Learning from Applications
The largest North American green roof of the last five years is that of the Vancouver Convention Centre in Vancouver, British Columbia, Canada. Aside from being very big—261,360 square feet, or six acres, to be exact—the green roof is also integral to the ground-up project’s overall ecological mission and LEED-Canada Platinum rating; in essence, it recreates a coastal grassland ecosystem, typical of the Pacific Northwest. “The design of the living roof is ambitious in integrating into the broader ecology and landscape of the Vancouver waterfront,” says Mark Reddington, FAIA, of design firm LMN Architects. “It’s big enough to house an ecosystem. There are a number of different local plant types, as well as birds, field mice, and other creatures who live there. It’s also home to 250,000 bees in hives, whose honey is harvested and served in the building.”
The roof, which undulates in elevation based upon the needs of the interior space it shelters, features a number of custom-designed aspects, many of which were locally sourced by PWL Partnership Landscape Architects. The plants, for example, were derived from seeds, sedum cuttings, and plugs that were collected from Pacific Northwest grasslands. It was the first time that many of these were ever grown commercially in a nursery, or used for a green-roofing application. In all, the project used 400,000 plugs, 108 kilograms (238 pounds) of seeds, and 80,000 sedum plants. The growing medium—which was applied 8 inches deep across the roof and settled to 6 inches deep—was also locally sourced and engineered from sand dredged from the nearby Frazer River, organic matter, and lava rock. The project also employs a mat drainage system, the better to contend with Vancouver’s wet winters, and a triple-ply liquidized-rubber waterproofing layer topped with a granulated cap sheet for protection during construction.
The Vancouver Convention Centre system was designed to impose no more than 39.6 pounds per square foot upon the roof. But at the Target Center in Minneapolis, Kestrel Design Group was asked to deliver a system on top of an already existing building that would impose no more than 17.4 pounds per square foot. While that is an unusually low number for a public building, the arena’s original designers never expected the roof to have to bear anything other than snow, and so delivered the barest minimum of structure to serve that purpose. But when it came time to replace the roof, Minneapolis’s sustainable-minded city council members wanted to go green. “Some city council members strongly encouraged putting on a green roof for stormwater management,” says Kestrel designer Nathalie Shanstrom. “If we had any less loading capacity at all, though, they would not have considered it.”
To minimize weight, Kestrel kept the growing medium as thin as possible—2-3/4 inches deep in the middle of the roof and 3-1/2 inches deep at the perimeter. They also added a thin layer of a recycled geo-tech material designed to retain water and a drip irrigation system to give the plants the most favorable conditions possible in the meager soil. The plants were delivered with the medium in pregrown vegetation mats that included basic green-roof sedum augmented with plugs of 20 native prairie species adapted to shallow soil, drought, and windy conditions. The mats were pregrown in a nursery for two years, instead of the usual one—long before work began on replacing the roof’s waterproofing membrane—in order to ensure a solid green cover the moment they were installed. Beneath the plants, medium, and water-retention layer, the project features a mat drainage layer, PVC waterproofing, and a leak-detection system.
While weight was of paramount concern at the Target Center, in calculations for the roof of the Laurie M. Tisch Illumination Lawn at Lincoln Center in New York, the weight of a different variable had to be accounted for: people. Diller Scofidio + Renfro and FXFowle wanted to encourage visitors to walk on top of the hyperbolic-paraboloid-shaped grass lawn set atop a restaurant. Making this green roof fit for occupancy began with the roof itself, a 6-inch-thick concrete-on-metal deck slab that is rated for the dead load of the roof system and a live load of 100 pounds per square foot. Atop this substrate is a 14-inch-thick roofing system that includes a green roof.
The system features a waterproofing membrane sheet, a 1-inch-thick root barrier, 4 inches of insulation, and a 1/2-inch-deep drainage mat with filter fabric. The growing medium itself is 8-1/2-inches deep. Since the lawn is at a significant slope, between flat and a 1:8 pitch, the growing medium incorporates an integrated cellular confinement system, basically an expanded plastic mesh that keeps the soil from rolling down the hill. “We tested a lot of different grasses, looking at things like durability, appearance, water retention,” says FXFowle partner Heidi Blau, AIA. “We wanted something that would be green as long as possible during the year and also comfortable under a bare foot.” They ended up choosing a blend of tall fescue and Kentucky Blue grasses.
While it’s durable enough, the lawn has attracted more visitors than expected and within the first couple of weeks the grass got trampled. To prevent a dirt trail from forming at the favored entry spot, the designers worked with Lincoln Center to set up barriers to change this entry point from time to time, thus giving the grass time to recover. “Grass is a natural material,” Blau says. “It can get destroyed. Lincoln Center is learning as each season goes by how to care for it.”
Measuring the Benefits
As with any piece of building technology or infrastructure, the feasibility of installing a green roof—whether on a new building or as a retrofit—comes down to cost-benefit analysis. One problem with conducting this analysis, however, is that there are no well-defined metrics for understanding the performance characteristics of these systems. For example, green roofs are often cited for improving an enclosure’s insulation values. In reality, you cannot apply an R-value to them because, while a green roof may deliver some insulation when dry, when wet, temperature will move right through. On the other hand, studies at Chicago City Hall have determined that the greening of the roof lowered the air temperature on the roof in summer to 85 F as compared to 110 F on a typical black-tar roof. That means that intake air has to be cooled that much less by the building’s air-conditioning system. “Green roofs will have an ecological benefit from an aggregation of services; there are very few discrete services that will justify the cost,” says horticultural consultant Snodgrass. “Those are things that are hard to calculate. The formula is so long and difficult that engineers just don’t want to think about it.”
Perhaps the easiest way to measure the cost-benefit of green roofs is the extra life they lend to the integrity of the shelter. “We did a life-cycle cost analysis of a green roof versus a regular roof,” says Shanstrom of the Target Center project. “It ended up being very close in cost when you considered the extra life span the green roof gave to the roofing membrane. Actually, the green roof was a bit more favorable.”