Inverted Roof Membrane Assembly (IRMA) concrete roof, insulated externally with a moisture resistant high compressive strength XPS insulation that serves to protect the membrane from UV exposure and maintains the roof slab at a stable indoor temperature. The insulation is weighted down by a stone ballast loaded above a drainage layer with integrated geotextile to manage rainwater above the insulation layer. Primary drainage is above the insulation layer, but this system can include a protection and backup drainage layer under the insulation if required.