To construct an envelope such as wall, roof, or the like consisting of architectural board in the form of a folded board, with which it is possible to preclude sound generation caused by thermal expansion and contraction originating from a temp. difference in the coupling part to couple together adjoining boards.

A single-layer or multi-layer envelope is composed of an architectural board A1, a cap A2, and a non-metal soft material 5, wherein the board A1 is equipped with bent parts 3, 3 for fitting on both sides across the width of the uppermost layer. A plurality of such boards A1, A1... are laid in line. The non-metal soft material 5 is interposed between the mating fit bends 3, 3 of adjoining plates and the cap A2, and the cap is fitted on the bends 3, 3.