The great successes of air and space crafts during the last two decades are due, to no small extent, to a new cell technology, the sandwich structure. Cells in sandwich structure consist of two dense outer cover layers and an inner shear-transferring core layer of lesser density, e. g. in honeycombed structure. The thought suggested itself to adapt this new technique with its optimum stiffness-to-weight ratio to the building industry; the utilisation of steel sheets 0.5 mm to 2 mm thick for the cover layers and of polyurethane hard foam 30 mm to 150 mm thick for the core layer proved particularly favourable. With this steel-plastic system combining the stiffness of coated plates with profile or shell stiffness, panels more than 10m in height and roof trusses with a span of up to 20 m and more may be made. In trapezoid profiles, bent work and shells in sandwich structure the main task of the polyurethane hard foam, with greater depth of profile, is to support the thin-walled metal cover layers and to protect them against instabilities.