Building elements of relatively thin form for the construction of parts of buildings, e.g. sheet materials, slabs, or panels (materials or manufacture, see the relevant subclasses, e.g. B27N, D21J; made in situ E04B; specially designed for insulation or other protection E04B1/62; load-carrying floor structures E04B5/02, E04B5/16; roofs consisting of self-supporting slabs E04B7/20; roof or like covering elements E04D3/00; for lining or finishing E04F13/00)