Method of insulating metal deck roof structures

A method for providing a roof structure having a plurality of trusses spaced apart from one another in a parallel arrangement is disclosed. The trusses each have a top portion. A support sheet is payed out so that the support sheet depends from the top portions of adjacent trusses. Flexible insulation material is applied above the support sheet so that the flexible insulation material is supported by the support sheet. Metal deck sheets are fastened to the top portion of the trusses, thereby forming a metal deck. A temporary waterproofing sealant is applied to the metal deck to protect the flexible insulation material, thereby creating a partially completed roof structure. Rigid insulation board and a waterproofing membrane are subsequently applied onto the metal deck, thereby forming a completed permanently sealed roof structure.