An improved insulated roof system for enclosing and insulating a building structure in which a flexible membrane is attached to extend tautly between a first support portion and a second support portion of the building structural assembly. A tertiary structural assembly is connected to the building structural assembly, a portion of the tertiary structural assembly extending from the structural assembly and supporting panel members which are attached thereto. Insulation material is supported by the flexible membrane between the panel member and the underlying building structural assembly.