A covering system for roofs and for the outside of building walls is provided in which, on top, a high range of various decorative materials and elements can be fixed without penetrating the watertight layer. The system contains protruding elements connected to the support and flexible watertight membranes arranged so as to cover and contact essentially the complete support surface, including the protruding elements. Panels, preferably rigid insulation panels with excellent resistance to water, are arranged so as to hold down the watertight membranes onto the support, thus protecting the membrane from thermal cycling, UV rays and physical damage. The panels can be fixed with non-penetrating fastening means to secure the complete system to the protruding elements and to the support.