“Most trays are plastic,” says Kephart, owner of Rana Creek Habitat Restoration, Carmel Valley, Calif. But museum design architect Renzo Piano wanted an alternative.
Kephart has designed more than 30 living roofs in the past two years. He says that at $17 per sq ft, the roof costs less per sq ft than many more conventional roofs, which can cost more than $25 per sq ft.
He attributes that to economies of scale and the simplicity of the planting design.
The rest of the system was not simple to design, thanks to slopes as steep as 55°. To minimize erosion, Rana and landscape architect SWA Group, Sausalito, Calif., designed a framework of gabions that crisscross the bigger mounds as well as the flatter roof areas. The gabions are small wire cages filled with lightweight volcanic rock. The cages are fastened together and reinforced by epoxy-coated rebar.
Reinforced-nylon straps bind them in a belt-and-suspender system that keeps them in place. The gabions channel excessive water to large cups in a reservoir board underlying the roof’s flat areas where it can be reserved for irrigation, says SWA.
Most of San Francisco’s annual precipitation occurs in December and January. Should a downpour exceed 4 in. and surpass the soil’s saturation point, the cages also direct potential run-off into an underground recharge chamber. By the time water seeps into groundwater through Golden Gate Park’s sandy soil, the roof’s dirt and plant roots will have filtered out pollutants.
Piano wanted an exposed ceiling in much of the museum’s interior, so builders installed polystyrene thermal insulation on the roof instead of inside. Crews could then notch gabions into the insulation.
Rock-climbing equipment was used for worker's stability.
The biggest challenge for Jensen Corp. Landscape Contractors, San Jose, Calif., was getting the 3-in. layer of soil beneath the 3-in. soil trays to the steeper slopes. Like a bucket brigade, workers lined up in a row, roped to one another for stability. “We had to train them to use rock-climbing equipment,” says John Vlay, Jensen’s president.
Rana has been adding native plants and seed since last fall, when installation was complete. “It’s growing very well,” says Kephart. The roof provides San Francisco with a vision of the urban restoration of ecological function, he adds.aul Kephart describes himself as an ecologist, designer, artist and horticulturalist. With the 2.5-acre planted roof and its seven hills capping the new California Academy of Sciences in San Francisco under his belt, he can add “inventor” to his list. To install the garden’s 1.7 million plants, Kephart created a biodegradable, soil-based, modular planting tray made from waste coconut fiber and infused the tray with a species of fungi that aids in plant growth and health.