Class A Fire Rating

A Class A fire rating is the highest of three fire classifications for roof coverings under the North American test standards ASTM E108 and UL 790 (which are technically similar). It indicates that a roof covering, as tested on a particular deck and underlayment, can withstand severe fire exposure from outside the building.

The classes

ASTM E108 defines three classes:

• Class A: effective against severe fire test exposure; provides a high degree of protection to the roof deck.
• Class B: effective against moderate exposure.
• Class C: effective against light exposure.

For all three classes, the covering should not slip from position and is not expected to produce flying brands (burning pieces that could spread fire).

The tests

ASTM E108 includes several test methods, applied as required for the product type:

1. Intermittent flame exposure: a gas flame is applied and removed repeatedly to see if the deck ignites.
2. Spread of flame: measures how far flame travels across the roof surface.
3. Burning brand: a burning wood block is placed on the roof to see whether it burns through to the deck.
4. Flying brand: checks whether the covering produces burning debris.
5. Rain test: for treated products, checks whether fire retardancy survives weathering.

Stand-alone versus by assembly

Some materials, such as clay tile, concrete tile, slate and many metal roofs, are often described as Class A on their own because they are non-combustible. Others, including some asphalt shingles, treated wood shakes and some metal or composite products over a wood deck, achieve Class A only as an assembly with a specified underlayment or fire-resistant layer. The listing for each product states the conditions.

Use in codes

Building codes in the United States require minimum roof classes in certain occupancies and in wildfire-prone areas; wildland-urban interface codes typically require Class A in the highest hazard zones. Wood shakes and shingles are restricted in many fire-prone jurisdictions unless treated and listed.

Other regions

Europe has its own classification system for the external fire performance of roofs, and Australia addresses roofs in bushfire-prone areas through AS 3959. Ratings from different systems are not interchangeable.