Learning Objectives - After this course, you should be able to:
- Describe the benefits of green roofs and walls.
- Explain how green roofs and walls are created.
- Describe current projects using vegetated surfaces.
A tree grows in Brooklyn-and in Chicago and Baltimore-but not only in parks or on streets. Increasingly, the greening of dense urban cores in the United States is occurring on rooftops and, to a lesser extent, on walls. According to a recent survey by Green Roofs for Healthy Cities-North America, a not-for-profit industry association, over two million square feet of roof and wall surfaces were greened in 2007, 30 percent more than the year before. Leading the trend in the U.S. are Chicago; Wilmington, Delaware; Baltimore; Brooklyn, New York; and Virginia Beach, Virginia.
This recent interest, though growing fast, is still far behind surface greening in Europe and Japan. In Germany, for instance, 10 percent of all flat roofs were vegetated by 2002. Japan has become a leader in facade greening in high-rise urban cores where the exterior wall area greatly exceeds the roof area. An excellent overview of the state of the technology throughout the world can be found in Planting Green Roofs and Living Walls by Nigel Dunnett and NÃ¶el Kingsbury.
There are many reasons for this interest, both economic and environmental. By effectively replacing hard surfaces, vegetated roofs and walls reduce the urban heat-island effect, making cities more comfortable and buildings easier to cool. The added layer of insulation lessens building heating and cooling loads in some cases. Vegetation also cools via evapotranspiration: The plants and growing medium hold water, which absorbs heat as it evaporates from the ground or is released by the leaves, cooling the air in the process. Plants absorb noise, trap dust, consume carbon dioxide, and produce oxygen. With proper design, plants can protect-and even extend the useful life of-the architectural surface beneath. They reduce or eliminate rainwater runoff, improving the water quality in rivers and streams and decreasing the danger of flooding. Vegetated surfaces provide a habitat for birds and other creatures, enhancing urban biodiversity. They can also become usable, enjoyable, harvestable gardens for humans, while improving the cityscape as viewed from high-rises above.
With all these advantages, why is urban greening not happening faster? One obstacle may be the reluctance of professionals concerned about increased roof loads, problems with leaking, or damage that plants can inflict on architectural surfaces. To learn about avoiding these problems, we need only look at the extensive research and practices of Europeans as outlined by Dunnett and Kingsbury. Another obstacle to North American adoption is that developers and building owners need to learn about the economic benefits. Indeed, municipalities and regulatory agencies at all levels should consider implementing "carrots" (development incentives) and "sticks" (taxes) to provide additional incentives. Chicago and Portland, Oregon, have succeeded with carrots.
Ateliers Jean Nouvel's bold MusÃ©e du Quai Branly in Paris is covered with over 150 species of plants, giving it the appearance of an untamed animal.
Photo Roland Halbe