Nashville builds a rooftop public square

Nashville, with its tradition as a hotbed of country music, is no stranger to showmanship. So, it wasn’t surprising that when the city constructed a new downtown courthouse complex a few years back, it felt the need to play to the crowd. The typical, stark sea of concrete wouldn’t do—so, atop the five-story parking garage, Nashville added a 2.25-acre, state-of-the-art green roof.

Many green roofs are designed to support growing medium and plants, but not people. However, the Nashville parking garage green roof, with a prime location in front of the Metro Courthouse, now serves as the city’s “Public Square.” The parking garage is underground, with the roof essentially at street level, allowing residents and visitors to gather on the grass plaza to relax and to enjoy various public events. The turfgrass and landscape plants add a critical environmental component to the urban environment.

Photos courtesy of Green Roofs for Healthy Cities (www.greenroofs.org)
Nashville’s 2.25-acre Public Square green roof is a popular spot for large events.

The green roof was designed by landscape architects from Nashville-based Hawkins Partners, Inc. and architects from Tuck Hinton Architects, also of Nashville. The project won top prize in its category in the 2007 Green Roofs for Healthy Cities international green roof competition. “The Nashville Public Square is a landmark project for the city of Nashville from a civic and sustainable perspective,” says Steven Peck president of Green Roofs for Healthy Cities (www.greenroofs.org). “This project raises the international bar for municipal green roof projects, and all parties are to be commended for their efforts on this award-winning project.”

In presenting the award, the group outlined some of the specifics of the project: “The cost of the green roof averages approximately $30 per square foot, exclusive of the vertical circulation structures. The project utilizes Tremco’s hot applied, rubberized asphalt waterproofing membrane, TREMproof 150 HRA and lightweight aggregate (expanded slate), manufactured by The Carolina Stalite Company, incorporated into three different forms (lightweight aggregate, rooftop mix and firelane mix) for areas supporting different intensities of traffic. The depth of planting media varies from a minimum of 8 inches along the central ridge of the structure to a depth of 5 feet towards the edges. A diverse community of landscape materials is used, totaling 43 different species of which 81 percent are native to the Southeast region and 63 percent are native to middle Tennessee.”

Elevator and stair towers emerge from the parking garage below to a rooftop deck covered in turfgrass and granite landscape features.

The green roof is designed to capture and filter rainwater, routing it underground to a 57,000-gallon underground tank where it can be stored and then used to irrigate the landscape above. That design feature is both an asset and a challenge, says Mike Bays, assistant director with Nashville’s Metro Parks and Recreation division, which oversees maintenance of the green roof.

“The green roof uses a structural soil that’s designed not to hold water,” he explains. “The whole idea is that the water gets through the soil, hits an impermeable barrier and is channeled off to the reuse tank that’s about three levels down. Because of that, we need to irrigate frequently because it dries out quickly. It takes high levels of nutrient input to make it grow, because it doesn’t hold anything in the soil. We get soil tests frequently to help us balance the nutrition. It’s not the typical approach you find in a parklike grounds setting.”

In many ways, it’s like maintaining a golf green, says Bays, who has a background in that field. Fittingly, the turfed area of the green roof features an improved fairway-type bermudagrass, a change from the common varieties more typically found in other Nashville parks. “It’s a learn-as-you-go process,” says Bays of calculations that go into creating the irrigation, fertilization and turf care regiment on the green roof. Some people say that bermudagrass will grow in anything, but this is testing that theory. It takes some time to develop a program that will produce the best results.” In addition to the turfgrass, there is also a mix of plant materials, trees and shrubs in the surrounding landscape.

Within the parks and recreation department, about 65 full-time people handle groundskeeping maintenance for approximately 10,000 acres across Nashville. The crews assigned to care for the green roof are given special instructions in its care. Because of the high-profile setting of the green roof, it’s important to keep the area looking great. “We mow at least weekly, or more frequently if needed,” says Bays.

The city uses walk-behind mowers on the green roofs. “We start early—we get there before the public does. There are a lot of events in front of the courthouse, so we need to make sure we’re not interfering with those.”

The high-profile locations are even overseeded with perennial ryegrass in the fall. “If there’s a news helicopter filming overhead, we want it to be green,” says Bays. Because the turf sits atop the relatively warm concrete of the parking garage structure, the bermudagrass stays green later into the fall, he adds. “And, it greens up faster in the spring, which makes it a little more susceptible to frost damage. Our latest frost date here is around April 15. This year we had a frost a week later than that.”

Nashville boasts another green roof parking garage, in front of the Country Music Hall of Fame and next to a new, state-of-the-art symphony hall. “We do a lot of events at that location, so it gets a lot of traffic,” says Bays. “The only issues we’ve had have to do with the loading of the deck. Groups set up huge tents, and there are restrictions on the amount of weight and ballast they can bring in. We have a lot of engineering information about per-square loads for the decks and that type of thing. I find it interesting, but I know way more about their engineering than I want to.”

It’s a challenge that most parks maintenance managers don’t have to worry about, but he and his staff must constantly be aware of how much weight trucks, tents and other loads are placing on the roof deck. They must be on guard to protect the irrigation and complex drainage system, as well as the impermeable barrier below the turf. “One guy with a tent crew last year wanted to drive stakes in,” says Bays. “We had to explain there was no way to do that.”

While it creates moisture-retention and fertility problems, the structural soil at the Public Square green roof and the sand-based soil at the Hall of Fame green roof do help prevent compaction issues during big events on the turfgrass, he explains.

For those who enter the Public Square green roof from the street, the area looks and feels like any other city park. However, those who park their cars in the garage below get an instant sense that there’s something unusual when they emerge up above. “The garage towers and steps come out right on the deck, so they know they’re on the roof of the garage,” says Bays. And that, he adds, makes the sight of turfgrass and landscape plants all the more impressive. “There’s two very elaborate fountains, as well, and a reflecting pool up there,” he says. “The architectural features include a lot of stone pavers; all the walkways are pavers. And there also are a lot of granite benches and steps and edging.”

Fortunately, there’s a service area from the edge of the courthouse that allows easy access for pickup trucks and mowers and equipment. “It’s actually very easy to get to and work on,” says Bays. “The two biggest challenges the green roof presents are the soil-nutrient holding capability and water management.”

Patrick White is a freelance writer and editor who is always on the lookout for interesting and unusual stories.