The invention is an improvement of extruded polystyrene (xps) for the inverted flat roof. The extruded polystyrene can interact under certain conditions of pressure and temperature, otherwise usual for this type of covers, with different types of impermeable membranes, causing degradation in them. The present invention consists of the adhesion of an aluminum sheet on the face of the extruded polystyrene that would be in contact with the waterproofing, which prevents the interaction between the materials, increasing the durability of most of the waterproof membranes for the cover inverted flat (Machine-translation by Google Translate, not legally binding)