Plant Roof Pioneer

Green roofs are sprouting in Chicago as environmental and structural engineers find ways to accommodate them on new and existing buildings

By Rebecca Little

The green roof atop Chicago’s City Hall serves as the centerpiece for the city’s initiative Courtesy City of Chicago

Hummingbirds hover over wildflowers. Bees buzz around their honeycomb hive. Stray crabapples litter the base of a small tree. It’s a typical garden, save for the backdrop of skyscrapers, parking garages, and honks coming from the urban streets below. The rooftop garden on Chicago’s City Hall building may seem out of place, but it ranks as an engineering and environmental feat retrofitted atop the 11-story structure, “greenroofed” a full 90 years after it was built in 1911.

With no small amount of help from the city’s tour-de-force mayor, Richard Daley, Chicago has become the greenest city in North America. Green Roofs for Healthy Cities surveyed cities in 2005 and found Chicago had 295,000 square feet of finished green roofs. While that is substantial, compared to what’s coming, the city has only begun. City officials say 2.5 million square feet have been approved on more than 200 public and private buildings, currently in various stages of construction and development.

“The engineering community is very conservative when it comes to something new, and what the mayor achieved was demystifying green roofs,” says Marcus de la Fleur, a landscape architect with Elmhurst, Illinois-based Conservation Design Forum, the firm that worked on City Hall as well as many other green roofs around the city.

Landscape architect Marcus de la Fleur, shown here on the roof of the Kresge Foundation headquarters building in Troy, Michigan, has had a hand in many green roofs
Courtesy CDF

Green roof advocates say they offer many environmental advantages to counteract heat, pollution, and wastewater runoff, particularly in urban areas. The roofs combat the heat island effect in cities — where asphalt absorbs heat and raises temperatures — because plants absorb the sun’s heat. For example, the City Hall side of the Chicago building is 40 to 60 degrees cooler, on average during the summer, than the adjacent Cook County portion of the building, which still has a traditional roof, says Larry Merritt, a spokesman for the Chicago Department of Environment. The roofs can also reduce energy costs by acting as an extra layer of insulation, and they improve air quality because the plants filter pollutants that would otherwise remain in the atmosphere.

Advocates also point to a reduction in stormwater runoff. Merritt estimates the roof absorbs 70 to 75 percent of rainwater hitting the roof, keeping it from overflowing storm drains and preventing overflows of stormwater and wastewater from entering Lake Michigan, which has happened in the past. As another environmentally friendly component, strategically placed barrels around City Hall’s green roof collect rainwater for use in watering the plants.

European Influence
In becoming a pioneer in green technology, Chicago now rivals European cities that served as the inspiration for the changes. Daley was so impressed by green roofs on a German tour in the late 1990s, he imported the idea back to his city and figuratively started in his own backyard by making City Hall the test subject. Work started in 2000, and the verdant garden officially opened in 2001. (Unfortunately, the roof is not open to the public or city employees because it is not ADA compliant, but workers in adjacent buildings have a view of a park rather than an asphalt roof.)

The 20,300-square-foot roof cost $2.5 million, including fortification of the beams and installation of the garden, fittingly funded by a refund from Commonwealth Edison after a series of power blackouts in 1999. The city reports it saves an estimated $4,000 a year in energy costs due to the roof.

The garden is surprisingly lush, with 150 species of plants, prairie grass, and two crab apple trees strategically placed above columns. Two beehives serve to pollinate the plants, and as an added bonus, the city sells the honey in a local store.

Many layers of materials were applied to
the City Hall roof before the garden
Courtesy City of Chicago

So what does it take to install a rooftop garden? It’s complicated, a fact that de la Fleur says surprises many clients. For starters, there are two types of green roofs: intensive and extensive. Extensive roofs have shallow soil (typically two to six inches) planted with hardy, low-maintenance plants and are not what leaps to mind when most people think of a rooftop garden. They can be as simple as tray plantings, low trays of basic greenery that can be easily switched out. On the other hand, intensive roofs require more maintenance, have deeper soil (up to 40 inches), are generally sloped, and are akin to a traditional garden.

However, even that explanation is simplistic because the local climate affects the definition of intensive and extensive systems. Four inches is extensive in a dry climate, but not in Seattle, for example, with heavy rainfall. “It’s really all about tolerant vegetation that has the capacity to fall dormant and spring back,” de la Fleur says.

City Hall has three different kinds of systems—extensive, intensive, and semi-intensive. The extensive systems (simple tray plantings) are on the areas with the lowest available load capacity, semi-intensive systems are in areas with a medium load capacity, and intensive systems are located atop sturdy columns that run through the entire building, de la Fleur explains.

Most Roofs Work
Generally, green roofs can be placed atop most roofs, as long as they are structurally sound and have a slope no greater than 40 degrees. The engineering has been transplanted from Europe, with few changes. “It’s a decade-old technology there, and there’s very little need for reinventing except for accommodating for climate,” de la Fleur says. “The Germans have mastered specifications, and those still guide green roof design.”

As a civil and water resources engineer, Eric Otto plays a key role in designing green roofs
Courtesy CDF

The easiest scenario, obviously, comes with new construction. “If you know you want a green roof in the early design stages, all you have to do is make sure the column spacing is right, decide what kind of green roof you want, and build it to support the load of the roof that has been chosen,” de la Fleur says. “Deciding to build a green roof in the later stages is trickier, because you might have missed opportunities for money saving that could have been gleaned earlier in the process. The earlier you get the green roof into the agenda, the better the chance for success and the more economic advantages you’ll realize.”

However, while new-construction green roofs are almost a no-brainer, de la Fleur says retrofitting green roofs to existing structures comprises the majority of the green roof business. “Retrofitting is a much bigger market than new buildings.”

The first step, says Eric Otto, civil and water resources engineer at Conservation Design Forum, is a structural analysis to determine what weight the building can withstand as well as a basic roof check for leaks or structural weaknesses. “If necessary, the engineer can do additional structural work and reinforcements like beaming or framing to provide additional capacity and support,” he says. However, that is more cost-prohibitive, and many clients opt to tailor the roof garden to the load capacity the roof can accommodate.

Included in the weight restrictions is the water capacity of each roof, with hydrology comprising a delicate balance. “It has to be engineered so that if it rains, it drains,” de la Fleur says. “If it holds too much water, it will be too heavy, not to mention kill everything, and if it holds too little, it will still kill everything.”

Structural engineers determine the load capacity of each section of the roof and whether beams should go down through the building or whether the load will be spread out among the entire roof. “We have a detailed map of what load is available in which spot,” de la Fleur says. “For instance, directly over a column can sustain a higher load than father away. If we determine, say, that it can sustain 40 pounds per square foot, then we start the design process and see what our options are for the kind of green roof possible for those contingencies.”

De la fleur says his firm just finished a green roof on the Jardine Water Filtration plant in Chicago, with some areas blank without any green roof material because of the limitations of the roof. “We work with the diversity of the load availability and turn it into a diversity of green roof typologies.”

According to Otto, “CDF does not have a structural engineer on staff. We always work with an outside firm.” Halvorson and Partners Structural Engineers, with offices in Chicago and Atlanta, served as the structural engineer on the City Hall green roof.

Many Layers
City Hall added roof reinforcements, strengthening the beams for the added weight. Once the structural concerns are taken care of, the roof must be fitted with a waterproof barrier as an initial layer, followed by a root barrier to keep the plant roofs from digging into the roof surface. The next layer is a drainage layer, designed to ensure that precipitation is filtered through the plant surfaces, with any excess moving laterally across the slope. The final layer is a filter to prevent the soil from eroding.

The City Hall roof has a Styrofoam lining for initial protection, topped by two inches of inorganic material, with each individual plot sloped for irrigation purposes. The soil on any rooftop garden is not traditional garden dirt but rather a material called seedum, which weighs a fifth that of traditional soil and tolerates drought better.

A variety of plants thrive on City Hall, reducing the building’s energy costs and stormwater runoff
Courtesy City of Chicago

Delicate engineering is only one of the considerations of green roofs. They tend to cost twice as much as traditional roofs, as most estimates peg the cost at an additional $12 to $15 per square foot. But proponents argue that the initial expensive is recouped in energy savings and less-frequent replacement costs. Green roofs last twice as long because they’re not susceptible to heat or freeze-thaw damage.

Chicago continues to push green roofs as a major initiative, offering incentives, and in some cases requirements, for developers. In 2006, the city offered all 40 of its $5,000 incentive grants for green roofs on residential and small commercial properties while requiring any major project with city funding to offset its environmental impact with a green roof. “The mayor has targeted developers by saying if you want so many floors, you have to get a green roof,” de la Fleur says. Just a few of the city’s other high-profile green roofs come atop the Chicago Cultural Center, the Apple Store on Michigan Avenue, the McDonald’s in River North, CTA Headquarters, and two big-box stores, Target and Wal-Mart.

Other cities like Portland, Oregon; Washington, D.C.; and Toronto are catching up, but Chicago remains the U.S. pioneer. Even in Chicago, though, acceptance has been slow. “I get a lot of reactions from people who say, ‘You want to put plants on a roof?’ It’s a logical reaction,” de la Fleur says. “But it’s not as nuts as it sounds. You just have to mitigate the growing environment from the extremes to something more moderate. The results can be really beautiful, and there’s no arguing with the environmental benefits.”

For more information on Chicago’s green roof initiative, visit or

Rebecca Little is a freelance writer in Chicago, Illinois

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