Extensive in San Francisco

One new example of an extensive roof is on San Francisco's California Academy of Sciences building, designed by Renzo Piano Building Workshop in conjunction with Chong and Partners Architecture, SWA Group, and ARUP Engineering. The primarily flat rooftop has dozens of skylights and several large mounds that reflect the dome-ceilinged spaces below, such as a planetarium. Taking the lead in planting issues was Rana Creek Living Architecture, headed by ecologist Paul Kephart. Kephart and his team spent four years researching and testing appropriate plant species. They also wrote the educational material about the planted roof, which now supports one of the institution's main exhibits. The 197,000 square feet of planted roof area will be off limits to the public, but visitors will be able to see and learn about it from an observation deck.

 

The extensive roof of San Francisco's California Academy of Sciences in Golden Gate Park can absorb over two million gallons of rainwater, saving about 70 percent of the total rainfall from becoming runoff. Photo Nic Lehoux

 

 

Using a full-size mockup of a roof section, Rana Creek studied the effects of the mounds on the local microclimate, discovering that while orientation had little impact, slope made a difference: The higher, more exposed slopes dried out faster. Eventually, Kephart says, some species will thrive better on the sloped areas and others on the flat areas. His firm chose native species: sedums, self-heal, sea thrift, which grows on edges of bluffs, and wild strawberries, which will provide food for birds. "Most important," says Kephart, "is a little annual plant, Plantago erecta, or annual plantain. It's the larval food plant for the Bay Checkerspot, a rare butterfly. To me, it's a field of dreams. Maybe the butterflies will come back."

In the process of developing the planting scheme, Kephart invented the BioTray, a biodegradable, modular planter. The 17-inch square tray is made of natural latex and organic coconut coir, a fibrous, rapidly renewable product derived from coconut husks. The coconut fiber decomposes over time, turning into soil. The plant roots will knit themselves together, growing better rooting structures than if they were in the more commonly used plastic pots. The coconut coir allows air and water to move through the trays and into the growing medium. The entire roof can absorb two million gallons of rainwater, preventing an estimated 70 percent of rainwater from becoming runoff. Any water that escapes absorption is collected in cisterns for later irrigation. The building is expected to achieve a LEED Platinum rating from the U.S. Green Building Council.

Intensive in Washington

A recent example of an intensive roof is actually at ground level. The National Cathedral in Washington, D.C., is a popular tourist attraction, hosting 180,000 visitors per year. In the 1990s, the long lines of tour buses were impinging on views of this impressive gothic structure, so it was decided to build a depot for them underground, then landscape the depot's roof for pedestrians. Architects in the Washington office of SmithGroup worked with Michael Vergason Landscape Architects Ltd., of Alexandria, Virginia, structural engineer Tadjer-Cohen-Edelson Associates Inc., of Silver Spring, Maryland, and construction manager Advanced Project Management Inc., of Chantilly, Virginia.

 

The grounds of the National Cathedral in Washington, D.C., were renovated to create a below-grade bus depot and parking garage. Visitors can stroll atop a 75.000-square-foot garden without realizing that it doubles as a green roof. Photo Donovan Marks

 

 

The project team excavated 20 feet below grade level, rerouted utility lines, and erected a post-tensioned concrete roof structure, minimizing the need for interior columns while still providing support for the green-roof loads. Over the structure, they installed a hot-fluid-applied membrane system with a protection board, drainage board, filter fabric, and planting medium. Some of the soil was several feet deep, enough for planting trees and shrubs. One of the many construction challenges was protecting the waterproofing membrane while adding the growing medium and avoiding soil compaction. The landscape architect worked with an agronomist to develop a custom mix for the growing medium that would provide steady percolation, while still retaining enough moisture for plant growth.

The lower level now has parking for 411 cars and 18 buses. At ground level is a 75,000-square-foot landscape of manicured walkways, gardens, lawns, and flowering trees for the enjoyment of cathedral visitors perhaps unaware that they are not walking on solid ground.