Green-Roof Design

Green roofs are not new, but they are gaining in popularity in the United States. Over the past 30 years, there has been a steady increase in the application of this technology.

Green roofs are growing in popularity.

Rooftop-garden systems help mitigate problems traditionally associated with roofs, such as heat gain and loss as well as stormwater runoff.

During the summer months, for example, a rooftop absorbs and radiates heat from the sun, creating higher air-conditioning costs. In winter, heat escapes through the roof.

The plantings of a green roof act as an insulating barrier while absorbing stormwater and providing wildlife habitat.

“Green roofs were developed as a stormwater-management practice in Germany and then extended throughout much of Europe before being imported to the United States,” says Charlie Miller, owner of Roofmeadow, a design firm specializing in green-roof designs and strategies.

“Green roofs were developed as a way of forestalling improvements or enlargements to existing stormwater facilities. However, the ancillary benefit of creating a green roof is transforming the look of the environment.”

A green roof can be anything from a simple garden that produces fruits and vegetables to a full-scale operation with low-lying sedums and prairie-grass gardens.

Mastering Stormwater

In the Performing Arts District of Kansas City, Mo., one green roof project includes more than 2.5 acres of lush gardens atop a four-story parking garage. The parking areas are maintained by the city, and the parks and recreation department maintains the green roof.

During the planning phase, the decision was made to capture or reduce the amount of rainwater flowing into the stormwater infrastructure. One of the ways to accomplish this was to install two 44,000-gallon underground cisterns. The water collected is used to irrigate the rooftop plantings during the drier seasons.

“We worked with a plumbing engineer to design a green roof that captures all the rainwater that falls on the roof,” says Brian Davis, senior project landscape architect with Jeffrey L. Bruce and Company, the architectural firm that worked on the Kansas City project.

“The green roof, combined with the rainwater-collection abilities of the cisterns, makes this a huge stormwater-reduction project.”

Davis projects that the cistern system will pay for itself in about eight years due to the savings in not having to purchase domestic water for irrigation needs.

Of course, controlling water is one of the concerns with a green roof; this includes preventing leaks.

A rooftop landscape can help with stormwater runoff.

“We needed to make sure that the spaces beneath the green roof aren’t affected negatively in any way from the green roof,” Davis says. “We did this by having the best waterproofing that we could afford.”

What Makes Green Roofs Different?

One consideration of green-roof design is the creation of a landscape environment open to the broad expanse of sky. With a green roof, one can manipulate and do plantings differently than on the ground.

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