PURPOSE: To easily and surely arrange precast roof-forming bodies having stepped parts in the vertical or the horizontal direction in waterproof structure.

CONSTITUTION: The rear part of a precast roof-forming body 11 made up by placing roof panels 14 on a base board in desired shape is arranged to the lover side of the front end of another precast roof-forming body 11 so that the rear part of the former precast roof-forming body can be connected with the front end of the latter precast roof-forming body with a stepped part 7 provided to the connected part. Upwardly projecting parts 17 provided in projection to the upper side near the rear end of the roof-forming body placed in the lower part are arranged so that they can be placed between a plurality of downwardly projecting parts 15 provided in dangling in parallel with one another at desired intervals to the underside near the front end of the roof- forming body placed in the upper place. Both of the roof-forming bodies are connected with each other with a gap S provided between the upwardly projecting parts 17 and the downwardly projecting parts 15.