A Class 1 roof designation requires approval from either Factory Mutual Research (FMR) or Underwriters Laboratory (UL). The roof design is tested, and then if it gains approval, must be installed exactly as tested.
Inspection is detailed and exacting. No building material substitutions or assembly technique can vary from the prescribed protocol, not even the thickness or dimensions. Insurance rates and coverage are affected, and building codes may be violated by breaking with installation guidance.
Each substituted component or technique must be approved by UL or FMR.
Confusion occurs when roofs are discussed as A, B, or C and then 1. Class 1 is a more rigorous standard.




