A roofing system for a roof deck able to retain mechanical fasteners includes a layer of roof insulation boards overlaying the roof deck and a plurality of spaced apart hold-down straps overlaying the roof insulation boards. The hold-down straps have pre-formed mechanical fastener openings therethrough on preselected center to center spacings and mechanical fasteners pass down through the mechanical fastener openings in the hold-down straps, through the roof insulation boards, and into the roof deck to secure the roof insulation boards to the roof deck. Preferably, the roof insulation boards each have guidelines thereon for placement of the hold-down straps relative to the roof insulation board.