Photo by Furbish Co. of Baltimore
Sidwell Friends School, located in D.C., installed a green roof on an addition built in 2006. The roof, which was redone in 2008 after the original roof failed, was constructed in three parts with varying soil depth.

Graduating to a Green Roof

April 1, 2014
A green roof can also help with cooling costs, producing energy savings over time. There are also the benefits that are harder to quantify, like aesthetics and the aid provided to the storm water system.

A growing number of urban schools and universities are utilizing a commonly ignored campus asset–the roof.

These educational institutions are part of a nationwide trend toward choosing a green roof, a roof structure incorporating waterproof membranes, a growing medium (usually soil), and living vegetation, as an alternative to a conventional roof. That increase will likely continue and the cost of installing and maintaining green roofs will go down, says Dr. Brad Rowe, a professor at Michigan State University who has studied green roofs extensively.

“The industry’s growing like crazy,” says Rowe. “(In) 2002, there were actually less than 50 green roofs in north America. Now there are more than 10,000.”

The upfront costs associated with a green roof generally make it twice as expensive as a traditional roof, Rowe said. The cost also depends on how much weight the structure can tolerate and what will be grown on the roof; the deeper the soil, the greater the expense.

But green roofs usually last twice as long as traditional roofs because the organic material protects the roof membrane. A green roof can also help with cooling costs, producing energy savings over time. There are also the benefits that are harder to quantify, like aesthetics and the aid provided to the storm water system.

Friends of the Environment

These were attractive perks for Sidwell Friends School, a private school in Washington, D.C. A green roof seemed like a natural fit for a school that prides itself on its environmentally conscientious campus, which it uses as a teaching tool.

“Our whole campus is a little bit different,” says Steve Sawyer, Sidwell’s plant manager. “It’s not a green, manicured golf course look. It has a lot more native plants and it looks wilder than what people might expect, and we’re OK with that. We’re OK with doing the educating that comes along with that: ‘Look this is not supposed to look like a corporate headquarters. This is an educational place here.’ It takes a little bit of explaining.”

But even with the right mindset, the school’s first attempt at a green roof didn’t go as planned.

When the school embarked upon a major renovation and addition project in 2006, it took the opportunity to incorporate a green roof on the addition (the original building’s structure could not handle the weight). The school hired a landscape architect and installers to turn the space into a rooftop garden.

Two years later, the roof was on a steady and obvious decline. The plants, chosen because they were native to the area, were withering in the harsh environment. In short, the green roof was well on its way to becoming a brown roof.

“We were pretty early in this whole green movement, and we were pretty aggressive about it,” says Sawyer. “We knew we were going to make some mistakes. We were doing some groundbreaking work… There were people who would say, ‘What do you think you’re doing? What are you spending this money on?’ There were some non-believers out there.

“I think we were pretty fortunate in our timing to do this and get some support behind it,” he says.

Undeterred, Sidwell scraped the roof and started from scratch in the winter of 2008. This time, the school hired a company that specializes in green roofs, and instead of native plants, the roof was populated with plants that thrive in the unfriendly conditions of a roof. The company also maintains the roof for the school at a cost of about $3,500 annually to Sidwell.

“It really is a specialty, and I think it really needs to be treated as such,” Sawyer says.

The second roof has thrived, says Sawyer. It provides insulation, reducing the burden on the building’s air conditioning system. It absorbs rainwater, purifies it and deposits it in a pond—all very clearly demonstrated for educational purposes.

Sawyer emphasizes the roof’s role as an educational tool at Sidwell. It serves as a habitat, giving students the opportunity to observe 21 different bee species on the roof alone. Sawyer says it’s important that, when installing a green roof, schools make them accessible to fully realize the benefits.

“You’re going to do all this work up there. Make sure you’re not putting it on top of a building that no one is ever going to go out and see,” he says.

“Don’t just do this in the dark. Have people see what it is and what it’s doing for you.”

Digging Deeper

The range of green roof opportunities depends on how much weight the roof will hold and how much money schools are willing to spend, according to Rowe.

“If you have four inches or less, then I would say go with succulents, like sedum,” Rowe says. “Then you don’t really have to water it. If you wanted to grow grasses and perennials in four inches, it would have to be watered.

“If you can make it a lot deeper, then that expands your plant palette,” he adds. Native plants, for example, need plenty of soil depth for their expansive root systems. Likewise, even though Rowe has seen vegetables grow in four inches of soil, he says eight to 10 inches of soil is ideal.

“That’s a lot of weight,” he warns.

Nevertheless, vegetables are quickly becoming a rooftop favorite. The Calhoun School, a private school in New York’s Upper West Side, uses some of its rooftop-grown herbs in its lunchroom. The roof is also sturdy enough to support as many as 120 people at one time.

Varying soil depths supports biodiversity. At Michigan State University’s plant and soil science building, sedums grow in an inch and a half of soil in one area, since that was all the building could handle. Meanwhile, grasses and perennials grow in eight-inch-deep beds installed on top of a newly constructed area.

Natick High School in Natick, Mass., recently put in a 7,000-square-foot green roof, which has a depth of about three inches and was planted mostly with sedums.

Sidwell’s roof also is three to four inches deep, on average, and these areas are mostly covered in sedums. But some areas that are more highly visible—like near the walkways—have six to seven inches of soil, allowing for more variety.

“The variety in planting depth and plant diversity in these locations makes the roof look more like a landscape than a strict utilitarian vegetative roof,” Sawyer says.

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