The expanse of native plants and grasses isn’t a park, but the top of Chicago?EUR??,,????'???s City Hall, the premier green roof in a city that is making green building a civic cornerstone.

Six years ago, when Mayor Richard Daley had the roof installed, it was an oddity. Today, more than 200 green roofs in the city have been constructed or are under way, covering some 2-1/2 million square feet of tar with plants – by far the most of any American city.

Now other cities, hoping to cool and clean their air and help with storm drainage, are beginning to emulate Chicago, and the city is taking key steps to encourage – and in some cases require – private developers to follow City Hall’s example.

Chicago’s City Council just announced a pilot program that will provide up to $100,000 in matching funds for developers who retrofit existing downtown buildings with green roofs.

“You look down on the prime real estate areas of this country – downtown Chicago, Manhattan – and so much is unutilized, all these rooftops,” says Sadhu Johnston, Chicago’s environment commissioner.

Green roofs may be surprising in a city still more known for manufacturing than composting, but they are relatively common in Europe. Germany – the country that gave Mayor Daley the idea – has green roofs on about 20 percent of all flat roofs, according to one estimate.

In Chicago, they sit atop the Apple store, a Target, and a McDonald’s . Even Chicago’s soon-to-open Wal-Mart will have one – the company’s first.

Nationally, green roofs grace the Gap headquarters in San Bruno, Calif.; a Ford Motor plant in Dearborn, Mich.; and the American Society of Landscape Architects building in Washington.

“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 side of the roof is often 50 to 70 degrees cooler than that on the county side, still traditional black tar.

“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.