Blue roofs are non-vegetated systems that focus on collecting stormwater. A blue roof system detains rainwater directly on a rooftop and slowly releases that water to the sewer system, allowing for some depression storage and evaporation losses. The water collected can be used for irrigation, a site infiltration system, a rain garden, or slowly discharge into the sewer system.
Blue roofs are less costly than green roofs due to the lack of materials required. They are most effective and practical when installed on relatively flat surfaces, which are often associated with commercial or industrial buildings. Blue roofs do not provide benefits such as energy use reduction or habitat and aesthetic appeal, but due to the light colored roofing material they can also provide sustainability benefits through rooftop heat reduction.
Special structural considerations are necessary to ensure that adequate support is provided for the detained water and blue roof materials themselves.
Green & Blue Roofs
Green roofs are specially designed roofs with waterproofing membranes covered in growing media and plants that capture, store, and slow the flow of rain water runoff. Green roofs provide a plethora of environmental benefits like absorbing and evapotranspiring rainfall, filtering pollutants, moderating heating and cooling, enhancing aesthetics, prolonging the lifetime of a roof, and providing wildlife habitat.
Green roofs can be planted in place, or they also can be purchased in pre-grown modular trays.The cost of a green roof can vary greatly, depending upon the type, depth of growing medium, selected plants, use of irrigation, size of instillation, and accessibility needs.
Green roofs are most effective in urban areas where space is limited and options for rain gardens and on-the-ground treatments may not be possible. Before installing a green roof, you must work with a professional who can evaluate the strength of your roof and its ability to accomodate the added weight of the growing medium, plants, and water it will store.
The Basics of Blue
General Information about Green Roofs:
NYC Department of Environmental Protection Blue Roof and Green Roof Study: http://www.nyc.gov/html/dep/html/stormwater/green_pilot_project_ps118.shtml
University of Connecticut Green Roof Gallery:
Green and blue roofs help to capture and slow the flow of rain water before it becomes harmful stormwater runoff.
Blue and green roofs can vary in cost. This blue roof in NYC used reclyled cafeteria pans filled with gravel to capture, evapotranspirate and slowly release rain water after a storm. Source: Gowanus Canal Conservancy.
An example of a modular tray style green roof on Science Hill at the Yale University Campus. Source: Kendall Barbery