Up high and down to earth

A beauty of a green roof comes with benefits

A green roof in Washington, D.C., is a sign of things to come in Boston. The owner of the roof - the American Society of Landscape Architects - has turned the site into an educational project. (courtesy of asla)

WASHINGTON - In a small space, with a modest budget, a new kind of green roof here is so inventive it changes the way you think about what a roof can be.

Green roofs are a hot item in architecture. Mayor Menino, for one, advocates them for all new buildings in Boston. But they're often uninteresting as architecture. The usual version is a flat surface planted with sedum, looking like a big rug or maybe a stretch of semi-desert landscape.

This one is different because it's three-dimensional. As you emerge from a stairway onto the roof, you find yourself flanked by two green mounds, each about 8 feet high. You don't feel exposed or threatened. The mounds embrace and protect you, and they shape a small social space. Someone, you realize, has created a landscaped enclosure up here, an outdoor room. It's a place you can inhabit, not merely stand on top of.

Looking down at your feet, you realize you're walking on a light aluminum grating, three inches above a field of, yup, sedum. That often boring plant feels, as you walk above it, like a magic carpet.

And the planting isn't all sedum, of course. The two mounds receive sun and wind differently in different places. One sloped area is called "meadow and upland," another is drier and hotter. The owner and the designer are continually measuring and experimenting to see which plants work best under what conditions.

The designer is Michael Van Valkenburgh, a nationally known landscape architect with offices in Cambridge and New York. He's perhaps best known locally for planting, believe it or not, 250 new trees in Harvard Yard. The client - the owner of the roof - is the American Society of Landscape Architects (ASLA), led by its CEO, Nancy Somerville. The building is the ASLA headquarters, at 636 Eye St. NW, not far from the Capitol.

Americans are laggards in green roofs. The leading country is probably Germany, where there are 50 square miles of green roof. That's bigger than the entire city of Boston. And that figure is probably already out of date. Every year, the Germans add rooftop gardens totaling four times the area of New York's Central Park.

The ASLA roof is intended to change that. It is, above all, an educational project. It's tiny, only 82 by 35 feet, but that's enough to transmit the lesson. Groups of schoolchildren come to see it. So do professional landscape architects. So do government officials.

In a quiet way, it's quite beautiful. Among the plants doing well (I love plant names) are flame sumac, trumpet vine, pasture rose, purple lovegrass, nodding onion, and thread-leaved tickseed. The mounds change color with the seasons. Where the building's structure can bear the most weight, which is atop an elevator shaft, the soil is 21 inches thick and planted with sumac that will eventually grow as tall as 30 feet.

More important than its looks, though, are the tasks the roof silently accomplishes. Cooling, for example. "There's an identical building next door, built at the same time 15 years ago, which has a flat black roof," says Van Valkenburgh. "It's a definite kind of hell." In the hottest weather, tests reveal, the ASLA roof is 32 degrees cooler than its neighbor.

Some other advantages:

The green roof retains 75 percent of all rainfall, thus keeping 27,512 gallons a year from flowing into the city sewer system. Since much of Washington still uses old-fashioned combination storm and sanitary sewers, the roof is helping to keep overflow waste out of lakes and streams.

It cuts the building's winter energy cost by 10 percent.

It should last at least twice as long as a conventional roof, because the planting forms a protective blanket over the waterproof membrane.

It helps insulate the interior from sound.

It cleans the air as the plants absorb carbon dioxide and give off oxygen.

It helps reduce the "urban heat island" effect, the tendency of built-up cities to be warmer than rural areas.

It's an aesthetic amenity, not only for the users but their neighbors.

The mounds help conceal the ugly rooftop utility boxes.

The plantings may provide biological habitat for some species.

The ASLA doesn't, however, wish to offer habitat to rats. There are a lot of restaurants nearby, and the ASLA is afraid that nocturnal rats may scale the building and feast on leftovers. Nancy Somerville has banned snacking, except for organized events. In architecture, there's always an unanticipated problem.

Van Valkenburgh compares his roof to the work of the great modernist architect Le Corbusier, who furnished his with daycare playgrounds, small parks, and other social spaces, thus creating a vital architectural skyline as well as a useful roofscape.

There's one more advantage to green roofs. They allow us to build more densely, without losing recreational open space. And as every study shows, the more densely we build the more energy we save, because people walk, bike, and take public transit rather than drive.

Boston, too, may soon be sprouting some interesting greenery. The Boston Architectural College, which owns a big flat roof on Newbury Street, has decided to green it. Ted Landsmark, the college's president, says that unlike the ASLA's, this roof will be accessible by elevator. And a German leader in green architecture is now the architect of a big Harvard science complex, soon to begin construction in Allston. He is Stefan Behnisch, the designer of the fine Genzyme Building in East Cambridge. At Harvard, Behnisch plans both planted roofs and solar panels. He's working with the Harvard Green Campus Initiative, which is an independent action group within the university that hopes to make Harvard "a global model of sustainability." This month is National Landscape Architecture Month, and Saturday will be the 186th anniversary of the birth of Frederick Law Olmsted, the father of the profession. Tuesday is Earth Day. Green is in the air.

A superb book about the ASLA roof "Green Roof: A Case Study" has been published by the Princeton Architectural Press.

Robert Campbell, the Globe's architecture critic, can be reached at camglobe@aol.com.