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.