Abstract: A heat insulated roof structure is composed of parallel beams between which a non-combustible, foraminous supporting structure is fitted. The supporting structure may be lengths of wire net, but on occasions glass-fiber fabric may be sufficient. A first layer of heat insulating material rests upon the support structure, the thickness thereof being less than one third of the total insulation thickness. On top of the first insulating layer a diffusion barrier, preferably a sheet of plastics foil, is spread, the sheet having a breadth sufficient to extend un-broken over two adjacent beams. A second layer of heat insulation material is located on top of the barrier, and uppermost there is a watertight sheeting, usually of metal plate. The thicknesses of the two layers of insulation material is selected so a slight compression occurs when the watertight sheeting is fitted, whereby air pockets below the sheeting are avoided.