Green Roofs retain a high amount of rain water and drastically reduce the amount reaching the urban sewage system. The excess rainwater is of good quality and can be used in toilet systems or for irrigation purposes with no problem. It is also possible to attain clear run-off water but special substrates need to be used.
Intensive Green Roofs depend mainly on additional irrigation. To install an irrigation system which does not use fresh water, a water dam-up irrigation unit is recommended.
In case of heavy rain the reservoir is filled primarily and any excess water is collected in the cistern. During dry periods the water on the roof is used first, then water is pumped from the cistern onto the roof and supplied to the plants.
This process can be carried out either manually or electronically. The water in the cistern can also be used for other purposes, provided the reservoir is big enough.