Egyptians and the Japanese have them. So do Scandinavians, Germans and the British. Even the Ford Motor Co. has one in Michigan that’s nearly 500,000 square feet.

They’re roofs with plants growing on them, known variously as green roofs or ecoroofs. Your basic tar-and-shingle roof covered with a fuzzy layer of Northwest moss does not count. These are roofs that have been purposely built and planted to have green things growing on them all the time.

Yes, they’re pretty, especially if they have flowers. But they aren’t built just for looks. Ecoroofs add insulation, making houses warmer in winter and cooler in summer. When the skies open up, ecoroofs capture 10 percent to 100 percent of the rainfall, reducing runoff into the storm-water system. And with life spans of about 40 years, ecoroofs last longer than conventional roofs, which usually need to be replaced every 20 years or so.

They also lower what’s known as the “urban heat-island effect,” the tendency of cities to be up to 10 degrees warmer than the surrounding countryside. Studies in Toronto, Singapore and Chicago have concluded that ecoroofs can reduce the surrounding air temperature by several degrees.

In Portland, a city with a growing international reputation for promoting sustainable development, ecoroofs are beginning to catch on, in part because of a city program that encourages them. The program, operated by the city’s Bureau of Environmental Services, began in 1999. It offers such incentives as storm-water fee discounts and density bonuses to builders to encourage them to include ecoroofs in their projects.

“The city got involved because I had been doing research on storm water,” said Tom Liptan, the city of Portland’s ecoroof program manager.

Because the city’s sewer and storm-water systems are combined, heavy rainfalls create overflows that wind up in the Willamette River. Any reduction in storm-water runoff – including from roofs – means less sewage in the river.

“I had identified this concept of vegetated roof systems,” Liptan said. “Since soil and vegetation hold water well, it struck me that these roofs must hold water, too.”

Still, ecoroofs aren’t for the casually inclined. They weigh 10 to 25 pounds more per square foot than conventional roofs, meaning that existing buildings aren’t always strong enough to hold them up. They take a good deal of initial maintenance, including weeding and watering, before the plants become established.

And while a conventional roof costs $3 to $20 per square foot, according to the city of Portland, ecoroof costs start at $10 per square foot and can run as high as $25 per square foot. “That’s a heck of a lot of money on a 100,000-square-foot building,” Liptan said.

Even with the city’s incentives, Liptan admits that the 30-plus ecoroofs in the Portland area have been built less because of the financial breaks than for environmental benefits. The existing ecoroofs total more than two acres of green stuff up in the air, filtering pollutants, attracting birds and insects, absorbing carbon dioxide and emitting oxygen.

Extreme conditions take toll

One issue with ecoroofs is finding the right plants for them. Even in the moderate Pacific Northwest, rooftop temperatures are much higher than those on the ground, especially in the summer.

“It’s really warm up there,” Sydney Mead said of the ecoroof covering part of Ecotrust’s Jean Vollum Natural Capital Center in the Pearl District. Mead, the center’s manager, said that the five-year-old ecoroof originally was seeded with plants native to the Willamette Valley, but rooftop temperatures in summertime Portland proved too intense.

“Now we’re going to more Central Oregon grasses and seeds,” she said. “The grasses and wildflowers from here are just not equipped to deal with the extreme conditions of an ecoroof.” Those conditions include rain and wind as well as heat, all of which can damage a tender, young ecoroof.

Low-maintenance, self-sustaining plants, especially drought-tolerant sedums, are generally recommended for ecoroofs. Shrubs and trees require much more care as well as room to grow, so they’re better off in roof gardens, where residents can enjoy and tend them more easily.

Scott DeSelle, landscape services supervisor at Portland State University, said that the ecoroof atop the university’s Native American Student and Community Center is really more of a roof garden. It’s designed to be visited, not just admired from afar. And it’s had more complicated issues than the typical ecoroof.

“A lupine just went rampant and took over the entire place,” DeSelle said. “Other plants were too big or died off. So we’re in the process of replanting some of it right now.”

The center’s ecoroof, which was built about three years ago, was designed to showcase native plants significant to Northwest Native Americans. But the heat-reflecting white paint and stainless steel on the southwest-facing roof made for an extreme microclimate.

“Sagebrush and some small manzanita did fairly well,” DeSelle said. “Salal did not do well up there.”

PSU’s Broadway Building has the largest ecoroof – more than 15,000 square feet – built to be self-sustaining, with minimal weeding and a temporary irrigation system just to get the plants established. The Native American center’s ecoroof, however, requires steady maintenance and a permanent irrigation system.

“The Broadway Building, with its sedums and grasses, is pretty bulletproof,” DeSelle said. “The Native American center is ongoing.”

Mead says that 2 inches of dirt – about the bare minimum required for an ecoroof, and the most that can be used without needing structural renovations to the building – weighs about 12 to 18 pounds per square foot when saturated with rainwater. The Ecotrust building was ecoroofed with the 2-inch minimum; PSU’s two ecoroofs each have a hefty 6 inches of soil. And none has leaked.

Garage experiment went well

The oldest recorded ecoroofs actually were homes dug into the earth, basically making mounds with grass and flowers growing on top; they range from early medieval Viking homes to prehistoric Native American cities in the Midwest.

Modern ecoroofs rely less on dirt and more on technology, including a waterproof membrane to prevent leaks, a drainage system to usher away excess water, and mulch to avoid erosion. Flat roofs are the easiest to turn into ecoroofs, but slopes up to 40 degrees are feasible.

Liptan’s garage, in fact, may have the first ecoroof built in Portland, created one day in 1996 when Liptan dumped dirt from his garden on top of his garage. On a later day that called for rain, he put on a jacket and went outside to stare at his garage.

“I’m out there watching the downspout,” Liptan said. “It’s raining, and I’m getting soaked. I keep watching, and no water’s running off the roof.”

On a typical garage roof, about 98 percent of the rain runs off, and nearly half an inch of rain fell in a few hours that day. “Finally, a little bit of water started dripping out, about a gallon or two. I said, ‘Wow! This thing really works!’ ”

Two years later, Liptan and his wife did another test on the roof. “We simulated rainfall – a high intensity of water in a short duration of time,” he said. In other words, his wife sprayed the roof with the garden hose while he held a measuring cup and a stopwatch.

“It was just great,” he said. “It proved that not only does it hold a lot of water, but it reduces the peak intensity of large storms.”

And it also provides the little spark of delight that comes from the unexpected. Since Liptan simply shoveled dirt from his yard onto his garage, all kinds of plants have sprouted there, including hyacinths and coral bells.

“There’s one tulip that, for eight years, put out one leaf every spring,” he said. “This year it’s got three leaves, so maybe it will actually bloom.”

Maybe this year, he’ll find out what color it is.