Roofing Fire Ratings: UL 790, ASTM E108 and Classes A, B and C

The Standards

The test method is published in parallel as UL 790, Standard Test Methods for Fire Tests of Roof Coverings, and ASTM E108, which carries the same title and substantially the same procedures. In Canada, CAN/ULC-S107 is the corresponding national standard for fire tests of roof coverings. Any of these may be cited on a product listing depending on where it was tested and for which market.

What the Tests Simulate

All of these methods are concerned with fire attacking a roof from the outside — an ember landing from a chimney, a neighbouring building or a wildfire, and flame spreading across the roof surface. They are not tests of fire resistance from inside the building, which is a different question addressed by fire-resistance ratings of assemblies. A roof covering classification tells you how the roof responds to external fire exposure.

The Three Tests

Classification involves a set of separate tests, each attacking a different aspect of performance. A specimen is built as a full assembly on a test deck at a specified slope and subjected to each.

The Burning Brand Test

A wood brand — a small assembled crib of kiln-dried lumber — is ignited and placed on the sloped test assembly while air is directed across it. The brand size is what separates the classes: the Class A brand is a substantial crib roughly twelve inches square, the Class B brand is smaller, and the Class C brand is a small piece only an inch and a half or so square. The assembly must not sustain flaming, allow flame through to the underside, or produce burning material falling from the deck. This test is the closest analogue to the real-world scenario of an ember landing on a roof.

The Spread of Flame Test

A gas flame is applied continuously to the lower edge of the sloped assembly while air is blown across the surface at a controlled velocity, for a specified duration. The distance the flame travels up the deck is measured, and each class permits a different maximum spread. This test is what catches coverings that are individually non-combustible but allow flame to run along a seam, an underlayment or an air space.

The Intermittent Flame Test

The same flame source is applied in a series of on and off cycles rather than continuously, exposing the assembly to repeated heating and cooling. This finds materials that survive a single sustained exposure but degrade, delaminate or open up under cycling. Some materials are also weathered or subjected to a rain test before classification, so that the rating reflects aged material rather than only new.

What the Classes Mean

Class A is the most resistant classification, representing effective resistance to severe external fire exposure. Class B represents resistance to moderate exposure and Class C to light exposure. A covering that does not meet Class C is unclassified. The lettering runs opposite to the impact classes discussed for hail testing, where Class 4 is the highest — a genuine source of confusion, and worth keeping straight when reading a product sheet that quotes both.

Terms That Are Easy to Confuse

Several different fire properties get quoted interchangeably in sales material and they mean different things.

Where This Matters in Ontario

The Ontario Building Code addresses roof covering classification, and what is required for a given building depends on factors such as the building's use, its size and its spatial separation from property lines and adjacent buildings. Most single-family residential re-roofing in Southern Ontario is not a demanding case, but the requirement is not universal and the authority on it is your designer or the local building department, not a contractor's recollection. Where it tends to become a live issue is on tightly spaced urban lots, on additions and infill, on garages and accessory buildings close to a lot line, and on rural and Northern properties where wildfire exposure is a real consideration.

Practical Fire Considerations Beyond the Rating

Two things are worth thinking about independent of any classification. First, a non-combustible roof surface does not help if embers can get into the assembly, so ridge and eave ventilation details, soffit construction and the treatment of open valleys and penetrations all matter. Second, fire services sometimes raise questions about ventilating a metal roof during interior firefighting. Metal roofs are cut in the field routinely with the right equipment, and this is a factor to be aware of rather than a reason against metal, but it is a fair question to ask your local service if you are curious.