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After a hard day at work, weather permitting, Mark Masters heads for the third-floor deck outside his apartment and pulls up a chair to take in the view: Center City skyline in the distance and, right there in front of him, on the roof, what looks like a yard.

It has grass, weeds, a few tree seedlings, and a thick groundcover, but this isn't a yard exactly. It's a green roof.

"I'm doing my bit for the environment," says Masters, president of the Fencing Academy of Philadelphia in West Philly, whose green roof is his "yard."

Once considered an environmental curiosity, European-style green roofs -- also called living roofs or eco-roofs -- are slowly catching on in the United States. There are about 1,600 here, covering 5 million square feet, atop hospitals, schools, government and office buildings, and some private homes.

Since his 3,000-square-foot green roof was installed for $10,000 nine years ago, Masters hasn't had to replace anything. He doesn't even water. During that same period, he spent about $10,000 on his apartment's black asphalt roof, which is one-third the size of the green one.

"What tears up a roof is hot, cold, rain, sun, and I don't have that problem on my green roof," he says.

Planted roofs absorb up to 70 percent of rainwater, greatly reducing the runoff that floods streams and sewer systems. The roof's soil neutralizes acid rain so, as Masters says, "the water that comes off my roof is cleaner than the water that falls on it."

Because it doesn't expand and contract with temperature spikes, a green roof lasts longer. And its temperatures hover around 75 or 80 degrees in summer, which can lower air-conditioning bills by 10 percent or more, according to Robert Berghage, director of the Pennsylvania State University Center for Green Roof Research.

"If you're really trying to solve global-warming issues, you're far better off thinking about putting in a forest," he says. "But any green we put in is good."

Pretty, too.

Thin green roofs like Masters' have only 3 inches of soil over the root barrier and roof membrane, but 6 soil inches with reinforcement can support a meadow of flowers, grasses, shrubs and herbs.

Aesthetics can be a major draw, says Charlie Miller, who installed Masters' roof for free as a model for his new business.

"The people I know who have done this in Germany come home from work and go out on their private little roof area with a deck chair and a beer and look out over the city and look at their plants and unwind," says Miller, president of Roofscapes Inc., a green-roof firm in Mount Airy. "That in itself is a valuable thing to do."

Green roofs have long been popular in Europe, especially Germany, where 10 times the green-roof square footage that now exists in the United States is installed every year.

"Right now, in this country, everyone is a little too freaked out about the possibility for leaks and roof collapse and this sort of thing," Miller says. "We're just too early on that curve."

Actually, he says, a properly built and maintained green roof is unlikely to leak or collapse, and a new electronic mapping process quickly pinpoints holes in the waterproofing layer so they can be repaired with minimal disruption.

"A good green roof could last 100 years, easily," says Miller, whose firm has done many high-profile installations around the country, including Chicago City Hall, Boston's World Trade Center, and the Heinz 57 Center in Pittsburgh.

Someday, he predicts, green roofs will be so common, "individuals will feel empowered to make these changes on their own or hire their own home-renovation carpenter to do it."

When that happens, Ed Snodgrass likely will have a cut of the action. This lanky fifth-generation farmer from Street, Md., between Wilmington and Baltimore, is believed to be the only nurseryman in the country making a living growing just greenery for green roofs.

Six years ago, he started Green Roof Plants at his 145-acre Emory Knoll Farms. Last year, he sold almost 1 million plugs, or seedlings.

Given that trajectory, Snodgrass, too, is convinced green roofs soon "will become a common building practice, especially for new construction." It's far more economical to plant them on new buildings or cluster residential developments than to retrofit, say, Philadelphia rowhouses one at a time.

Already, the U.S. Department of Defense and the General Services Administration require their new buildings to be "green," which could mean green roof, and about 15 municipalities around the country are considering, or have on the books, similar mandates.

In the Philadelphia area, besides the Fencing Academy, in the 3500 block of Lancaster Avenue, green roofs can be found at Swarthmore College, Temple University Ambler, the Schuylkill Center for Environmental Education, and the new Radnor Middle School under construction in Wayne. New Jersey green roofs include the Hostess Cupcake Factory in Hoboken and Seapointe Village oceanfront condominiums in Wildwood Crest.

Most of a green roof's green derives from the humble sedum, a cactuslike groundcover with shallow roots and fleshy stems and leaves that retain water. Sedums thrive in harsh conditions, making them perfect for saving the planet, one roof at a time.

They're light and low-growing. They spread well, forming a mat that keeps soil from blowing away and smothers weeds. And they're as colorful as they are tough.

Just cruise the aisles of one of Snodgrass' 14 greenhouses, with its chilly air and toasty dirt, and prepare to be charmed.

"Garden designers are always amazed when they see what I have," he says. "It's a much broader palette than they imagined."

You have to look closely, because the plugs -- 28,000 of them in this greenhouse, selling wholesale for up to 65 cents each -- are so small. Not all are sedums, but many are.

Sedum reflexum 'Blue Spruce' is aptly named for its tiny silvery limbs. Sedum rupestre 'Angelina' is golden yellow, with chubby needle foliage. And Sedum sexangulare has six spirals of tiny, spoon-shaped leaves. All three have yellow flowers in summer, but sedums bloom in pink, white and red, too.

The wildflower look doesn't come cheap, however. Like quality slate or tile, a green roof can cost $30 to $40 a square foot, according to Angie Durhman, green-roof specialist with Magco Inc. of Jessup, Md., who planted sedums, chives and alliums on top of the new Radnor school.

But there's no need to convince Masters of the beauty or benefits of his third-floor aerie, which draws birds, butterflies, ladybugs and squirrels. And how about this: A green-roof garden requires none of the work its earthbound cousin does.

"It's very enjoyable," Masters says with a smile.