Pasque flowers, snowberry shrubs, Boston ivy and a prairie crabapple tree are just a few of the more than 20,000 plant species that sit 11 stories high in Chicago’s Loop.

The plants and grasses that make up the green space aren’t a park but a green roof built in 2000 on Chicago’s City Hall. Now, hundreds of building projects are underway in the city and its suburbs that will incorporate the vegetation-filled roofs.

“You can use all that space that alternatively isn’t programmed for anything since it’s designated as roof space,” said Andrea Cooper, a landscape architect with the Elmhurst-based Conservation Design Forum, which designs green roofs.

Long popular in European countries, green roofs, which advocates say reduce energy costs and help ease storm water management, are growing in popularity in Chicago, its suburbs and across the nation.

Green roof space in the U.S. increased by 25 percent from 2005 to 2006 to more than 3 million square-feet, according to a study from Green Roofs for Healthy Cities, a nonprofit industry association. Chicago led all U.S. cities, with nearly 360,000 square-feet of green roofs constructed in 2006.

The Calamos Investments building in Naperville, the Church Street Station in Evanston, the Evelyn Pease Tyner Interpretive Center in Glenview and the Oak Park Library make up just some of the green roofs in suburban Chicago.

“The green movement is really going,” said Kevin Crist, lead project manager for Intrinsic Landscaping, a Glenview-based landscaping construction company. “Oftentimes, a green roof is the easiest way to retrofit a green element into your building.”

There are many ways to build a green roof, but one is generally composed of growing material with a sturdy roof underneath and drought-resistant, shallow-rooting plants. Builders classify the roofs in three categories: extensive, semi-intensive and intensive, Cooper said.

An extensive roof is built on a thin layer of growing material—less than 6 inches—meaning only hardy, self-sustaining plants, such as the sedum variety, can survive, Cooper said.

Semi-intensive and intensive roofs require more structural support and an irrigation system is generally recommended. But with a growing material of six to 10 inches for semi-intensive and more than 10 inches for intensive, they can support the widest variety of plants, including some tree species and woody plants, Cooper said.

“When you have a thick system, you have a much more lush, dense growth of plants—a rooftop garden,” she said.

Benefits of such roofs include reduced energy costs, which stem from a reduction in the urban “heat island” effect. Green roofs absorb the sun’s ultraviolet rays, reducing the cost to cool the building interior during the summer. During the winter, green roofs reduce heating cost by providing insulation, Cooper said.

Storm water retention is also better with green roofs, which can absorb about 70 percent of one inch of rainfall, greatly reducing the storm water pressure on municipal sewer systems, said Tom Cooper, national sales manager for Green Roof Solutions, a Glenview company providing green roofing products.

“Developers can devote less area to storm retention ponds and more to retail space,” he said. “They also have an aesthetic benefit.”

Aside from natural beauty to otherwise unused spaces, green roofs provide habitat benefits for wildlife. A green roof on a parking lot at Chicago’s McCormick Place has become a major stop for food and water for birds migrating across Lake Michigan, Andrea Cooper said.

But there are downsides. Green roofs can be twice to four times as expensive as regular roofs, although their energy savings and longer life span—two to three times longer—usually result in a payback. Additionally, some buildings are just too old to support a green roof.

With roots dating back to Babylon’s hanging gardens, green roofs are relatively common in European countries like Germany, where energy costs are high and residents can face additional taxes if storm water is not managed onsite, Andrea Cooper said.

“There are roofs in Germany that were constructed 50, 60, 70, 80 years ago,” she said.

In the U.S., most green roofs are being implemented on commercial buildings, but the residential segment has shown signs of growth in the past few years, said Andrea Cooper. For instance, her colleague, Marcus de la Fleur, put up a small green roof on his Elmhurst home to help reduce water runoff.

While a home like de la Fleur’s is not common, residential will likely catch up, especially as manufacturers continue to innovate and drive down costs for green roofs, Andrea Cooper said.

An increased focus on environmental issues also has homeowners looking to make their homes more environmentally friendly, she said.

“There’s a green eco-conscious; it’s becoming almost pop,” she said. “People are wanting to do their own part, and this is one small way to do that.”