The "hot roof" type is made with bituminous or synthetic membranes, but the thermal insulation is always laid under the membrane. The advantage of this solution lies in the fact that is well known and has been widely used for a long time thus reducing any possible errors. The insulation is always protected by the waterproofing and its characteristics and performance are preserved at length without any washout effect. 

The type of insulation required in this type of application must have the following characteristics:
- excellent insulation properties
- good compressive strength
- resistance to heat
- resistance to solvents contained in bitumen

The most suitable product for this type of application is undoubtedly rigid expanded polyurethane. Please bear in mind that extruded polystyrene and expanded Styrofoam CANNOT RESIST to temperatures over 80°C that can be reached not only during laying, but also in operating conditions. Special attention is to be paid when laying these products in contact with artificial and bituminous membranes with a high content of plasticizers and solvents. Fiber products DO NOT HAVE adequate resistance to humidity to ensure constant performance over time. Fiber products DO NOT HAVE adequate compressive strength resistance to humidity to ensure constant thickness and resistance to treading over time. Expended polyurethane panels are the optimal solution for this type of application. Synthetic coatings are blends of resins mixed with plasticizers, stabilizers and pigments produced by extrusion, calendering and spreading. The structure can either be homogeneous, stabilized and reinforced or even fiber-reinforced. The use of a synthetic membrane at sight requires the use of anti-UVA additives. The polyurethane panels can be laid dry or with compatible additives; the synthetic membrane must always be left free from the insulation. The joints between the coating’s components are achieved by hot welding or with solvents. The various layers can be laid dry in case of ballasted coatings.When dry laying is not recommended, the coating can be fastened mechanically or glued with adhesives. Hot bitumen is one of the most widely used. A quantity of about 1.5-2 kg/m² is spread evenly on the surface to be bonded. The separation layers (non-woven fabric of about 500g/m²) inside the package protect the vapor barrier, normally composed of a polyethylene sheet and synthetic coating, against the roughness of the laying surface and abrasions caused by the ballast layer. POLIISO^® VV expanded polyurethane panels with glass fiber supports are the recommended type of insulation for this type of application, because they ensure good compressive strength values, excellent thermal insulation values and resistance to the high temperatures that develop under the membrane both during laying and service life.