To provide a new lateral roofing method having a device for making roofing work simpler and easier.

A suspending portion 3 provided at the upper side edge portion of a laterally long strip-shaped board, an attaching piece portion 4 provided at bottom edger of the suspended portion 3, a portion 1 comes into contact with the bottom edge portion of the strip-shaped board, a hook edge 7 provided at the tip edge portion of the portion 1, and a long-sized roof board equipped with a guide portion 8 and a guide receiving portion 9 achieving the set free function when the portion 1 provided at the suspending portion and the hook portion is subjected to a downward external pressure are constituted, a sealing member is adhered to the laterally facing portion 1 is adhered, thereafter, a first long-sized roof board is placed on the lateral facing parallel lines of the roof, and an attaching piece portion on the long-sized roof board is secured with nails, then the portion 1 of the second long-sized roof board is located above the suspending portion of the first long-sized roof board, at this position, a downward external pressure is applied to the portion 1 and the hook edge at the second long-sized roof board is hooked to the suspending portion of the first long-sized roof board.