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A Living Roof

Photo by <a href="" target="_blank">GreenGrid Green Roof Systems</a>

Called an eco-roof, garden roof, living roof, or vegetated roof, a green roof is a lightweight, layered roofing system that allows virtually maintenance-free plants to top a waterproofed substructure. If you've been hearing more about them lately, that's because the technology has become simpler, making installations, especially residential ones, more common.

Runoff Reduction

Photo by Jay Stearns/Photo Design

The green roof of this unheated porch in the Chesapeake Bay area absorbs 27 percent of the rain that falls on it, reducing storm-water runoff that might pollute nearby rivers and streams. Homeowner Mark Gaulin, a roofer, installed "chicken ladders" to facilitate twice-yearly weeding of the roof, which is planted mostly with sedums, a family of hardy, drought-tolerant perennials.

Primitive Genius

Photo by Ian Worley

If you're thinking that green roofs are some kind of tree-hugging, 21st-century eco-design trend, think again. The earliest settlers of the Great Plains lived in sod houses. So did 18th-century Germans and 12th-century Icelanders, who carved these sod-covered homes out of hillsides; they found that the pesky vegetation that grew naturally on their rooftops provided needed insulation.

Hitting the Slopes

Photo by <a href="" target="_blank">Nancy Rottle</a>

For many years, green roofs were installed only on flat surfaces, such as the ubiquitous black-tarred tops of a cityscape. Today, sloped suburban roofs as steep as 6 in 12, or 26.5 degrees, can go green—thanks to a mesh-like screen technology that keeps the growing medium and plants on the roof and out of the downspout.

Lower Heating Bills

Photo by Emilio Ancaya, <a href="" target="_blank">Living Roofs, Inc.</a>

Eco-roofs help to purify the air by absorbing pollutants; soak up rainwater, thereby reducing storm-water runoff that pollutes rivers and streams; and keep a rooftop 10 to 20 degrees cooler in the summer and warmer in the winter, which means lowered air conditioning and heating bills, fewer fossil fuels wasted, and a healthier environment. Green roofs also allow creative types to show some personality, as Emilio Ancaya did with his inventive birdhouse design.

Urban Trends

Photo by Doug Folgelson

Today, green roofs are mandatory for new buildings in some European countries. In the late '90s, Chicago Mayor Richard M. Daley began searching for ways to reduce his city's urban heat-island effect, which occurs when dark surfaces such as parking lots and rooftops absorb and retain heat, spiking local air temperature and increasing smog. In 2001, he added a 21,000-square-foot green roof to the top of Chicago City Hall. The mayors of Atlanta and Portland, Oregon, have since followed suit.

The Greening of Corporate America

Photo by <a href="" target="_blank">GreenGrid Green Roof Systems</a>

Corporate America has helped bring the green roof trend to the United States. Wal-Mart, Target, The Gap, and the Ford Motor Company have planted green roofs on some of their buildings. This 33,500-square-foot green roof was built atop a Kohl's retailer in Chicago, Ill., in 2005.

Playing Catch-Up

Photo by Bradley Rowe, <a href="" target="_blank">MSU Green Roof Research Program</a>

While the U.S. has a long way to go to catch up with the estimated 12% of flat-roofed German buildings that are covered with vegetation, experts are confident that the American movement will continue to expand, especially since it's a potential lucrative market for landscape contractors and designers, architects, and nurseries. Dr. Brad Rowe, Associate Professor of Horticulture at the Michigan State University Green Roof Research Program, built this green-roofed shelter for his dogs, Finn and Cooper.

Extensive Roofs

Photo by Bradley Rowe, <a href="" target="_blank">MSU Green Roof Research Program</a>

While it might be possible to design your own green roof, it's advisable to hire an architect or landscape professional who has had some experience with green-roof systems. That expert will want to consult with a structural engineer who can advise you as to what kind of system you are installing. "Extensive" systems are the lighter, low-maintenance variety, usually planted with hardy, drought-tolerant perennials such as sedums and other succulents. Because an extensive system requires no watering or special care, it's environmentally preferred.

Intensive Roofs

Photo by Jozanne Rabyor

"Intensive" green roofs are more elaborate: deeper, heavier and sprouting everything from grass to small shrubs, trees and walkways, like this beach-like retreat installed in Washington State. Such an intensive system is really a roof garden and requires watering, fertilizing, or even mowing.

Let It Snow

Photo by Emilio Ancaya, <a href="" target="_blank">Living Roofs, Inc.</a>

A good landscape professional will help choose plants that will provide an attractive roofscape for your region in all seasons. "Some sedums are deciduous and lose their leaves in winter, while others just turn a different shade, such as deep russet or bright red," says Ed Snodgrass, an expert in green-roof plants and owner of Maryland's Emory Knoll Farms. Many roofs mix both types of sedums with other perennials for variety. As for snow, most sedums are happy to hunker down under a cozy blanket of the white stuff.

Recipe for a Green Roof

Photo by Ian Worpole

Obviously, green roof technology is a bit more complicated than just throwing a bunch of dirt and seeds on your roof. Though systems vary, most have some similar elements. Before it's planted, the surface must be covered with an elaborate, cakelike assembly. The bottom layer is usually made of plywood, steel or concrete, sometimes topped with rigid foam insulation (the pink layer). Then, a rubberized liquid asphalt is laid down, followed by a polypropylene root barrier, a drainage layer, and a filter mesh fabric that will keep the lightweight growing medium in place. This "soil" consists of compost mixed with expanded shale, slate, or fragments of volcanic rock. An optional wind blanket can help hold baby plants in place. Finally, your drought-tolerant perennials, ideally 2 to 6 inches tall, can be planted.