Active and passive rooftops
Blue roofs often try to mimic natural systems to catch and temporarily store water so they can reduce or slow the effects of rainwater overflow from storms. They attempt to slow the rapid discharge of water runoff bouncing from
hard surfaces that can overflow sewers and create veritable epidemics for natural bodies of water. Instead, rooftop detention ponds, acting more softly and slowly like sponges, can imitate the natural absorption and slow moisture-release mechanisms of soil.
Blue roofs can consist of an array of equipment, including physical tanks, shelves, leaves, pipe systems, valves, catchments, pools, barrels, and trays that function in either an active or passive capacity (or somewhere in between) in order to collect rainwater. Active roofs can use techniques to store, slow, move, and reuse water according to a pre-designed mechanism. Active blue roofs, which are also sometimes called automated roof runoff management systems, can be anywhere along the gradient from highly sophisticated to more limited in scope.
Some actively controlled systems will use programmable, hydraulically controlled valves to control the retention and release of water for reusable collection, drainage, or subsurface storage into a series of containers. Some systems even use communications or data tools such as forecast integration—
a technology employing sensors and Internet-based data feeds to help estimate rainfall quantities. Projects can use such tools to help schedule and record their real-time rainwater collection to meet user demand.
Whereas passive blue roofs typically need little to no upkeep, one of the primary considerations of active blue roofs is their need for regular maintenance. Passive blue roofs simply catch and hold water, and may also function as temporary holding tanks for storm events by later releasing water via the process of evaporation.
Generally speaking, the volume of water capable of being retained by a given blue roof system depends on several factors, the most significant being roof size, quantity and depth of trays/containers being deployed, and geometry of the overall configuration. The total volume contained can ultimately be calculated using the lengths, widths, and heights of the total collection of all containers.
In addition to the geometry of a blue-roof system, water collection will also ultimately depend on the climate conditions at that particular site at that time. According to the annual precipitation based on weather data collected from 1981 to 2010 for the (NOAA) National Climatic Data Center, the average rainfall in New York City was 1268 mm (50 in.) whereas the average rainfall in San Francisco was only 525 mm (20.7 in.).
Safety is of primary importance for maintenance personnel. Blue roof trays and basins are commonly less than 0.3 m (1 ft) deep, but maintenance crews must still be able to reach components of the system that require regular tuning, repairing, or cleaning. As maintenance can be costly and intensive, systems requiring limited upkeep should generally be prioritized. The NYDEP was one of the first city departments in the country to use a passive and essentially automated tray design incorporating special fabrics to laterally transfer and filter water. Using highly limited supervision to increase efficiency and enhance function, NYDEP made effective choices to decrease maintenance.