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Stephan Brenneisen, a Swiss green roof specialist and ecologist, claims that many of the thin substrates on sedum roofs lose their effectiveness over time. Some of the substrates used tend to become acidic and do not sufficiently provide minerals and nutrients for the long-term health of the sedum plants. Owners rarely fertilize their sedum roofs on a regular basis and the plants suffer after a couple of years. Brenneisen also found that many sedum roofs have minimal positive effects on storm water retention, energy use, or the urban climate — core arguments that have been used to justify the implementation of green roofs. Moreover, ecologists attribute rather limited biodiversity values to sedum roofs and prefer roofs that offer a secondary habitat to endangered plant and animal species.

For example, Brenneisen created a 16,000-square-foot (1,500-square-meter) green roof on top of a new hospital building in Basel, designed by one of today's leading architecture firms, Herzog & de Meuron. He likes to deviate from the thin and lean artificial substrates of the industry toward thicker (more than three inches, or 7.5 centimeters) and richer soils that mimic local site conditions. The flat roof of the seven-year-old Rossetti building is coated with a substrate that varies in thickness from three inches to small hills of fifteen-inch (38-centimeter) depth. Compared to the lightweight soil compositions used by the industry, Brenneisen's mix seems old-fashioned and underdesigned. However, the soil was highly successful for habitat creation. Six years after the roof was seeded with a mix of local grassland plants, a colony of rare spiders and fifty-two beetle species have been found. The building is close to the river Rhine in Basel, and some of the species found on this roof are typical inhabitants of river banks.

Brenneisen ascribes the roof's success to the fact that he used local materials for the substrate and that the varying soil thickness enabled a richer pant and animal life. The small mounds enable insects to survive the winter, whereas they would freeze to death in thinner substrates.

Moos Water Filtration Plant

Brenneisen believes that his roofs will get richer in biodiversity over the years. He likes to show the potential long-range benefits of his designs by referring to a green roof in Zurich that is now ninety years in service.

The lake water filtration plant Moos built in 1914 was a product of engineering excellence of the time. Water from the lake of Zurich is pumped into the building and is cleaned to drinking-water as it slowly filters through a layer of sand. It was the first reinforced concrete building in Zurich and was outfitted with a nine-acre (3.6-hectare) earth roof for temperature moderation.

During its ninety year lifespan, the roof had to be reworked only on its edges; the rest of the waterproofing remained functional.

The three-inch- (7.5-centimeter-) thick concrete ceiling is sealed with a three-quarter-inch (two-centimeter) mastic asphalt membrane and then overlaid with six to eight inches (15 to 20 centimeters) of topsoil. Today, the roof supports a stable meadow community with 175 different plant species and six thousand specimens of a rare orchid. Many plants are on the endangered or rare species list of Switzerland.

The protected location of the roof and the continuous maintenance regime of the meadow preserved a plant community that does not exist on the ground anymore.

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Christian Werthmann is currently an assistant professor in the landscape architecture department at the Graduate School of Design at Harvard University. He received his master of landscape architecture degree with a specialization in urban design at the University of Kassel in Germany. Before moving to the United States in 1997, he taught in the landscape architecture department of the Technical University Munich and worked for the landscape architecture office of Latz + Partners. In the U.S. he worked at Hargreaves Associates in San Francisco and as an associate of Peter Walker and Partners, where he led a series of major design projects, including the green roof for Parking Structure #4 at Stanford University, which won an ASLA design award in 2005.

This article is excerpted from Green Roof—A Case Study: Michael Van Valkenburgh Associates' Design for the Headquarters of the American Society of Landscape Architects by Christian Werthmann, copyright © 2007, with permission of the publisher, Princeton Architectural Press.

SUBSCRIPTION SAMPLE Atelier 5 was inspired by Le Corbusier's example and covered all roofs of the experimental Halen Estate project with four to six inches of soil, 1958. Photo: Balthasar Burkhard (courtesy of Atelier 5) Extra Large Image Exploded view of the new roof for the American Society of Landscape Architects (ASLA) Headquarters, with its green roof by Michael Van Valkenburgh Associates (MVVA). Image: MVVA Extra Large Image The root barrier of the ASLA Headquarters green roof includes rubberized asphalt enforced with polyester and treated with a root-repelling chemical. Photo: Greg Scheider (courtesy of ASLA) A water retention and drainage mat manages water that percolates down from the growing substrate in the new roof for the ASLA Headquarters in Washington, D.C. Photo: Magco Inc. Expanded shale was used to fill the drainage mat where the green roof is thickest. Photo: Magco Inc. A fabric filter overlays the water-retention area to isolate the growing medium above from the water below. Photo: Magco Inc. The growing medium consists of lightweight aggregate, expanded shale, and compost. Photo: Magco Inc. Green Roof—A Case Study: Michael Van Valkenburgh Associates' Design for the Headquarters of the American Society of Landscape Architects Image: Princeton Architectural Press   Click on thumbnail images to view full-size pictures.