Sprinkling water on the roof to reduce the indoor temperature is one way to improve the hot thermal environment. This method is particularly effective for steel roofs which offer poor thermal insulation, and which are often used for factories and warehouses. Based on measurements from experiments and actual roof sprinkler systems, this paper describes results concerning the following: (1) planning for and characteristics of a rotary sprinkler system with nozzles that sprinkle water over a circular area; (2) the potential of the intermittent and sequential sprinkling methods for saving water and efficient cooling, and the potential of the sprinkled water recovery system and rainwater utilization system; and (3) simulation of the running cost of the roof sprinkler system, and comparison with conventional space cooling. It is concluded that a roof sprinkler system is effective for indoor cooling provided that an appropriate water source is available.