Most living roofs aren't flashy. Usually they are no more than nondescript greenery on top of a building. Sometimes you can't even see them. So when SWA Group in Sausalito was hired to build the living roof for the California Academy of Sciences in Golden Gate Park, the company had an opportunity to help create one of the most remarkable living roofs in the world.

Not that the business hadn't had similar experiences before. SWA has been landscaping high-profile projects for more than 50 years. It's the company behind the Golden Gate National Recreational Area, which spans 90,000 acres and 59 miles of coastline. Today, SWA's projects range from landscaping the campus at Google headquarters to restoring superfund sites in Texas. And so the firm was the perfect candidate to take on architect Renzo Piano's challenging vision for the museum roof.

Piano's design incorporates seven mounds, a nod to San Francisco's seven hills. Since some mounds slope at a 60-degree angle, keeping the plants from sliding off the roof when it rains would take innovation.

"We had done a lot of green roofs before," says Lawrence Reed, a principal at SWA Group. "Most of them were sections on flat roofs, but we had just finished a new project at the Library of Congress in Virginia that had a sloped roof."

SWA partnered with Rana Creek, an ecological design company in Carmel Valley, to come up with a way to make the roof work. They did extensive testing, including making a full-scale model, before coming up with a plan. The idea was to cover the roof with gabion baskets--crisscrossing wire containers full of volcanic rock. The plants were placed in 17-inch trays made of coconut husk and tree sap. The trays were then installed like tiles over the top of the gabion baskets. Over time, the trays will break down and the plants roots will intertwine and give themselves support.

The resulting roof is a broad expanse of plants that rises and falls for 2.5 acres. Visitors aren't allowed to walk through the plants, but there is a deck where they can take in the view. The roof also recycles rainwater and cools the building by 10 degrees.

While it has often been reported that there are 1.7 million plants on the roof, the number is actually closer to half-a-million.

"There's no way there could be 1.7 million plants," says John Loomis, another SWA principal. "That would mean there would have to be 35 plants in one of those 17-inch trays. There are really about 10. I challenge anyone to get 35 plants into one of those trays."

Picking the right plants was essential to the success of the roof. First Rana Creek made a list of 27 natives to the San Francisco region. Not only would using native plants create a habitat for local wildlife, they would be best at handling Golden Gate Park's climate. From there, the list was narrowed further. The plants had to be a certain height to give the roof a look of uniformity and they had to be able to survive in the roof's soil and environment.

In the end, four perennials were chosen to cover the majority of the roof: self-heal, sea pink, strawberry and stonecrop. Along the viewing deck is a garden with five more annuals: miniature lupine, tidy tips, goldfields, California plantain, and California poppy.

In the four months since the museum opened, the plants have begun to adapt. Insects, hawks and other wildlife are interacting with the plants and some new plants have popped up. Among the perennials, the self-heal and strawberry seem to be taking over.

"The stonecrop are still present, but they are being pushed aside by the other plants," says Loomis. "The botanists at the facility want to see what will come in naturally with the birds. It is a science museum, after all. They are treating the roof like a giant experiment."

Another prominent feature of the roof is the way it collects storm water. For 10 months out of the year, the plants take up all the water from the roof. In the remaining two months, the water drains down into the rain forest inside the museum.

Storm water management is the number one benefit of living roofs, according to Emilio Ancaya, owner of Living Roofs, Inc. in Arizona. Studies suggest living roofs absorb 60-100 percent of storm water, thereby preventing flooding and drainage problems.

Living roofs also improve air quality because plants take in carbon dioxide and release oxygen. And because the roofs aren't covered with asphalt or other heat-retaining material, they are cooler, which lowers air-conditioning in the summer, but can increase heating in the winter.

In cities, they can reduce the "urban heat island effect," a phenomenon where the rooftops trap the heat from the sun and make the cities hotter than they naturally would be. Living roofs can even make the city more harmonious. The sight of green space in urban environments has been proven to lift people's moods and increase work productivity.

Because of all of this, Ancaya has seen enough demand for living roofs to start a company specializing in them.

"Green Roofs for Healthy Cities (www.greenroofs.org) did a study that said there is a 30 percent increase for green roofs in general," he says. "That's pretty big for one green product. It's growing pretty fast."

For most roofs, Ancaya uses sedums, a succulent variety that can handle drought and high temperatures. Because sedums grow in rocky environments, they don't require much soil. As one of the few plants that can survive on a roof, they are used for 75-85 percent of green roofs.

"When most people think of a green roof, they think of turf or sod," says Ancaya. "That's the least appropriate plant for a roof. Not only would people have to mow it, it needs thicker soil. You don't see that very much because a roof is a harsh environment."

The roof on the California Academy of Sciences is an important example of how beautiful living roofs can be. At 36 feet in the air with the forest canopy hanging above it, it is part of both the ecosystem of the building and the ecosystem of the park, connecting the two. It reveals the potential of green roofs not only to people today but to the next generation as well.

"Someone wrote about being on the roof and overhearing a woman saying, 'So, it's a green roof, what's exciting about that? Let's go,'" says Loomis. "But at the same time, I've been up there and heard a child asking, 'Why don't we have a green roof?' and all kinds of other inquisitive questions. That is the audience for the living roof."

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