For thermal insulation of flat roofs, flat roofs and terraces, various types of boards made of various thermal insulation materials can be used. Rigid polyurethane foam plates are most often used. These boards are clad on both sides with kraft paper. The boards are placed with a tongue-and-groove connection, and sticking with bitumen mastic to the substrate. A layer of waterproofing made of various types of bituminous roofing papers or waterproofing membranes is laid directly on the boards.