Extensive green roofs have soil, or other growing medium, less than four to six inches deep and provide aesthetic and ecological benefits to the building and surrounding area. These roofs are often self-sustaining and require minimal maintenance. For extensive green roofs best plants are generally those that are low-growing, shallow rooted perennials that tolerate heat, cold, sun, wind, drought, salt, insects, and disease. Hardy succulents, which offer good ground cover, are often the primary plants and work horses on an extensive green roof.
Understanding the popularity and interest in green roofs might best be done by considering the benefits of green roofs and how they can accomplish multiple goals. Green roof benefits can include:
• Reducing storm water runoff (by as much as 75%), as well as providing filtration that improves water quality
• Providing a habitat for animals and insects that is otherwise eliminated by development
• Reducing energy costs, providing insulation, and reducing roof replacement costs
The aesthetic benefits of roofs have long been a design concern in architecture. In the late nineteen-hundreds, William Morris wrote on the topic of the external appearance of roofs, saying: "And where... new buildings must be built, by building them well, and in a common-sense and unpretentious way, with the good material of the countryside, they will take their place alongside of the old houses and look, like them, a real growth of the soil."
Peggy Notebaert Nature Museum extensive green roof (photo credit: © Conservation Design Forum)
Millennium Park intensive green roof
Peck, Steven W. Award Winning Green Roof Designs: Green Roofs for Healthy Cities (Atglen, PA: Schiffer). p.139. Crerar SB419.5.P43 2008
Searle Chemistry Laboratory’s extensive green roof
5735 S. Ellis Avenue. Photo by Cheryl Rusnak.