Abstract: The present invention relates to an insulated one-piece roof for use over heatable rooms comprising at least three layers, the first layer being a solid or other load-bearing construction, the second layer being an insulating board and the third layer being a prepared roofing, said one-piece roof further characterized in that the characteristics value .mu..multidot.s where .mu. is the diffusion resistance index and s is the layer thickness of the constituent material of the vertically adjacent layers, decreases outwardly, said prepared roofing being firmly joined to the underlying insulating board and having a characteristic value .mu..multidot.s of less than 4 m.