Teresa Williams and David Leppanen's roof is alive. Last fall, the Bellingham, Wash., couple installed what they believe to be one of the first green roofs in the area, and now it's in full spring bloom. Already popular in Europe for their environmental benefits and aesthetic pleasures, green roofs are in-demand in the United States.

Today, when they look out of their livingroom window, they see saxifrage, flowering rosettes of houseleeks and eggplant-colored tulips.

For Williams and Leppanen, the practical and philosophical bonuses of the garden over their heads had appeal.

"I hate the idea of paving over the Earth," says Williams, a prosecutor for the Lummi Nation tribe.

This, she says, is the opposite.

The idea of a garden roof is nothing new — they go back to the hanging gardens of Babylon and centuries-old turf roofs of Viking settlements. In Europe, they're well established: An estimated 10 percent of all roofs in Germany are green.

But in the United States, they're just catching on.

Green roofs, eco-roofs or vegetated roofs, as they're called, are increasingly being touted as a way to beautify concrete-bound urban areas, regulate global warming and spare rivers from stormwater contamination.

The couple wanted to expand their home with a shop. Williams didn't want to look out on a bare roof, so they opted for a green one: Something they hoped would be beautiful and functional.

A green roof can help regulate temperature and absorb stormwater, which runs off slowly instead of pouring down in buckets and overwhelming sewer systems.

"Green roofs are definitely possible in Hawai'i," says Honolulu architect Patrick Tozier, who specializes in environmental design and planning. "I see opportunity in urban areas of Hawai'i, retrofitting existing buildings, and places like universities, government buildings and strip malls."

Bill Brooks, a principle with the architecture firm Ferraro Choi and known for his sustainable architecture, agrees, and points to two existing Honolulu examples of vegetated roofs: "the park at the city & county building and the big landscaped park at Kukui Plaza." He believes that when it comes to residences, it makes more eco-sense to use roofs for energy production.

Green roofs' mitigation of storm water runoff is "very important in Hawai'i," points out Tozier, as we are susceptible to flooding and have burdened water facilities. Another plus here, he says, is that the elevated gardens "increase the life of roof membranes by protecting them from temperature fluctuations and ultraviolet light."

The drawback: "It's not cheap, but the expense is worth it," says Leppanen, a general contractor.

"You can't do this in a weekend with plastic bags and dirt and just hope for the best," says Williams.

High-quality materials are essential — a poorly built green roof can cause structural damage to a home by compressing the roof or leaking. It will increase building costs 20 to 35 percent, Leppanen says. A project like their green roof might cost around $30,000, says Leppanen.

Because Leppanen did much of the work and design himself, their total cost was lower.

There's a potential to recoup some costs, because the vegetation can act like insulation.

"I would think that if it was over living space ... a green roof is going to give you more insulation and help keep it cooler in the summertime, and warmer in the wintertime," says Leppanen.

It's also beautiful. The 750-square-foot roof garden is the focal point of the whole house, says Williams.

To build theirs, Williams and Leppanen consulted with Patrick Carey, a Seattle-based expert on the topic.

He told them that a green roof must have three main elements to be successful. It can't leak, can't be too heavy, and must sustain the greenery growing on it.

The couple used a rubber lining (much like a pond liner), a drainage mat and a soil layer (a mix of crushed lava rock, sand, composted soil and water-retention granules). They chose plants that tolerate drought conditions, require only six inches of soil and thrive in heavy exposure.

They covered the area with more than 500 flower bulbs, several grasses and varieties of sedum and saxifrage. After a rainy winter, their roof is in full bloom.

But the best part, says Williams, is morning coffee in the living room while looking out on the green patch.

"(It's a) mediation garden that flows directly out from the windows," she says, "and provides a beautiful screen from the street."