PROBLEM TO BE SOLVED: To provide a roof unit which is used for desalting salt water such as seawater and ground water and supplying the desalted water to a hothouse for cultivating plant life as irrigation water, the roof unit being compact and providing excellent maintainability, and to provide a roof and a hothouse. SOLUTION: A roof unit is constituted so that a casing is used which has a horizontal bottom surface part and a light-transmissive inclined upper surface part, a salt water storage tank is arranged in the casing and a salt water supply port and a salt water discharge port are arranged respectively on the rear surface and front surface of the casing, and the inside surface of the upper surface part is made to play such a role that steam evaporated from the salt water is condensed to obtain freshwater and the obtained freshwater is stuck and guided to the front surface. The roof unit constituted thus is arrayed in multiple stages to constitute a roof in which the salt water discharged from the roof unit on the upper stage side is introduced into the roof unit on the lower stage side and desalted by sunlight in each of roof units to recover freshwater and the recovered freshwater is made to fall by gravity, for example, in the hothouse for cultivating plant life and used as irrigation water. COPYRIGHT: (C)2008,JPO&INPIT
