Green at the Top

When space is scarce, building owners plant vegetation on their rooftops.

By Eric Schneider

March 2013


In every city and town, there are millions of square feet of rooftops. Few see them, and these sometimes expansive and often empty spaces tend to remain neglected as they bake in the sun and send storm water gushing into the sewer system.

But some building owners are taking advantage of the empty spaces above and giving a nod to nature at the same time. They are covering these forgotten surfaces in greenery, providing aesthetic, environmental and even financial benefits to each structure. Popular in Europe, “green roofs” are becoming more of a presence on buildings across the US—and offer benefits that go beyond their attractive looks.

High-Rise Gardening

Simply defined, a green roof is any roof covered with vegetation, says Robert Berghage, PhD, associate professor in the horticulture department at Pennsylvania State University. “There are two basic types,” Berghage says, “intensive rooftop gardens with 12 inches or more of [soil], which…can support trees, shrubs and larger plant material. The other type is the extensive variety, which is a thin layer or veneer of vegetation with 6 inches or less of media.”

Ed Snodgrass, president and founder of Emory Knoll Farms in Maryland, says intensive green roofs are designed as “an amenity space for people.” Essentially elevated gardens, these green roofs have deep soil, irrigation systems and even gardeners, and feature a wide variety of typical garden plants. On the other hand, Snodgrass refers to the more common extensive green roof as a “functional living machine” that requires little maintenance.

Snodgrass’s farm, also known as Green Roof Plants, specializes in extensive spaces, which are primarily created using sedum, succulent groundcover plants that require little soil and have leaves that store water. Not only are these plants attractive, with their curved geometric shapes, many of them can withstand almost any kind of weather, from heat-induced droughts to bitter cold winters. To date, Snodgrass and his company, primarily using a variety of sedum, have installed more than 100 acres of green roofing for more than 700 projects. Installations also include decking and drainage.

Green Advantages

Building owners and occupants benefit from green roofs, as does the surrounding community. For building owners, the pluses are reduced energy use, since plants keep the roof and the building under it cooler, and extended roofing life, with better wear because of the combination of water-retaining plant material and the necessary waterproofing membrane.

“The other kind of benefit accrues to the community at large,” Snodgrass says. Among the big-picture perks are storm water management, with the succulents retaining considerable precipitation that would otherwise contribute to flooding and erosion, and a reduced “urban heat island” effect, which raises temperatures in cities because the amount of heat-retaining asphalt and concrete heavily outnumbers foliage. Green roofs also aid in building biodiversity by providing needed habitat for birds and insects, some endangered.

While not all green roofs are easily viewed or accessed, those that are tend to provide a certain meditative ambience. Amidst the lively environment of the Boston Children’s Museum, which unveiled its impressive eco-friendly renovation and expansion in 2007, there are three green roofs.

Two can be seen from the building’s upper floors. With their tidy connected squares of low-lying sedum, the roofs are attractive and help the museum’s many environmentally conscious measures as they collect water that can be reused in the facility’s low-volume toilets and irrigation for the building’s grounds.

The museum’s visible green roofs also provide a tangible way for city dwellers to see environmentally conscious measures at work, a notion that adult visitors can pass on to curious children. The most easily viewed green roof was even built in part by museum visitors, who helped the staff establish the plants in their new home.

Jo-Anne Baxter, a museum spokeswoman, says the green roofs need little maintenance. “The first two years we needed to do some watering, but now they are pretty much self-maintained—just a little watering and weeding as needed,” Baxter says.

Although green roofs are relatively rare in much of the country, demand is consistently growing, particularly since many local governments provide tax advantages for building owners who install them. They also help owners garner prestigious Leadership in Energy and Environmental Design (LEED) status, an eco-friendly and highly marketable designation.

Snodgrass says the green roof market has grown steadily for the last 10 years, and he expects it to grow at least 20% annually. And while these rooftop oaises are most commonly found on commercial and government buildings, it may not be long before more homes in the United States are sheltered under a pleasing green layer that benefits the world in a myriad of ways.

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