In more recent history, settlers on the American Plains built sod houses to insulate themselves from temperature extremes. The technology that has made green roofs a realistic solution for today's structures was developed in Germany, where some cities use them to combat flooding problems caused by stormwater runoff. Green roofs are starting to catch on in the states, says Matt Carr, Garden Roof product manager for American Hydro Tech, a company that manufactures roofing membranes, including the one at City Hall. Today, we're overseeing about a hundred green roofs throughout the states. They're really looking to green up the city.
Green roofs are a step forward from the old style of rooftop garden associated with penthouses.
Most of those rooftops feature a deck over the roof and lots of large, lightweight containers that hold plants. While those gardens are better than a bare black rooftop, they are high-maintenance affairs that offer fewer environmental benefits than the newer green roofs. The green roof planned for City Hall includes a seamless membrane made of hot, rubberized asphalt designed to last the lifetime of the building. It incorporates German irrigation technology, called a floradrain, which retains water and reduces the need for irrigation of green roofs. During a typical rainfall, this roof would retain 50 to 70 percent of the rainwater, says Carr.