Class 1 vs. Class A Roof Assemblies

Are you familiar with Class 1 and Class A commercial roof assemblies, what they mean and how they differ? Learn more about these important ratings below.

Class A Rating

Class A, B or C rating is a measurement of the external spread of flame on a roof surface. A number of fire test laboratories, including Underwriters Laboratories (UL) and FM Global (FMG), conduct this test using either the ASTM E 108 or UL 790 Standard. The best rating achieved is Class A, which is described by UL as, “Effective against severe fire exposure.”

Class 1 Rating

The Class 1 rating is an exclusive designation that can only be awarded by FM Global, which indicates the successful testing of the entire roof assembly using FM Approval Standard 4470. According to the scope of FM 4470:

“The performance of a roof assembly depends on all components that make up the assembly, beginning with the deck and its securement to the structure to the uppermost layer exposed to the weather. It is therefore necessary to evaluate the entire roof assembly as a single entity.”

The FM 4470 standard is comprehensive series of tests that include:

  • Internal Fire/External Fire
  • Simulated Wind Uplift
  • Metal Component Corrosion Resistance
  • Hail Damage
  • Water Leakage
  • Foot Traffic Resistance
  • Puncture Resistance (Optional)
  • Solar Reflectance (Optional)

What is the difference between a Class 1 and Class A roof assemblies?

 A roof is classified as a Class A as the result of one test, the external spread of flame test (ASTME 108 or UL 790 Standard). A Class 1 roof has undergone a series of tests that evaluate the entire roof assembly, including the external spread of flame test utilized for the Class A rating. All Class 1 roof assemblies are Class A roofs, but not all Class A roof systems meet the Class 1 rating.

*Note: Substitution of any component not specified in a Class 1 assembly will nullify the Class 1 rating.