Rooftops Slowly, but Steadily, Start to Sprout

Technology begins to take root on roofs in North America as new research supports claims of environmental benefits, and policies encouraging implementation multiply.
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From Architectural Record
Joann Gonchar, AIA

The Portland Building roof will include a system to monitor how well the installation mitigates storm-water runoff. City officials hope to ultimately use the data collected, along with data from four other roofs it has been monitoring for several years, to fine tune requirements for various incentives aimed at increasing implementation of the technology. Results thus far show that the installations mitigate between 30 and 60 percent of total runoff, depending on factors such as the thickness and composition of the growing medium, and the type of plant material, according to Tom Liptan, the city's environmental specialist.

Portland offers a floor area ratio (FAR) bonus to building owners who incorporate green roofs into buildings in the center of the city. About 12 projects have taken advantage of the policy since it was instituted in 2001. "We are now starting to evaluate how we might improve the FAR bonus or develop policy to encourage green roof development in other areas of the city," says Liptan.


The plots each employ a different green roof system. Performance under the same conditions varies (above). Data from one Seattle test plot (below) demonstrates that even after successive storms, green roofs typically mitigate at least some runoff.


Green roof advocates say incentives are necessary to offset the expense. Even the most utilitarian extensive installation adds about $7 to $10 per square foot to the cost of a roof, estimates Drew Gangnes, director of civil engineering at Magnusson Klemencic Associates, a Seattle-based consulting firm.

Some jurisdictions allow a green roof as a substitute for other storm-water-management measures. However, even elimination of an underground retention tank will cover only 30 to 70 percent of the roof's cost, says Gangnes. "The public sector should provide incentives since there are public benefits," he says.

Gangnes's firm, along with a construction firm and several local developers, is monitoring the performance of five green roof test plots installed on top of four existing buildings around Seattle in February 2005. The 8-by-12-foot test plots each employ a different proprietary system, with a different thickness and composition of growing media and different plant material. So far, the data collected indicates that even in soggy Seattle, green roofs can be an effective storm-water-management strategy. The plots, even after successive storms, generally held water long enough for it to evaporate from the soil and transpire from the plants-a process called evapotranspiration.

Some researchers are developing predictive modeling tools to help building designers and owners calculate the retention and detention potential of a green roof with specific characteristics on an individual building. Earth Pledge, a new York City−based nonprofit environmental organization, is developing such a tool. The program will also have macro capabilities, allowing urban planners and government leaders to evaluate the aggregate impact of green roof development in a given sewer shed and direct resources. "It is a policy-making tool that will help identify the neighborhoods where green roofs will have the most significant impact," says Leslie Hoffman, executive director.

 

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Architectural Record
Originally published in August 2006

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