“Wet” green roofs are used to store rainwater and to delay its runoff to the rainwater system, thus protecting the spaces concerned from local flooding (stormwater runoff control). This also reduces the costs associated with draining water from rainfall into the public sewage system. This water is not stored in the roof top layer, but in a special storage layer underneath the growing layer.
A blue roof of 700 m2 (50 mm storage level) can collect 35,000 liters of water, which can then be used for other operational purposes of the building (an example of the De Boel building at Boelaan in Amsterdam). The blue roofs are much more absorbent than traditional green roofs, so it is worth using them in areas exposed to flash floods and large precipitation. Their use also reduces the costs associated with the use of air conditioning, as the building becomes less heated.
Construction:
An important element of the structure will be an absorbent layer, e.g. in the form of a sponge, which keeps water on the roof for some time – which allows to control the runoff of water from the roof. The aggregate layer or permeable surface is applied in the form of cobblestones. These roofs can also be combined with vegetation layers. An important element of the structure will also be designing an adequate fall in the roof and a sufficient number of pipes for drainage and sufficient number of drains.
Greening:
Greening with plants of high water evaporation properties.
De Boel at Boelaan, Amsterdam
The roof on the De Boel apartment building in Amsterdam has a surface area of 700 m2
The rain water storage layer is made of Permavoid 85S, which allows 35,000 liters of water to be stored for subsequent watering (capillary irrigation is used here).
Sources and additional information:
Nagarajan K., 2017, Sustainable Smart Blue Roof Network System with application of Geographic Information System (GIS), Conference: International Journal of Engineering Research in Mechanical and Civil Engineering (IJERMCE), 2, 3, s. 423 – 428
https://livingroofs.org/introduction-types-green-roof/blue-green-roof-cities-stormwater/
https://www.smartcitiesworld.net/news/news/amsterdam-installs-blue-green-roofs-3454
https://www.phpsd.com/blog/everything-you-need-to-know-about-blue-roofs
https://stormwater.wef.org/2017/10/new-blue-green-roof-design-demonstrates-promising-results/