Roofing Experts | ACS | NorCal

How Fireproof is My Roof? The 4 Fire Ratings Explained

California’s wildfire season is just around the corner, and many building owners are wondering if their commercial roof will protect their building from fire.

There are four commercial roof fire ratings: Class A, B, C, and Unrated.

Class A ratings are obtained when material survives a severe fire test. Class B ratings are awarded after successfully withstanding moderate fire exposure and Class C demonstrates survivability of only the lightest fire test.

The International Building Code (IBC), states that all roof coverings are required to be tested in accordance with ASTM E 108 or UL 790.

According to Carlisle Syntec:

The Standard Test Methods for Fire Tests of Roof Coverings, UL 790 (ASTM E108), includes three key tests to provide fire classification of roof covering materials and systems. These are the Spread of Flame Test, Intermittent Flame Test, and Burning Brand Test. During the Burning Brand Test, heat is transferred directly through the roof covering (single-ply, BUR, shingles, etc.) as well as the other roof components (insulation, underlayments, etc.), which protect the roof deck.

In most Burning Brand Tests conducted over a combustible deck (3/8- or 15/32- inch plywood), the roof components play a significant role in achieving the fire rating of the overall roofing system. Accordingly, many certifications of roof covering materials describe the specific types of roof components to be used based on the testing conducted.

That means the roof material is tested, as is the underlying insulation and roof deck, whether combustible or not.

Class A Roofing

Class A roofing is the preferred choice for any commercial building, especially if it is in an area prone to wildfires. To achieve a Class A rating, the roof must be effective against severe fire exposure. This is proven if it can:

  • Experience maximum flame spread of 6 feet
  • Withstand a burning brand measuring 12″ x 12″ and weighing 2,000 grams
  • Last 2 to 4 hours before ignition
  • Resist 15 cycles of a gas flame turned on and off


Common stand-alone Class A roof coverings include clay tiles, slate, asphalt glass fiber composition shingles, and concrete tiles.

PVC and TPO products are both available with a Class A rating, although TPO is the most common commercial roof.

Acrylic and silicone roof coating may have a UL Class A fire rating. However, they are part of a commercial roofing system and that entire system must be considered when determining the roof’s fire rating.

Assembly-rated Class A roof coverings are those that meet Class A standards when combined with other elements. For example, metal roofs are non-combustible, but it still requires the underlying material to meet specification in order to receive a Class A fire rating. For example, a plywood deck under a commercial roof may ignite due to heat transfer through the metal unless there is additional protection to prevent this.

When using assembly-rated roofing material, it’s crucial the customer read the manufacturer’s specifications carefully. These will detail exactly which materials to combine for your roof to achieve a Class A rating.

Class B Roofing

Class B roofing is effective against moderate fire exposures. This is proven when the roofing can:

  • Experience maximum flame spread of 8 feet
  • Withstand a burning brand measuring 6″ by 6″ and weighing 500 grams
  • Last 1 hour before ignition
  • Resist eight cycles of a gas flame turned on and off


Pressure-treated shakes and shingles are the most common roofing materials to fall under the Class B rating although these are rarely used in commercial roofing.

Class C Roofing

Class C roofing provides only light fire protection. Roofing with a Class C rating is able to:

  • Experience maximum flame spread of 13 feet
  • Withstand a burning brand measuring 1.5″ x 1.5″ and weighing 1/4 gram
  • Last 20 minutes before ignition
  • Resist three cycles of a gas flame turned on and off


Examples of common Class C building materials include untreated wood shakes and shingles, plywood, and particleboard. None of these materials is recommended on a commercial or residential roof.

Unrated Roofing

This type of roof provides little, if any, fire resistance and should be avoided. Most commercial roof building codes will not accommodate any type of unrated roofing material.

Understanding the fire rating for your roofing materials will help you determine how safe your commercial building is in the event of a fire, and whether it is up to code.

If you’re concerned about the threat of fire to your commercial building, feel free to call us at 925-724-2324.

Additional reference provided by Build Direct »

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