Finding a Viable Solution

Our m-Tray green roofs can “attenuate” (absorb) up to 50% of rainwater on flat roofs, and also slow down the initial deluge. By taking pout 50% of the rainfall and then slowing down both the start of drainage water leaving the roof / podium deck and entering the drainage pipes, and making that flow slower and over a longer time period. This puts a lot less pressure on the (often Victorian) drainage system.
http://www.wallbarn.com/government-v-builders-drainage-row/