Blue-green roofs can store more rainwater than “normal” green roofs. Under the green layer of plants is a crate system in which the water is stored. The water reaches the plants through the substrate (soil) and a filter layer. The roofs can also choose whether to retain the water or release it. Check here and here videos of how a blue-green roof is constructed!
THE TECHNICAL ASPECTS OF SMART BLUE-GREEN ROOFS
The smart blue green roofs are equipped with smart sensors and valves. As the sensors measure the water level, the valves respond to the weather forecast and open and close at the right time. To control the valves an ingenious system called Decision Support System (DSS) is used (read more information about the DSS here). The system knows exactly how much water a roof can or should store. This is useful when, for example, there is going to be heavy rainfall or in times of drought.
For example, heavy rains are expected. DSS signals the valve on the blue green roof to release the remaining rainwater before the showers arrive. In that way the smart roof can retain the highest amount of rainwater and thus relieve the sewer system. In times of severe drought, the water storage will provide a cooling effect and keeps the plants optimally hydrated. Pretty smart right?