Succulents, including cactus, are generally the plants of choice for green roof projects, like the demonstration garden on the roof of the American Society of Landscape Architects headquarters building in Washington. They can live on existing rainfall, withstand drought, wind and harsh winter conditions.

Washington ? The sod house pioneers who fashioned farms from the tallgrass prairies of the American Midwest probably would have scoffed had you called them visionaries.

Yet many of the natural components they used for their dwellings, including wildflowers and grasses and thick blocks of turf-covered soil that insulated everything indoors, are the stuff of today’s popular green-roof movement.

As in those earlier times, green-roof construction now is driven as much by its environmental benefits as by adding eye appeal to otherwise drab properties.

“It’s an ecological response to urban areas,” said Edmund Snodgrass, co-author with his wife, Lucie, of “Green Roof Plants: A Resource and Planting Guide.” These roof gardens help manage storm rainfall and insulate against city heat buildup, he said.

Builders classify modern green-roof designs as “intensive” or “extensive.”

Intensive roof gardens require a great deal of structural support for such things as trees and trails, seating and a growing medium more than 6 inches deep. They also require irrigation and frequent maintenance, in showcasing trees, shrubs and flowers.

An extensive garden, meanwhile, is built on a thin layer of growing material, requires little to no maintenance and supports drought-tolerant plants capable of surviving long periods of heat or winter cold.

Snodgrass owns and operates Emory Knoll Farms Inc. near Street, Md., which he described as the first nursery in the U.S. dedicated solely to the propagation and sale of green roof plants.

Roof gardens are built primarily over the flat roofs of large commercial buildings and plant sites, but growing numbers of residential developers are beginning to take an interest, Snodgrass said.

The green-roof phenomenon began to spread in the 1950s when some German cities encouraged building owners to convert their tar and pebble rooftops to vegetation. Since then, Germany has developed more than 50 square miles of green roof space and adds an additional five square miles per year, according to Christian Werthmann, an assistant professor of landscape architecture at Harvard’s Graduate School of Design.

The U.S. has only a fraction of the green roof space found in Germany, but a recent study by Green Roofs for Healthy Cities determined that green roof space in this nation grew 80 percent in the past couple of years.

Chicago leads the trend among all U.S. cities, having planted 300,000 square feet of green roof space in just one year, the nonprofit industry group said. That includes putting a vegetative cover over city hall.

The new Ford Rouge Dearborn, Mich., truck assembly plant has what its architects say is the largest “living roof” in the world, a 10-acre surface planted with a mixture of nine low-maintenance sedum varieties.

Now, Snodgrass said, green roofs are being seen increasingly on urban residential buildings like condominiums, which are more flat-roof. “The single-family home with a pitched roof is still a little ways off.

“People are tending to use green roofs more on additions and garages rather than on whole roofs. You can cut a door into the roof to give it some additional utility,” he said. “A roof turned garden gives your family some extra living space.”

Upfront costs may be higher ($10 to $20 a square foot vs. $5 to $10), but green roofs conservatively can be expected to last two to three times longer than a conventional roof, according to the American Society of Landscape Architects. They also can reduce the heating and cooling costs for buildings by at least 10 to 15 percent.

Green roofs also can retain up to 75 percent of a 1-inch rainfall, greatly reducing the pressure stormwater puts on municipal sewer systems, the society says.