PURPOSE: To make it possible to confirm easily whether or not roof boards jointed with each other on a roof are engaged at their seam joint engagement parts.

CONSTITUTION: A roof board 1 is provided with a plate 8 and a first seam joint engagement part 2 formed by folding one end of the plate 8 downward and a second seam Joint engagement part 3 which is capable of inserting a first seam joint engagement part of the other roof board. An insertion piece 7 which is inserted into the seam joint engagement part of the other roof board is installed to the tip of the second seam joint engagement part 3. A joint line 4 is formed in a position equivalent to the width of the insertion piece 7 from the edge of the first seam Joint engagement part 2 of the plate 8.