the thermal insulation directly to the metal deck, the fire only spread 60 feet down the building during the 30-minute test burn. This became, and still represents, an acceptable level of fire risk. Had the Livonia fire burned at that slower rate, the building’s fire brigade could have controlled the fire and saved the building.
The next step was to develop a laboratory scale fire test that replicated the “White House” and Livonia constructions.
FM Global uses a furnace (calorimeter) that measures heat released by the test specimen, with no more than 400 BTU per minute released during the steepest three minutes of the temperature curve. Successful constructions over steel roof decks are designated by FM Global as Class 1.
To establish burning on the underside of a test deck. UL uses a 25-foot-long test chamber called a Steiner tunnel and measures the progression of flames on the under-side of the roof deck. UL’s allowed criterion is that flames shall progress no more than 10 feet down the tunnel in 10 minutes and 14 feet during the entire 30-minute test. Successful test constructions are found under the classification of insulated metal decks.
Class A Doesn’t Mean Grade A
These tests pertain only to fire performance. In many cases, a thinner roof system will have better fire performance because it contains less fuel to feed a fire. However, in terms of durability, a thicker membrane (i.e. more plies or a thicker single ply membrane) may well be a better, more durable roof.
Richard (Dick) L. Fricklas was technical director emeritus of the Roofing Industry Educational Institute prior to his retirement. He is co-author of The Manual of Low Slope Roofing Systems and continues to participate in seminars for the University of Wisconsin and RCI Inc., the Institute of Roofing, Waterproofing, and Building Envelope Professionals. His honors include the William C. Cullen Award and Walter C. Voss Award from ASTM, the J. A. Piper Award from NRCA, the William C. Correll award from RCI, and the James Q. McCawley Award from the MRCA. Dick holds honorary memberships in both ASTM and RCI Inc.
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