Prefabricated modified polymer bitumen membrane composed of polyolefin thermoplastic stereospecific polymers with high molecular weight and special distilled bitumens, with excellent characteristics of resistance to ageing and phase inversion. These built in elements, integrating themselves, enhance the excellent qualities of flexibility, lightness, adhesion, resistance to ageing and to UV rays of the NORD-TEC MINERAL F.R. membrane. The waterproofing compound has a special inorganic and non toxic additive which confer to the product a flame retardant action.