Blue Roof

A blue roof is a roof designed to hold rainwater for a period and then release it slowly, rather than draining it away immediately. Blue roofs are usually flat or low-sloped and are used mainly in cities, where large areas of paving and roofs send stormwater quickly into sewers. By delaying runoff, they reduce peak flows, which helps prevent flooding and combined sewer overflows.

Types

Passive systems have no moving parts. Water is held on the roof and released through restricted outlets:

• Roof-integrated systems retain water on the membrane, often within a gravel layer, using weirs or flow-control devices on drains.
• Modular trays placed on existing roofs that hold water and release it through controlled openings.
• Roof dams or checks that hold water behind low barriers with weep holes.

Active systems use valves (manual, timer-controlled, pneumatic or hydraulic) to control drainage. Forecast-based systems that empty the roof ahead of expected storms were proposed in 2008.

Blue-green roofs

A blue-green roof combines water storage beneath a vegetated green roof. The stored water can irrigate plants, increase evapotranspiration and cooling, and still provide detention during storms.

Case example

From 2010 to 2012, the New York City Department of Environmental Protection tested blue roofs to reduce combined sewer overflows. The pilot used passive trays with filter fabric to control drainage, and monitoring showed lower peak flows and delayed timing in the sewer system.

Design considerations

• Structural capacity for the weight of stored water, including if outlets block. Water weighs about 10 kg per square metre per centimetre of depth (about 5.2 lb per square foot per inch).
• Waterproofing suited to prolonged contact with water.
• Overflow drainage so water cannot rise beyond the design depth.
• Drain-down time so roofs empty before the next storm and do not become mosquito habitat.

Limits

Blue roofs detain water but do not treat it; they do not remove pollutants by themselves. In drier climates, stored water can be harvested for reuse, such as irrigation or cooling.