PURPOSE: To prevent the leakage of rain water to the room side of girder surface wall interface plate 3 even when rain water enters the room side of flashing 4 due to poor workmanship.

CONSTITUTION: The upper edge 4a of flashing 4 is supported by a girder surface wall 2 of upper floor, and the lower edge 4e of flashing 4 is brought into contact with the upper surface of roof member 6 of lower floor roof 1. An internal flashing 27 is disposed on the room side of flashing 4. The upper edge 27f of internal flashing 27 is supported by the girder surface wall 2, and the lower edge 27d of internal flashing 27 is positioned between a water leak proof plate 9 and a roof surface layer 22. The water entering the room side of flashing 4 is allowed to flow along the outdoor side surface of internal flashing 27. The water is received by the water leak proof plate 9, and discharged from the eaves to the outside of the roof 1.