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. Several German cities already require so-called “green roofs” on all new commercial buildings. Here in the U.S., the idea is just catching on. And we found one of its proponents. Tom Liptan is a storm water specialist for Portland, Oregon — a city where sewer overflows are common. Two years ago Mr. Liptan built a green roof on top of his garage.