Plants, grass on the rooftop? No longer an oddity.

The idea is simple: bring back some of the organic material displaced by buildings, streets, and parking lots. Advocates tout benefits that range from reducing the urban "heat island" effect – which makes cities several degrees warmer than surrounding areas and can translate into millions of dollars in energy costs – to lengthening the life span of a roof, providing community garden or recreation space, and contributing to a building's energy efficiency.

"Cities are just going to keep getting hotter," says Steven Peck, president of Green Roofs for Healthy Cities. "So you take away hot surfaces and turn them into air conditioners. Green roofs do that very, very well."

On City Hall, for instance, the ambient temperature on the planted, city side of the roof is often 50 to 70 degrees cooler than that on the county side, still traditional black tar. Commissioner Johnston acknowledges that it's hard to know how many such roofs are needed before the effects become felt throughout the city, but he's determined to keep encouraging them until Chicago gets there.

"It's like turning off the water when you brush your teeth," he says. "Every building that does it this way has an effect."

This is why Chicago is doing its best to push private developers to follow City Hall's example. Every new roof in the city is already required to be reflective – another step to minimize urban heat island – but the latest matching-funds initiative is designed to show existing buildings that they, too, can establish green roofs.

"There are certain preconceived notions that it's easier to do it with new construction than with existing construction," says Constance Buscemi, spokeswoman for the city's Office of Planning and Development.

But there can be drawbacks. It's often twice as expensive to install a green roof, though experts say that's usually recouped through the roof's lengthened life span (they can last 40 or 50 years instead of the typical 20 or 25) and energy savings for the building. And some buildings simply aren't designed for the additional load, even when that's just a few inches of lightweight soil.

In a recent survey by Green Roofs for Healthy Cities, Chicago was followed by Washington and Suitland, Md. (home of a huge green-topped National Oceanic and Atmospheric Administration building) in green-roof square footage. But the amount of space is increasing rapidly – up 80 percent in the US between 2004 and 2005.

"What we've seen in Europe is that once the technology was understood and people saw that it worked, combined with incentives from the regulatory side, it really blossomed as an idea," says David Yocca, a senior partner at the Conservation Design Forum in Elmhurst, Ill., who has designed a number of green roofs. "It's the sort of idea that makes a lot of sense."

Page: 1 | 2