Green roofs are gaining popularity in U.S. commercial real estate development. Advances in technology and improved payback periods are expected to drive even greater adoption. When integrated into the early design stages, they provide sustainability and solid returns on investment.

The use of soil and planted material to cover commercial and residential buildings is not a new concept, but the interest in sustainable development has fueled more innovation and implementation. The largest green roof on the East Coast, in Culpepper, Va., outside Washington D.C., was recently completed atop the National Audio Visual Conservation Center of The Library of Congress.

In Manhattan, Rockefeller Center is installing an 18,000 sq. ft. green roof to be completed this year. In San Francisco, Hines' winning proposal for a mixed-use complex of office towers 1,000 feet high at the Transbay terminal was aided by inclusion of a huge five-acre green park 70 feet above the main bus platform.

Ford, meanwhile, has been a corporate real estate innovator. Its one-acre green roof atop the carmaker's Premier Auto Group headquarters in Irvine, Calif. in 2001 was the first Ford building to receive LEED certification and the first LEED-certified project in Orange County. The 10.4-acre “living roof” on Ford's Dearborn, Mich. truck plant is the world's largest.

Behind the curve

While a 2005-2006 survey by Green Roofs for Healthy Cities estimated the amount of U.S. green roofs grew by 25% from the year before, representing more than 3 million sq. ft., the U.S. is still playing catch-up to other nations. Tokyo now requires 20% of roof surfaces to be green. In Germany, 7% of roof space is already green.

A green roof provides a number of benefits:

  • Reduced energy costs: A conventional roof will heat up to as much as 180 degrees Fahrenheit, while a green roof may only heat up to 80 degrees. Thus, HVAC systems can be downsized and operating costs reduced by as much as 25%, reports Swinerton Builders, which constructed Gap's headquarters and its 69,000 sq. ft. green roof in San Bruno, Calif. in 1997.

  • Reduced heat island effect: In New York City, the U.S. Department of Energy estimates that reflective roof surfaces and urban re-vegetation could lower summertime temperatures by 3.6 degrees Fahrenheit. Each degree can translate to millions of dollars in increased air-conditioning demand.

  • Longer roof life and lower long-term costs: Green roofs protect buildings from ultraviolet-ray damage and relieve the problem of fluctuating temperatures that can damage roofing. Researchers from the University of California at Davis reported that green roofs can “double the life of a roof from 15 years to 30 years.”

  • Stormwater capture and pollution filtration: Green roofs act as a filter for many airborne pollutants that fall with the rain. A green-roof surface of 3 to 5 inches of soil or growing medium absorbs 75% of rainfalls that are one-half inch or less. This “sponge” effect reduces stormwater runoff and helps relieve the pressure on urban storm-drain systems.

  • Improved air quality: Green roofs filter air by absorbing and converting carbon dioxide and producing oxygen. Research indicates that a 1 square meter plot of grass can remove 0.2 kilograms of airborne particulates from the air. U.S. cities are creating green-roof incentives, led by municipalities such as Chicago, New York, Cambridge, Mass., and Portland, Ore. Chicago provides a 50% matching grant for adding green roofs to existing private-sector buildings downtown.

Assessing the costs

Green roofs carry higher upfront costs, but researchers indicate they are competitive on a life-cycle basis because of reduced maintenance and replacement costs. The City of Portland estimates a new green roof costs $1 to $6 more per sq. ft. than a standard roof.

Although energy efficiency will offset operational costs, developers typically have not been able to incorporate the higher costs into the project's pro forma.

Green roof owners can break even on upfront costs in five to 10 years based on energy savings and maintenance. A study by the University of Michigan concluded that the average payback period for more extensive green roofs was about 20 years.

In other words, green roofs not only pencil, they can pay off in long-term cost savings and environmental benefits.

John Loomis is a principal with SWA Group in Sausalito, Calif. He can be reached at jloomis@swagroup.com