“Green roofs are what they sound like,” says Hunt, Extension specialist in the Department of Biological and Agricultural Engineering. “Other names for green roofs include garden roofs, eco-roofs and roof gardens.”
Regardless of what the roofs are called, architects, engineers and scientists from Portland to Chicago to Atlanta are intrigued by the potential benefits of this tool for improving an area’s environmental quality.

A green roof works like a sponge atop a building. When it rains on a conventional roof, the water immediately runs off. The greater the rainfall, the greater the runoff. With a green roof, the soil on the roof retains most of the water temporarily and then releases that water at a much slower rate. In addition to the obvious flood-control benefit, Hunt’s team is examining the degree of nutrient removal, and that effort is what makes this a pioneering endeavor.

“Our sites in North Carolina are among the first in the world to try to discern if this is indeed the case with respect to nitrogen and phosphorus removal.”

Although green-roof research in North Carolina began in the spring of 2002, and green roofs remain a relatively new idea to the United States, the concept has been around for centuries. Native Americans used green roofs in their sod houses. Green roofs were also popular among the Vikings. Most recently, the concept has been extensively used in Germany and other northern European nations. Today green roofs are used to aid in the “heat island” effect in highly congested areas, stormwater retention and for aesthetic purposes, according to Hunt.

How it works

The green that most people see in “green roofs” is the top layer of plants. The bottom layer consists of a moisture retention and protective mat that maintains the waterproof quality of the roof. Sandwiched in between these end pieces are two important layers: Just beneath the vegetation is the soil mix layer, and beneath that is the filter sheet drainage layer, which looks like a small egg carton. The challenge that architects and engineers face when serving up this roof-top sandwich is weight gain, according to Chuck Friedrich, a horticulture and landscape architect with Carolina Stalite Company in Salisbury. Friedrich, a nationally renowned expert on green roof soils, says, “Maintaining a roof garden is very different from what is performed at the ground level. Soil for overstructures actually includes a lightweight mix of sand, compost and burned pebbles. The pebbles are burned to remove nutrients.”

When the appropriate soil mix is used, good drainage is a given, “and good drainage promotes deep roots, which reduces the need for excess watering once the plants are established,” says Friedrich.

Green roofs come in two varieties – extensive and intensive. The main difference between the two involves the type of plants used for the roofs.

“In general, roof plants should not only be drought-tolerant but should not have airborne seeds and be able to live a long time [at least 40 years] without adding too much weight to the roof,” says Ed Snodgrass of Emory Knoll Farms, a private nursery that is donating plants for Hunt’s research team.

Extensive green roofs are designed to be virtually maintenance-free and thus require hardier plants that can survive drought, extreme temperature variation and intense sunlight. These plants usually grow in soil that is 2 to 4 inches deep and reach heights of about 3 inches.

“It’s important that extensive plants be a ground cover and stay low and add as little weight to the roof as possible,” Snodgrass says. “In the Mid Atlantic and Southeast, this means plants like Sedums, Delspermas and Rosularias.” He adds that it is important that these types of green-roof systems be designed for each specific location and warns that “a cut and paste plant approach would result in low-performing systems.”

While extensive green roofs should need little more than some annual maintenance, intensive green roofs require a substantial amount of maintenance. Intensive green roofs are generally used more for “show” than function and require regular watering and fertilizing. The soil is always deeper for intensive green roofs because the plants grow taller. Plants can reach heights from 3 to 15 feet and include varieties such as Jovibarbas and Sempervivums.

“We are trying to identify which plants work best in eastern and central North Carolina,” says Hunt.
The purpose of the North Carolina green roof demonstration sites, located in Raleigh, Kinston and Goldsboro, is threefold: to determine the quantity, quality and temperature reduction of stormwater runoff, according to Mike Regans, area specialized agent of the Neuse Education Team.

“We want to examine the feasibility of green roofs as a stormwater management tool and decide if such a practice should be recommended to local governments,” says Regans, who adds that state law requires local governments to establish stormwater management programs.