to retain water on roofs of buildings with a water - retention basin , a vortex - type throttle valve is fitted on the roof and is connected to a drain pipe leading to a drain . the throttle valve makes it possible to control 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 .