PURPOSE: To hold watertightness definitely by changing partially how to place a waterproofing sheet on a roofing backing in a rising part of a roof surface and a frame member.

CONSTITUTION: A waterproofing sheet 33 is preliminarily interposed and mounted when a metal frame 6 of a top 1 light 1 is fixed to a wooden frame 5 while an asphalt roofing 32 is covered on a roof board 31 from the side of eaves. During the roofing layout, the waterproofing sheet 33 up to the site substantially half the eaves side on the side of the top I light 1 and its interruption area are covered with the asphalt roofing 32 by rolling up the waterproofing sheet 33. Then, the asphalt roofing 32 is laid on the waterproofing sheet 33 from the side to the ridge side unlike the side of the eaves. The roofing 32 positioned on the ridge side of an upper frame 5b is laid on the water-proofing sheet 33 while the roofing 32 positioned on the side of the eaves is laid on the lower side of the water-proofing sheet 33 so that they are laid one upon the other. This construction makes it possible to prevent rain water from flowing in a space between the roofing 32 and the waterproofing sheet 33 even when rain water may flow on the roofing 3 into the top light 1 area from the side of the eaves.