Green Roofs in the District: RiverSmart Rooftops
Green, or vegetated, roofs help to manage stormwater. Stormwater runoff is rainwater that flows off impervious surfaces such as rooftops, driveways, roads, sidewalks and sometimes even lawns. Stormwater runoff travels from these surfaces to our streams, picking up pollutants such as oil and grease from our roadways and driveways as it goes. Nutrients from lawn fertilizers and bacteria from pet waste may also be picked up by stormwater and carried to our streams. Once in the stream, stormwater causes erosion, poor water quality and destruction to habitat for fish and other wildlife.
Green roofs hold and delay rainfall, effectively preventing rainwater from becoming stormwater and reducing combined sewer overflow events. In addition, green roofs filter air pollutants from the rainwater and save energy in buildings.
Green roofs are especially effective in cities such as the District, where so much surface area is taken up by rooftops. Green roofs are most appropriate for flat rooftops of commercial and residential buildings; modern systems are lightweight but roof structures must be checked for adequacy.
Designs take many forms but modern "extensive" systems are simple to install and use proven, tested components. They consist of a drainage layer and root barrier plus about 3-4 inches of growing media and plantings of low-growing "sedums." The plants have been developed and cultivated for local climatic conditions, they survive long dry periods and they perform well in absorbing heavy rainfalls, with little maintenance after establishment.
Green Roof Demonstration Projects
The District recognized the technical and economic benefits of green roofs in the early 2000s, and began supporting their development through financial incentives to building owners.
The CBF Green Roof Demonstration Program – Final Report* (2003-08) summarizes the results of this effort and provides data regarding the benefits of green roofs and the prospects for future expansion.
In 2007, DDOE initiated a $3 per square foot green roof subsidy program that has resulted in funding for 12 green roof projects throughout the District. That subsidy has increased to $5 per square foot, with a per-project maximum of $20,000.
A Green Roof Toolkit* was also produced as a result of this effort, to assist District building owners with practical information to assist decisions about designing and installing green roofs.
These and other efforts have resulted in the installation of numerous green roof projects across the District. We have identified a small group of Showcase Green Roof Projects – each of which provides practical green roof data and contact information for site visits or further investigations.
DDOE has also compiled an inventory* of approximately 75 green roofs in the District, each over 1,000 square feet, with total area coverage of ~350,000 square feet.
Green Roof Incentives
DC Greenworks administers DDOE’s green roof subsidy program. Several projects have been funded to date and projects will continue to be subsidized through 2009.
DDOE is also extending incentives for DC building owners to install green roofs. Please continue to check this space for announcements of these and other new initiatives.
Green Roof Technical Data
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Green Roofs for Healthy Cities (GRHC) is an industry organization that sponsors annual technical meetings and trade shows --- their web site provides information about green roof technology and businesses.
- Greenroofs.org contains encyclopedic references to green roof industry.
- Greenroofplants.com is a local grower of green roof plants and provides technical and business data.
- Green Roof Research Center at Pennsylvania State University is a local source of green roof monitoring data.
Green Roof Reports
Green Roof Virtual Tours in the District
