A frame of reinforced wooden sandwiched plates (23) is directly on the horizontal joist (15') fixed wall surface element. The rear wall (20) is formed of full length, jointly suspended wooden plates. These plates are arranged beside each other in a row and attached directly to the joist. The plates are held between the holder shafts of the support (5) end sections of the frame.At least the centrally located plate has a horizontal minimal width of at least 25% of the total wall field width and has a thickness of at least 20 mm. A vertical joist is attached by nails to the horizontal joist.
