Growing Green Roofs, City by City

Green roofs—rooftops that are partially or completely covered with vegetation growing in soil medium over a waterproof membrane—have gained momentum over the past six years as building owners recognize their advantages over conventional roofing in terms of better energy efficiency and reduced rain runoff. Now local governments are exploring incentives for moving the practice into the mainstream. A look at cities that are leading the country in green roof coverage reveals a growing range of policy tools.



Alexi Boado, low-impact development coordinator for Washington, DC's District Department of the Environment (DDOE), says the city began seriously examining green roofs for stormwater control five years ago, when the DC Water and Sewer Authority provided $300,000 for green roof development as part of a court-ordered settlement. Those funds, managed by the nonprofit Chesapeake Bay Foundation, seeded a program of incentive grants that encouraged eight builders to choose green roofs over other traditional devices as their primary stormwater control device (stormwater control plans are required for any new construction or redevelopment of more than 5,000 square feet in the District). Builders also have a procedural incentive: designs that include a green roof in the stormwater control plan receive expedited processing.

To build local engineering design and green construction capacity and catalyze interest in green roofs, the DDOE is working with the USDA Natural Resources Conservation Service to offer almost $800,000 in complete design-and-build services for select public and commercial properties. This program is slated to begin in the summer of 2007. In addition, as part of a cash grants program, DDOE and its sister agencies are in the process of installing green roofs on three new community recreation centers, two public schools, and one housing development. Previous grants have subsidized some of the first green roofs in the District, as well as the implementation of many other innovative stormwater control practices such as rain gardens and permeable surfaces. The District allotted about $500,000 in 2007 to innovative stormwater control grants in addition to the Natural Resources Conservation Service partnership.

Dawn Gifford, program coordinator of the nonprofit DC Greenworks, has seen a shift in green roof installations from mainly commercial buildings to a mix of commercial and residential. DC Greenworks has dedicated itself to installing green roofs throughout the city; a high-profile demo model they installed at 1425 K Street NW in 2004 has drawn more than 3,000 visitors and inspired similar projects across the metropolitan area.

Doug Siglin, director of federal affairs for the Chesapeake Bay Foundation, explains the public policy perspective behind the interest in green roofs: one problem in the Anacostia River, which runs through Washington, DC, and in the bay generally, is too much erosion, with silt increasing water turbidity. Most erosion comes from stormwater runoff; green roofs help moderate that blast of runoff from precipitation events, and therefore help local governments deal with rainwater by detaining, retaining, and absorbing it where it first hits.

Chicago officials see another public health benefit in moderating the city's "heat island" effect (defined as urban and suburban areas having temperatures up to 10°F higher than nearby rural sites). Heat islands spike energy demands, air pollution levels, and heat-related illnesses such as heat exhaustion and heat stroke. With climate change, says Sadhu Johnston, the city's commissioner for the environment, Chicago can expect hotter and drier summers—conditions that the heat island effect will only exacerbate.

Johnston says green roofs can help avert heat wave–related deaths, citing studies that show lower temperatures on green roofs compared with traditional roofs, and reduced air-conditioning use in buildings with green roofs. According to the 2004 Green Roof Test Plot 2003 End of Year Project Summary Report by environmental engineering firm MWH, which is posted on the City of Chicago website, the mean temperature of green roof areas in the heat of the day (between 12:30 and 4:30 pm) was up to 31% cooler than other roof types.

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