To carry out smooth roof board works and to prevent leaks from seams in its conventional works, a bending method and its instrument for roof board works are devised. Since bending parts of roof board seams are broken in a conventional way of bending metal roof boards, roof boards cannot be laid smoothly on top of another one. When a cutting part is arranged to prevent this inconvenience, a leak in the roof occurs from its part.
A paper band 60 is put between seams 71a and 71b which are formed at a metallic roof board 70. When this roof board 70 is turned round toward a bending direction in a freely movable situation with two wings, it is laid between a drag 20 forming a circular part 42 and a cope 40 forming a clearance 75 at its lower end. A paper band 60 is taken out from seams 71a and 71b in a roof board 70 after the drag 20 and cope 40 are turned round toward a bending direction and a roof board 70 is bent.