PURPOSE: To facilitate machining and positioning for a large board without the need for improving installation accuracy in the horizontal direction of the primary angle, and to prevent freezing damages while making finishing and repair works executable without employing special structures.

CONSTITUTION: A groove 24a that is aslant breadthwise is provided to the upper reverse side of a large board 24 in its longitudinal direction so that the slant looks down toward the surface of a building frame. A fixing hardware 23 is equipped with holding pieces 23b that look diagonally upward toward the front side of the surface of the building frame 1. The groove 24a of the large board 24 is made to engage with the holding pieces 23b of the fixing hardware 23.