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What's a green roof? It's what you see on top of Chicago City Hall, lots of tall buildings in Portland, Ore., and the vast 10-acre roof of Ford Motor Company's assembly plant in Dearborn, Mich.: A green roof is planted with a dense layer of vegetation that absorbs rainwater, filters air pollutants, provides insulation and can create wildlife habitat. If enough buildings with green roofs are in close proximity, the respiring plants even create a microclimate that can cool a city.

There are two basic types of green roofs: An intensive one more closely mimics a lush natural garden, complete with a base layer of 2 to 4 feet of soil. An extensive green roof has only 2 to 6 inches of growing medium (generally not soil), and so is much lighter. See examples of both types of green roofs.

Often, roofs are carpeted with a variety of short plants, but some roofs practically resemble a forest, as at the Coast Hotel in Vancouver, British Columbia. (See: hrt.msu.edu/greenroof). While the U.S. so far doesn't match Germany's zeal for green roofs — it's estimated that at least 10 percent of flat-roofed buildings in Germany are covered with vegetation — the concept has growing cache here. Examples abound, including the Social Security Administration Building in New Bedford, Mass. (tinyurl.com) and Atlanta City Hall (epa.gov/hiri/strategies/greenroofs).

Installing a green roof costs more up front, but it can save money in the long run: A green roof can be up to 100 degrees cooler than a traditional roof, and so cuts the cost of a/c by up to 50 percent. Because it can prevent heat loss through the roof of the building, it cuts heating costs by up to 25 percent. A green roof also extends the life of the roofing membrane by protecting it from the elements. And, when you're ready to resell, a green roof can increase property value by as much as 15 percent.

For more info:

*Green Roof Research Program at Michigan State University: hrt.msu.edu/greenroof

* Center for Green Roof Research at Penn State University: hortweb.cas.psu.edu/research/greenroofcenter

*Live web cam of a green roof, plus other resources:

naturemuseum.org/greenroof/planningaroof

* Live web cam, green roof tours, fact sheets and more from American Society of Landscape Architects: asla.org/land/050205/greenroofcentral

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