Rooftops are generally poorly used areas and therefore have enormous potential. The Polder roof uses the roof surface to create a water buffer and create a new roof surface for other rooftop functions. The Polder roof then controls the flow of rainwater to the downspout. When it is raining the rainwater runoff is stopped and when the rainwater in the sewage system is processed the Polder roof starts to empty the rainwater buffer, offsetting the peak of rainwater flow into the sewage system.