A living roof, also referred to as a vegetative or green roof, is a layer of soil and plants installed over a waterproof roof system. Studies on such roofs have shown that they provide a number of economic, aesthetic, and environmental benefits when designed and built properly: reducing annual heating and cooling costs, extending the life of the roofing material below the soil and plants, improving air quality, reducing outside noise, and controlling storm-water runoff. Author Roxi Thoren, an assistant professor of architecture and landscape architecture at the University of Oregon, explains how the layers of a living roof work together in a system. She discusses the options for waterproofing membranes, lists three ways of insulating a living roof, and discusses soil retention and drainage on sloped roofs. A sidebar describes three ways of building a living roof with modular products.