Blue Roof and Combined Sewer Overflow (CSO) prevention

Different approach to the same problem is brought by blue roof technology. Its name simply comes from the bluish color that a roof assumes during a rainfall. In fact, the water temporarily stored on the roof turns it into blue, like a pool.
A very few is known about blue roof under this name, while means of controlled flow devices seams make more sense to common awareness. As the name suggests, the idea behind is to reduce the flow of water running towards a conduct. In doing that, an appropriate volume for temporary storing is required.

Unlike the green roofs, blue roofs are non-vegetated surfaces and the runoff that comes to be during a rainfall is controlled by weirs properly set at the roof drain inlets. […]
Many brands are available on the market but the going-concern concept remains the same. An external steel cage prevents debris picked up by water to go into the inlet and make obstructions. In the worst case scenario such debris could reach the treatment plant causing more serious damages to the machineries. The cone at very center of the devise (it could have also a cylindrical shape) actually has two entrances. The first is on the side of the cone allowing water enter it during normal rainstorm. The second is at the very top of the inlet and it has a much bigger entrance which is meant to funnel more water into the inlet as soon as the water level rises until certain height. This system prevents overloading on the roof that otherwise could be damaged by the weight of the water standing on it. According to DEP the cone (or the cylinder) into the metal cage is 2 inches in height on average.
A weir […] is eight inches in diameter and it weighs less than 10 lbs. It is specifically designed to discharge 5 GPM per inch of water and it costs $534. (MIFAB).

Although each device is rather expensive, costs per unit are very low specially if compared with green roofs. The Department of Environmental Protection assumes $10 per square foot as a good approximation for a blue roof cost but other studies report costs even lower. The Center for Clean Air Policy has recently issued a document where costs per square foot of blue roof are reported to be as high as $4 (from $1 to $4 per unit) with considerable lower costs for maintenance particularly at the start-up point. (Center for Clean Air Policy, 2011).