5 Tips on Building a Sloped Green Roof

Experts on vegetated roof systems share their thoughts on designing and building green roofs that slant, slope, tilt, and turn.
August 11, 2010

As the popularity of vegetated roofs grows in the U.S., Building Teams are finding new methods for greening rooftops. A recent trend is to adapt green roof technology for low- and steep-slope applications.

While the basic components of a sloped green roof are similar to that of a flat system, Building Teams must pay particular attention to water management, erosion control, and maintenance.

BD+C asked several green roof specialists who have worked on sloped systems for their advice on designing and building these systems. They offered the following tips:

1. Make sure the roof will stay in place. The most crucial component of any sloped green roof is the confinement system, which holds the growth medium in place during the roof’s early stages of development. If not properly secured, a pitched vegetated roof can erode and slip under heavy rain, excessive irrigation, or poor plant growth. This is especially true for roofs with a pitch greater than 2:12, says James Kirby, AIA, senior technical director with the National Roofing Contractors Association, Rosemont, Ill.

“Once above 2:12, you need to have something in place to prevent slippage of the growth medium, because things are going to want to move,” says Kirby, who is currently authoring NRCA’s Green Roof System Manual, to be published in August.

There are several approaches to designing a confinement system, says Angie Durhman, green roof specialist with roofing contractor Magco Inc., Jessup, Md., a subsidiary of Skokie, Ill.-based Tecta America Corp.

Cellular confinement systems, such as GardNet from American Hyrdrotech, Chicago, and Geoweb from Presto Products Co., Appleton, Wis., are installed on top of the drainage mat and feature flexible open cells into which the growth medium is poured. The result is a fully supported growth medium up to eight inches in height, depending on the manufacturer and product.Geo-textile plastic webbing materials, such as Enkamat from Colbond, Enka, N.C., are embedded in the growth medium in a similar fashion. The webbing, which is no thicker than an inch, is sandwiched between two layers of soil. As the roots grow, they become entwined within the thin webbing material, producing a stable cover. “This material is good because the roots grab onto the material very quickly, usually within two weeks, and since it’s a continuous system, the growth medium stays in place,” says Durhman, who has worked on several sloped green roof projects, including the Walter Reed Community Center and Senior Center in Arlington, Va., and a demonstration roof for the American Society of Landscape Architects atop its Washington, D.C., headquarters.For low-slope systems, an erosion mat may suffice. Common with roadside landscaping projects, erosion mat is usually manufactured using organic materials such as jute, wood, or straw and held together with synthetic mesh or strand. The mat is installed on top of the soil surface; openings are cut where the vegetation is planted. Erosion mats are designed to decay after one to three years, when the vegetation is fully mature.Another method is to terrace the roof. Durhman employed this approach for the 20,000-sf, 3:12 pitched roof at Walter Reed Community Center. “Every 10 feet, we installed four-inch-high edging bar,” she says. “So if any soil slips, it will get caught by one of those bars.” Installation can also be tricky. Durhman advises that teams install the growth medium from the bottom of the roof upward. “That way the soil won’t shift because you have your anchoring points set at the bottom,” she says.

2. Pay careful attention to peaks and valleys. Irrigation can be difficult with sloped systems because the water tends to filter down the roof quickly.

“There’s not a lot of organics in the soil, so the plants have only a few moments to soak up the water,” says Meg Needle, AIA, LEED AP, staff architect and construction specialist with Atlanta-based Lord, Aeck & Sargent, design architect for the Gwinnett Environmental and Heritage Center in Buford, Ga., which features one the largest green roofs in the U.S. at 40,000 sf.

As a result, vegetation near the top of the roof can be neglected vital moisture, while plants near the bottom often get too much moisture as the water ponds and pools.

To address this issue at the Gwinnett Center, the team (which also included landscape architect Jaeger Co., Gainesville, Ga., supplier American Hydrotech, and installer ProLandscapes, Norcross, Ga.) planted water-loving vegetation at the bottom of the roof and varieties akin to arid soil at the top of the slope, says Needle.

Durhman recommends that Building Teams consider installing thicker growth medium near the top of slopes. “That’s going to dry out first,” she says.

Some manufacturers, such as Germany’s ZinCo, advise Building Teams to install a moisture-retention mat underneath the soil so that the vegetation has more time to feed.

3. Specify a high-performance waterproofing membrane. “Don’t think that just because the roof slopes that the water is not going to pool,” says Kirby. “Don’t treat it any differently than a low-slope roof—use a very good, continuous waterproofing membrane.”

4. Single-source the roof. Because sloped green roofs can be especially difficult and involve increased risk, Kirby recommends that teams select a sole supplier for the system. “Try to single-source the components and warranty as much as possible,” he says. “Mixing and matching system components is not a good idea because you’re taking full responsibility for all the things that could go wrong.”

Kirby says that even if a single manufacturer does not provide all services and products required for the design and installation of the roof system, many form strategic partnerships to provide one-stop shopping. “If you call up one of these companies and tell them you need a 4:12 pitched green roof, they should be able to supply everything needed, including a warranty,” he says.

5. Include a maintenance contract in the specification. “It really takes a couple of years for the vegetation to mature, so you want to make sure the roof is well cared for during that most-crucial time,” says LAS’s Needle, who specified a two-year maintenance contract for the green roof at the Gwinnett Center.

A sneak peak at Lincoln Center’s 'walkable’ green roof
When the redesigned campusfor the Lincoln Center for the Performing Arts opens in 2009, the famed New York institution will feature a green roof like none other in the U.S.

Located across from the Juilliard School in Lincoln Center’s North Plaza, a new “campus green” will consist of a glass-walled restaurant topped with a 10,000-sf, sloped vegetated roof that will double as a campus lawn. It will be one of the only truly “walkable” green roofs in the country.

“Lincoln Center has been a hardscape for a very long time, and this will introduce some softness to that environment,” says Heidi Blau, AIA, LEED AP, associate principal with FXFowle Architects, which is collaborating with Diller Scofidio + Renfro, both based in New York, on the Lincoln Center redevelopment. “It will be a dynamic, engaging kind of space that people hopefully will want to run up on, play Frisbee, or relax.”

The roof, in the form of a hyperbolic paraboloid, will reach a full height of 11 feet from the plaza level and 23 feet from the sidewalk level; it will extend down to the plaza on the south end, providing access for pedestrians. The slope varies from a 1:12 pitch at the landing point to a 3:12 slope at the edges. About 50% of the roof is at a 2:12 slope—providing a “fairly comfortable climb” for visitors, says Blau.

To support the weight and handle the wear and tear from constant foot traffic, the design team specified a beefed up version of a typical green roof system. A six-inch-thick composite poured-concrete/metal-deck structural slab rated for 100-pound live loads will support the 14-inch-thick vegetated roof. The slab will be supported on nine steel columns with five girders and 45 beams situated to create the saddle-like “hypar” roof shape.

The green roof will be composed of a liquid-applied waterproofing membrane, an inch-thick root barrier, three inches of insulation, a 1½-inch drainage mat with filter fabric, and 8½ inches of growth medium with an integrated cellular confinement system. The confinement system (Geoweb, from Presto Products, Appleton, Wis.) will anchor the growth medium during the early stages of the roof’s growth—a critical function for sloped green roof systems because the soil can erode before the vegetation has a chance to take root.

The team selected fescue grass over Kentucky Blue and other grass varieties because it proved to be more comfortable and rebound more quickly during testing. “Fescue seems to be hardy and a little softer to the bare foot,” says Blau. “The color and density are also nice.”

The team has been evaluating the performance of all facets of the system for the past year using three mockups built in Jersey City, N.J. Water management proved to be the biggest challenge, says Blau.

“Because the roof has an unusual shape, we want to make sure there won’t be ponding water in certain places and water running off in other places,” says Blau. The solution involved adjusting roof angles and integrating a series of check dams to help channel excess water to a trench drain along the lower edges of the roof.

If all goes as planned, students, musicians, and concertgoers will be basking in the sun on Lincoln Center’s new front lawn by fall of 2008.