Prefabricated modified composite waterproof membrane with differentiated waterproofing compounds. The upper face waterproofing compound is composed of distilled bitumen and elasto plastic polyolefin’s with high heat resistance and cold flexibility, while the lower face compound is composed of distilled bitumen and special polymers which give incomparable characteristics of adhesion to old membranes with mineral self-protection. A particular compound, specifically studied, is used to make the upper and lower face compounds compatible. The waterproofing compound has a special inorganic and non toxic additive which confer to the product a flame retardant action.