In these roof systems, as it can be understood from its name, the membrane is laid under the heat insulation, exactly opposite to the flat roofs. There is no need for mechanical fixing or full adhesion in this system. Stream gravel is spread such that it is minimum 50 kg/m2 over the minimum 200 g/m2 geotextile to be laid on the opposed heat layer against lifting power of the wind. The gravel sizes must be between 20 and 40 mm. Or concrete plaques with a thickness of minimum 50 mm are laid.