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. Green roofs are built when you need a green covering either for esthetic or environmental reasons. Garden roofs can pose hygrometric and static problems if special attention is not paid to these aspects during the design phase. Good thermal insulation fit for the purpose is absolutely necessary. In particular, the inverted roof allows for the preservation of the waterproofing membrane's quality and properties over time.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 and then fastened or welded according to the manufacturer's instructions. The X-FOAM^® HBT extruded polystyrene insulation boards are then laid on top of the waterproofing membrane trying to cover the roof's entire surface. 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 gluing of the X-FOAM^® HBT boards to the waterproofing. A drainage layer of gravel is laid on the insulation boards. If the particle size of the drainage layer is smaller than 10 mm, a filtering layer is to be laid as well to further protect the insulation. The roof is completed by applying the adequate quantity of soil or humus on the gravel layer depending on the type of vegetation to be planted.