The inverted roof is the solution to all those problems linked to building traditional flat roofs and it is characterized by the laying of the waterproofing membrane below the thermal insulation layer. When insulating non-treadable flat roofs, extruded polystyrene is normally used for its high compressive strength, almost no water absorption, and high resistance to frost/defrost cycles. Generally, in this type of roofing, there is a layer of washed gravel (16 - 32 mm) laid on top of the insulating layer. It is used both as finishing and to protect the extruded polystyrene boards against UV rays and wind. The drainage layer is to be laid by starting from the farthest point of access to the roof and covering the entire surface. It is recommended that the drainage layer be at least as thick as the insulating layer and anyhow no less than 5 cm thick. The weight of the gravel also contrasts the buoyancy of the insulation boards. It is also recommended to lay a separating layer made of non-woven fabric, for instance, as a filter between the extruded polystyrene and gravel. The concrete slab is duly waterproofed and the laying surface is leveled with a roll of expanded polyethylene. The X-FOAM® HBT boards can be laid on the sheathing without any need for adhesives. There is no need for vapor barrier because there is no forming of condensate.
In this type of application, thermal insulation is exposed to bad weather and rainwater, which, as it runs off the roof, reduces the thermal insulation value. During planning, a coefficient is to be applied to take this into account.