Green roofs are another green infrastructure concept that help reduce the harmful effects of stormwater runoff. Green roofs are roofs on buildings that are partially or completely covered in vegetation with a root barrier and drainage, which is planted over a waterproofing membrane. Along with being aesthetically pleasing, green roofs provide a plethora of environmental benefits like absorbing rainfall, filtering pollutants, moderating heating and cooling, increasing roof durability, and providing wildlife habitat. Although it is possible to install a green roof on a residential house, they are most commonly installed on commercial buildings with flat roofs. Green roofs can be planted in place, or they also can be purchased in pre-grown modular trays.
There is a lot to know before installing a green roof. Factors such as roof slope and structural loading capacity are most important, and should be evaluated by a structural engineer. Roof building materials, drainage systems, waterproofing, water supply, roof access, and maintenance are other considerations. Thankfully there are Green Roof Professionals (GRP) who can address these questions and provide design and native plants suggestions based on your roof and maintenance criteria.
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. A more cost effective option may be a “blue roof.”
(Green roof cross-section illustration courtesy of American Wick Drain.)
Blue roofs are yet another form of green infrastructure, but unlike green roofs they 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 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 they do slightly outperform green roofs for stormwater reduction. Due to the light colored roofing material they can also provide sustainability benefits through rooftop heat reduction. In some cases, special structural considerations are necessary to ensure that adequate support is provided for the detained water and blue roof materials themselves.
An example of a blue roof. (Photo courtesy of the Gowanus Canal Conservancy.)