PURPOSE: To cause snow to melt at good heat transfer efficiency for preventing a snow coverage by forming a pitch and providing a sprinkler header and a drain pipe on a roof comprising a roof top plate kept watertight with and detachable from a roof frame laid on a skeleton structure.

CONSTITUTION: In the snow season, a cooling fan 3 is stopped and a roof top plate 7 is laid on a roof frame 6. Also, a feed water pump 13 is operated and water is sprinkled on a roof from a sprinkler header 8. Sprinkled water flows down along the slope of the roof, while melting snow, and is collected from a drain hole 10 at the low position of the frame 6 to a feed water tank 12 via a drain pipe 10. In the case of much snow, steam is supplied to the tank 12 from a steam supply pipe 14 for heating feed water. In the summer season, the roof top plate 7 is demounted, and the cooling fan 3 is operated. According to this construction, melting efficiency is improved and no excessive load acts on a roof and a skeleton 4, thereby enabling construction cost to be reduced.