Blue roof

A blue roof is a roof of a building that is designed explicitly to provide initial temporary water storage and then gradual release of stored water, typically rainfall. Blue roofs are constructed on flat or low sloped roofs in urban communities where flooding is a risk due to a lack of permeable surfaces for water to infiltrate, or seep back into the ground. 
Water is stored in blue roof systems until it either evaporates or is released downstream after the storm event has passed. Blue roofs that are used for temporary rooftop storage can be classified as "active" or "passive" depending on the types of control devices used to regulate drainage of water from the roof. Blue roofs can provide a number of benefits depending on design. These benefits include temporary storage of rainfall to mitigate runoff impacts, storage for reuse such as irrigation or cooling water makeup, or recreational opportunities.
The term "blue roof" may also be used to indicate roofs that are blue in color.
Stormwater management and other benefits.
Flood mitigation.
Due to the density of urban development, there is a general lack of permeable surfaces in cities. With nowhere for water to infiltrate, or be absorbed back into the ground. This leaves cities vulnerable to flooding and overrun storm sewers. 
A number of blue roof pilot projects have been implemented around the United States, the results from which highlight their efficacy in reducing stormwater runoff during and after severe weather events.
Pollution reduction.
While blue roofs do not remove pollutants from water by temporarily detaining it, they do reduce the load severe rain events place on storm sewers which stops emergency overflow from combined sewer systems from discharging untreated wastewater into rivers, streams, and coastal waters. 
A significant blue roof pilot project intended to evaluate the potential of the systems for mitigating combined sewer overflow impacts was conducted between 2010 and 2012 by the New York City Department of Environmental Protection. The NYCDEP blue-roof projects are the first to utilize a novel passive blue roof tray design which relies on the lateral transitivity of non-woven filter fabric for drawdown control in a full scale pilot. Monitoring of these systems has demonstrated their performance as an effective means for mitigation of peak flows and alteration of timing in combined sewer systems.
Water scarcity.
On the opposite side of the spectrum, cities with limited rainfall are vulnerable to drought and extended periods of restricted water usage. In drier climates, blue roofs act as a water conservation tool harvesting the water that falls on a roof's surface and collecting it at a controlled rate.
Design compatibility.
Another major benefit of blue roofs are their ability to work alongside other rooftop systems such as solar panels (both solar thermal and pv), and HVAC mechanical equipment. 
Some recreational blue roofs integrate rooftop waterplay areas that can also be used to irrigate a green roof, or to cool the roof of a building on hot days, in order to eliminate or at least reduce the HVAC load placed on mechanical refrigeration equipment.
Some blue roofs utilize stored water for beneficial on-site purposes cooling of solar panels and irrigation of a green roof. One example of a blue roof that provide ancillary services was the winning entry (First Place, 10,000 Euro prize) in the 2004 Coram Sustainable Design Award, by Steve Mann.
Types.
Active blue roof.
Active blue roof systems control the rate at which water drains from a rooftop through mechanical means. Sometimes referred to as "automated roof runoff management systems," active blue roofs use valve configurations and controls to monitor and regulate the discharge of stormwater runoff from roofs. Water ponded on the roof can be released in several ways, including via a pneumatically or hydraulically actuated pinch valve, an electronically controlled valve connected to a timer, or manually opening the valve. Active blue roofs for stormwater detention using forecast integration were first proposed in 2008. 
Passive blue roof.
Passive blue roof systems control the rate at which water drains from a rooftop through non-mechanical means. Unlike active systems which inhibit water flow through drainage pipes, passive systems temporarily detain water on the surface of the roof by lengthening the path the water must take in order to reach outlet drains. Blue roofs can include open water surfaces, storage within or beneath a porous media or modular surface, or below a raised decking surface or cover. 
Blue colored roof.
A different type of "blue roof" has been proposed by researchers at the Lawrence Berkeley National Laboratory, who researched a pigment used by the ancient Egyptians known as "Egyptian blue." This color, derived from calcium copper silicate, absorbs visible light, and emits light in the near-infrared range, helping keep roofs and walls cool.