In this type of inverted flat roof, like any other inverted roof, the waterproof membrane is laid below the thermal insulation so that the insulating layer can protect the waterproofing and underlying structure over time. This type of roofing is treadable and the finish consists of cement or ceramic tiles laid on mortar. When insulating inverted flat roofs, extruded polystyrene is normally used for its high compressive strength, almost no water absorption, and high resistance to frost/defrost cycles.
Laying: a coating of lightweight mortar with a minimum gradient of 1%is applied on the roof's concrete slab. The waterproofing membrane is laid on the coating. It is then either fastened or welded according to the manufacturer's instructions. X-FOAM^® HBT extruded polystyrene insulation boards are then laid trying the cover the entire surface of the roof. We recommend laying roofing felt (layer of non-woven fabric) of a weight of at least 100 g/m² between the insulation boards and waterproofing layer to avoid the risk that they glue together.  Non-woven fabric, preferably white, is laid on the insulating layer followed by a layer of reinforced mortar at least 4 cm thick to ensure better gripping with the finishing tiles.