The invention relates to a room arrangement (20) comprising at least two superposed prefabricated load-bearing room units (1) which have been attached together. The room unit (1) has a ceiling (24), a floor (2), and at least two walls (3, 4, 5, 6), the ceiling (24), floor (2) and walls (3, 4, 5, 6) of a room unit (1) being connected together. Further, the said ceiling, floor and at least two walls are at least mainly of cellular board formed of two substantially parallel surface plates and of a core arranged between them, the core being firmly attached to the surface plates, wherein the surface plates and the core of the cellular board are made of metal. The invention relates also to method of constructing such room arrangement, and its uses.