In heavily-paved urban areas, a good downpour can quickly overwhelm a municipal sewer system. But an ancient building technique that’s popular in Europe suggests a way to alleviate the problem: add a touch of green to the roof. Roof-top vegetation provides insulation, cooling and waterproofing; and it can help prevent runoff, too. Here in the U.S., the idea is just catching on. Steve talks with Tom Liptan, a storm water specialist for Portland, Oregon. Two years ago Mr. Liptan built a green roof on top of his garage.