To simplify a sealing work by attaching a waterproof adhesive tape to the jointing ridge beam of adjacent roof boards at a ridge and the upper surface of a ridge cap fastener seal part covering the ridge side end of a roofing material.

Jointing ridge beams 14 are abutted right and left to the upper end of each roof board structural body 12 of roof boards 01 adjacent to each other at a ridge 20 and, then, a ridge cap fixture 17 is fitted to the ridge side end of the roofing materials 15 of the right and left roof boards 01. Furthermore, a waterproof adhesive tape 21 is attached, so as to cover the upper surface of the back board of the cap fastener 17, the end of a sheet 16 on the roof board structural body 12, and the jointing ridge beams 14. Then, ridge caps are jointed and secured for waterproofing a ridge 20 via the ridge cap fastening part of the ridge cap fastener 17 preliminarily provided on the right and left roof boards 01. According to this construction, the infiltration of rainwater into the body 12 can be prevented.