A case study was developed for the city of Rotterdam. In this account 25% of the roofs in Oud Mathenesse were provided with green roofs with a retention capacity of 25 mm. Hereby the annual overflow volume was reduced by 19.5% annually, which in turn has had a positive effect on the surface water quality. In this example, the contribution to the reduction of the maximal storm water overflow, based on a storm to happen on average once every 100 years, was 3.4%. This storm is the benchmark for water management policy Gemeente Rotterdam, 2006.
Despite the fact that green roofs have the capacity to retain close to 100% of water from small and average rain showers annually, the contribution to the water management cycle is limited. In terms of the water cycle it is most important to limit peak discharges. Green roofs do have a positive effect on water quality with regard to pollution on rooftops.
The rainwater retention capacity depends on many factors: the slope of the roof, the consistency of the layers, and the thickness and type of substrate. Gemeente Rotterdam, 2006