Roofs are one of the most significant sources of concentrated runoff from developed sites. A green roof, sometimes called an eco-roof, living roof or rooftop garden, is the roof of a building that is typically covered with a layer of waterproofing material, then with soil or another planting medium, and planted with grasses, flowers, groundcover, or even shrubs and trees. It may also include additional layers such as a root barrier and drainage and irrigation systems. Managing rooftop runoff has particular value in urbanized settings where space for other BMPs is limited.
Benefits of Green Roofs
Based on documented experience and studies, a green roof offers several important benefits not found in conventional roofing:
- Captures and evaporates from 10 to100 percent of the precipitation that falls on it. This reduces the volume and speed of stormwater runoff leaving the site, helping prevent sewer overflows and protect receiving rivers and streams.
- Lowers the temperature of stormwater runoff, which helps maintain the cool stream temperatures needed by fish.
- Improves outdoor air quality by reducing smog.
- Reduces the heat island effect in urban settings by cooling the air during evapotranspiration.
- Increases vegetation and wildlife habitat on urban sites that typically have neither.
- Provides insulation and lowers cooling and heating costs for the building.
- Provides an attractive alternative to a conventional roof.
- Lasts twice as long as a conventional roof, saving replacement costs and materials
- Creates a market for recycled materials, such as compost, mulch, soil and other green roof components.
- Creates jobs in multiple industries.
In 2004, 3 Rivers Wet Weather awarded multiple grants for the installation or monitoring of green roofs throughout the City of Pittsburgh and its surrounding neighborhoods. 3RWW considered both extensive and intensive roof systems. Extensive green roofs are ecological roof covers with limited human access and typically have limited sectional depth with thinner and less numbers of layers. Intensive roofs are more like the traditional roof gardens, with deeper sectional depths, and may provide access and recreation space.
The following projects were awarded grants:
- Shadyside Giant Eagle (Pittsburgh): Renovation and expansion of an existing commercial building. Extensive roof with five-inch growing medium using non-invasive, drought resistant plants. Excess stormwater is captured in cisterns and provides grey water for other uses. 3RWW funding: $240,000
- Hammerschlag Hall/CMU (Pittsburgh): Renovation of an existing building on the CMU campus. Extensive green roof that collects discharge from an adjacent main roof area; Pennsylvania native plants are used when possible. 3RWW funding: $25,250.
- 213-215 E. Eighth Avenue (Homestead): Renovation of an existing commercial/residential building on the main street of Homestead. Extensive green roof for residents of the lofts on the upper floors. Second side of attached building provides control side. 3RWW funding: $66,000.
Green Roof Stormwater Monitoring
In addition to funding the construction of the green roof projects, 3RWW funded the development of monitoring protocols and programs that will provide uniform standards to evaluate the performance of these roofs.
1. The project at Carnegie Mellon University titled, Development of a Green Roof Monitoring Protocol and Green Roof Stormwater Modeling Tool, was funded by 3RWW for $70,448. The final report provides a summary of the different methods, procedures, and sensors available to monitor stomwater runoff from conventional and green roofs. It can also be used as a guide to determine appropriate methods, procedures and sensors for monitoring roof runoff for different applications and roof types. The study identified best practices currently in use in Europe and the U.S. for monitoring stormwater retention and diversion and stormwater quality on green roof projects, and assessing the suitability of these monitoring practices for implementation in Pennsylvania. The recommended methods and procedures will then be applied to all green roof projects.
2. The University of Pittsburgh project titled, Green Roof Instrumentation, Data Collection and Analysis, was funded by 3RWW for $99,356. The measurement protocol monitored water quality and quantity with a combination of installed instrumentation, sample collection and laboratory testing for both the Shadyside Giant Eagle roof and the Homestead green roof.
The Allegheny County Office Building is another good example of a green roof in the City of Pittsburgh. Completed in June 2010, the roof, which uses four different types of technologies, showed a reduction of 40-50 degrees in temperature over the first two months and is expected to save 10-20 percent in overall heating and cooling costs. Eight dataloggers and a weather station were put in place to help measure the benefits of the green roof, and the County plans to partner with the Art Institute on growing hard to find herbs for their culinary program.