Extensive Green Roofs: Horticultural Considerations for Stormwater Management

An extensive green roof is a contained ecosystem with a soil depth of less than six inches (15.25 centimeters); the shallow depth results in a lighter-weight roof than a traditional intensive roof garden, thus reducing the cost of the building’s engineered structure. Such roofs have been shown to provide both aesthetic improvements (beauty and useable space) and economic benefits (reduced energy use and extended roof life). However, researchers, horticulturalists, and commercial developers are increasingly creating extensive green roofs that meet specific ecological goals, such as stormwater management, reduction of the urban heat-island effect, enhanced air filtration, and habitat creation.

Successful extensive green roofs result from a holistic design approach, influenced by regional climates, rooftop microclimates, plant needs, and, most importantly, the green roof goals. The composition of a green roof varies within and across regions, reflecting local considerations that may include building codes and neighborhood character but certainly bear upon the prioritizing of these ecological goals. In the Pacific Northwest, for instance, stormwater management ranks high among those goals. Whether they arise from structural constraints, seasonal dry periods, or client needs, there are many issues to be resolved in designing any extensive green roof to maximize a specific ecological goal. We still have a lot to learn, but increasing numbers of researchers and practitioners are busy focusing on the effort to make our roofs serve us in better and more effective ways.

For Further Reading

In addition to the references cited by the authors in their article in the April 2010 issue of Pacific Horticulture, the following may be of interest to those pursuing a deeper understanding of extensive green roofs.

California Academy of Sciences. The Living Roof. https://www.calacademy.org/academy/building/the_living_roof/

FLL: Guideline for the Planning, Execution and Upkeep of Green-Roof Sites. Bonn, Germany: Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau E.V. 2002.

Green Roofs for Healthy Cities, a non-profit industry association to promote the intelligent use of green roofs. https://www.greenroofs.org/

Inhabitat. A two-part interview with Paul Kephart, executive director of Rana Creek Habitat Restoration and Living Architecture. https://www.inhabitat.com/2006/07/18/interview-paul-kephart-of-rana-creek/. 2006

Klinkenborg, Verlyn. Up on the Roof. National Geographic, May 2009.

Miller, C. Moisture Management in Green Roofs. Proceedings of the Greening Rooftops for Sustainable Communities Conference, Chicago, IL. 2003.

Monterusso, MA, DB Rowe, CL Rugh, and DK Russell. Runoff Water Quantity and Quality from Green Roof Systems. Acta Horticulturae (ISHS) 639:369-376. 2004.

Nagase, A. and C Thuring. Plant Responses to Drought on Extensive Green Roofs: The Effects of Temperature, Substrate Type and Substrate Depth. Proceedings of the Fourth Annual Greening Rooftops for Sustainable Communities Conference. Boston, 2006.

National Institute of Building Sciences. Extensive Green Roofs, Whole Building Design Guide, https://www.wbdg.org/resources/greenroofs.php. 2009.

Simmons, MT., B Gardiner, S Windhager and J Tinsley. Green Roofs Are Not Created Equal: The Hydrologic and Thermal Performance of Six Different Extensive Green Roofs and Reflective and Non-reflective Roofs in a Sub-tropical Climate. Urban Ecosystems. 11:335-337. 2008.

VanWoert, ND., DB Rowe, JA Andresen, CL Rugh, RT Fernandez and L Xiao. Green Roof Stormwater Retention: Effects of Roof Surface, Slope and Media Depth. Journal of Environmental Quality. 34:1036-1044. 2005.

Wolf, D. and JT Lundholm. 2008. Water Uptake in Green Roof Microcosms: Effects of Plant Species and Water Availability. Ecological Engineering. 33:179-186. 2008