Stuart Brodsky likes to stare out of his office window in downtown Chicago. He enjoys his view of the City Hall roof, a 20,000-sq.-ft. green roof with 300 species of drought resistant plants, all native to Illinois. Mayor Richard Daley commissioned the roof after a tour of Germany, where green roofs have been in use for years.“The Chicago City Hall is a fine example of a green roof,” Brodsky says.“It is much more attractive than other roofs where all you see are membrane, drains and HVAC units.”

Mayor Daley envisioned City Hall’s green roof as a demonstration project that might inspire owners, architects and builders to consider more sustainable building designs in general. The Chicago Public School system picked up on the message and has made a commitment to sustainable design for renovations of all its existing school buildings and the construction of any new buildings. About a year ago, the district tapped the Chicago-based architectural and engineering firm of OWP/P to develop the program. As a senior project manager with OWP/P, Brodsky is directing the effort.

Since green roofs offer significant sustainable design features, Brodsky plans to consider the concept for appropriate Chicago school projects.

Properly designed green roofs create stable, living ecosystems. Their insulating properties conserve energy and reduce sound reflection and transmission. A collection of green roofs will reduce the warming effect of urban heat islands by as much as four degrees — a feature that attracted Mayor Daley to the concept.

One of the most significant benefits of a green roof is control of storm water runoff, an increasingly important environmental goal of sustainable design. According to Roofscapes, Inc., a Philadelphia-based firm that specializes in green roof design, green roofs control runoff by mimicking natural processes. They capture and hold precipitation in plant foliage, absorb water through roots, and slow the velocity of direct runoff. Roofscapes research suggests that a green roof can cut total annual runoff volumes by as much as 50 percent to 60 percent, compared to conventional roofing systems.

While green roofs offer many functional benefits to buildings, they can also provide schools with educational tools. “In some of the schools we’re working on, we are trying to place green roofs so that you can see them when you look out a window,” Brodsky says. “Teachers can show the roof to the kids, explain why it is there and how it works.”

A Fundamental Problem

Although interest in green roofs has grown in recent years, high costs have restrained their adoption in the U.S. In the world of K-12 schools, green roofs are rare to nonexistent, probably because of cost. Brodsky himself knows of no K-12 school that uses a green roof.

“Cost is the fundamental problem for any building owner interested in a green roof,” says Charles Miller, president of Roofscapes. Price ranges from approximately $11 to $30 per sq. ft., depending upon the thickness of the soil layer and the overall area to be covered, continues Miller. Thick soil layers covering wider areas cost the most. By contrast, roofs made with high-quality waterproofing membranes cost between $5 and $15 per sq. ft.

On the other hand, green roofs will conceivably last longer than conventional roofs. “Done right, I think these systems can last for 75 years,” Miller says. “No one knows for sure how long they will last. Green roofs have only been around for 35 years or so. But the good ones are still holding up well.”

A 75-year green roof could amortize costs and perhaps lead to lower roofing costs through the long haul, but school officials cannot ignore the immediate budgetary impact of a green roof. When up-front construction costs threaten educational programs, educational programs, of course, take precedence.

Chicago Schools Attempt to Move Toward Green Roofs

Indeed, high costs foiled OWP/P’s first attempt at a green roof for a Chicago school. The firm designed a green roof for a new high school. But when it came time to bid the project, school officials decided not to bid the soil and plant materials. “They went with a standard roofing system,” Brodsky says. “Nevertheless, the structure is there. They can install one in the future if they choose.”

To date, OWP/P has completed designs that could eventually place green roofs on four other Chicago school building, all K-8 elementary schools.

Set to go out for bidding soon, these 120,000-sq.-ft. school designs envision buildings of three stories that will fit into 50,000-sq.-ft. footprints. Greenery will not cover the entire span of these roofs. “These schools have been designed with green roofs that cover one-third to one-half of the roof surface,” says Brodsky.

The reason is structural. “When you cover a long span structure with a green roof, the cost premium gets higher because you are adding weight,” Brodsky continues. “If you plan to load a roof with dirt, water and snow, you need a very strong roof deck, one that is not susceptible to deflection through time. We advocate using a structural slab of concrete as the roof deck, in lieu of metal. This is the most reliable support system for plant medium.”

Green Roof Design

According to the Roofscapes website , green roofs fall into two design categories — intensive and extensive.

Intensive green roofs have been around for years. Commonly called roof gardens, they feature soil covers that are 12 to 40 in. deep. They can accommodate numerous conventional landscaping features, including small trees, fountains and ornamental ponds.

Newer, extensive designs are considered the wave of the future. The green roof atop the Chicago City Hall features an extensive design that follows a highly regarded German method. “The German approach is a breakthrough green roof technology,” says Miller. “German green roofs generally feature shallow, low-maintenance designs that require no irrigation. Plants for extensive systems tend to be succulents — perennial flowering plants with fleshy leaves that do not need much water. In northern climates, we use varieties of grass.”

In arid climates that require irrigation, Miller’s design philosophy is to introduce water at the of the soil layer with capillary or active trickle systems. “You can use spray irrigation,” Miller says. “But this method creates more weeds and greater maintenance requirements. Since low maintenance is part of sustainable design, we recommend against spray systems.”

Whether a system is extensive or intensive, all green roof designs require a high-quality waterproofing layer. According to Miller, green roofs can work with different waterproofing systems, but the highest standards must govern their construction. “For example, we’ll use a PVC membrane, but we want it to be at least 60 mils,” Miller says. “We’ll also use modified bituminous, but it has to be at least two ply. Other practical waterproofing systems include coal tar pitch. Bottom line, you need premium waterproofing. There are no bargain selections in waterproofing for a green roof.”

The bargain comes later, in the form of a long lasting, low-maintenance roof that provides a host of sustainable design benefits and makes it worth staring out the window at the rooftop.