The document specifies a method for the determination of thickness and mass per unit area of bitumen sheets for roof waterproofing. The measurement of thickness is appropriate for most bitumen sheets including sheets with a factory applied mineral finish. The determination of thickness is not applicable to sheets with pronounced surface texture or to sheets with a substantial fibrous backing. Where it is required to characterize these products, mass per unit area should be used instead. The determination of mass per unit area serves to verify the value stated by the manufacturer for information and is not applicable to perforated materials.