Abstract: In order to retain water on the roofs of buildings with a water-retention basin, a vortex-type throttle valve is to be fitted on the flat roof and is to be connected to a drain pipe leading to a drain. The throttle valve makes it possible to control the rainwater run-off at a given rate determined by the size of the throttle valve. If rain falls at a high rate, the excess is retained. Overflow protection is provided by fitting on top of the throttle, a length of pipe which permits the unrestricted flow of water through the throttle valve. Alternatively, the drain pipe can be extended upwards to the maximum permitted water-retention level, thus allowing the water which exceeds this level to pass directly into the drain pipe.