Understanding Roofing Fire Ratings

In a recent roofing column, we focused upon technical information that could be found on roofing material wrappers that could verify compliance with various building codes. This month, let’s take a closer look at one particular type – fire ratings.

Class A Does Not Mean Grade A
Roofing systems are rated by realistic fire tests. We must emphasize, however, that individual roof components are not rated at all! Our roof systems are rated by code for external fire resistance, under-deck exposure, and time-temperature resistance. One goal of this column will be to increase your understanding of each of these ratings, and what the rating does and does not do for your peace of mind.

External Fire Exposure—Class A, B or C
In catastrophic fires such as the great Chicago or London fires, close proximity of adjacent buildings coupled with combustible roofs resulted in block after block of each city being consumed in flames. Codes today take proximity into consideration, as well as building use and occupant density, when establishing fire resistance requirements.

According to ASTM Test Method E-108, the highest degree of fire resistance is designated Class A, with B being slightly inferior and Class C lower yet. For high-risk buildings such as theaters, hospitals and schools, the code may require the Class A rating, while adjacent, less critical structures might only need to meet a less stringent B or C. Remember, however, that Class A has nothing to do with roof quality or durability. Some compromise might have been made, for example, in the number of plies in a BUR membrane to squeak by the test requirement. Meeting code is necessary, but exceeding it could result in a thinner, less weather-resistant roof.

A rooftop fire can result in ignition of the underlying combustible roof deck if the mass of surfacing, membrane and insulation is inadequate to thermally protect the deck from reaching its ignition temperature. Aggregate and ballasted roofs perform well in this regard, as do many roof insulations such as mineral fiber, perlite, glass fiber, foamed-glass roof insulations, and gypsum cover boards.

For structures with non-combustible decks such as steel, concrete and gypsum, burn-through resistance is not evaluated. Surface burning, however, is tested regardless of whether the deck is combustible or not. The nature of the surfacing and the slope of the roof greatly affect spread of flame. On very low slope commercial structures, even a glaze coating of pure asphalt may meet Class A flame spread. Fibrated coatings and asphalt emulsions retain the Class A to higher slopes, as do the FR Modified and FR single-ply systems.

Another test to establish time-temperature resistance is ASTM E119: Establishing Resistance to an Internal Building Fire. A one-hour rating would see an internal heat load of 1600 degrees F. UL uses their Steiner Tunnel to establish under-deck spread of flame. UL Construction No. 143: Fire Classified sets rigid requirements for each component of roof covering materials.

It would normally be the duty of a qualified roofing QA inspector to verify that the roof project is conducted by the book.

How Did That Fire Start?
During construction, there are many opportunities for accidental ignition, including welding of steel and the use of torches on modified bituminous roof membranes. Some building jurisdictions now require a fire watch for a minimum of one hour after the last roofing torch has been extinguished. When the torches are not in use, the operator should burn off the propane in the line by closing the cylinder valve first. A 50-foot hose line can contain a lot of fuel that could cause a problem if the line is severed or develops a leak, even just over a lunch hour.

Arson and terrorism are also potential sources of ignition. Aggregate-surfaced roofs probably give the most protection against a Molotov cocktail, but obviously sandbags have been used in wartime for the same purpose.

A less obvious source of ignition is the use of the building, the nature of exhausted material on the roof, and even discarded cigarette butts. Vandals who climb on a roof may find buckets of adhesive are compelling and attractive nuisances, something to sniff to get a high and then something to pitch cigarettes into. Restricting the amount of combustibles left on the roof may be the only defense against this type of occurrence.

As rooftop solar panels become more common, there will be more rooftop traffic, and behavior of rooftop fire methods is not yet established.

Internal Fire Exposure
Under-deck fire exposure may also originate with an arsonist, process fire or the like. While a steel roof deck is considered non-combustible, it readily conducts heat. This in turn might fluidize or vaporize combustible roofing components that leak into the structure, spreading fire underneath the roof deck. Both FM Global and Underwriters Laboratories have tests to evaluate this type of exposure. FM calls their acceptable risk systems Class 1, while UL calls theirs Classified Insulated Metal Deck Constructions.

Time-Temperature Ratings
The third major roof system fire test is ASTM E-119. This is a system test that includes structural members such as beams, trusses and roof deck. UL and FM list many roof-ceiling systems in their publications. In the World Trade Tower collapse, heat load on the connections of steel trusses is suspect. This may result in changes in the E-119 method in the future. Fortunately, our roofing systems are located externally, and generally have little bearing on the fire performance under the E-119 test conditions.