What is a green roof?

Green roof systems are described as an extension of the existing roof which involves a high quality water proofing and root repellant system, a drainage system, filter cloth, a lightweight growing medium and plants.¹While there is some debate as to the effectiveness of green roof structures at addressing some of the larger issues associated with urban environmental degradations and climate change, green roofs have been found be beneficial. For example, these systems have been found to lower rooftop and building temperatures, thereby reducing the urban heat island effect found in so many of today’s cities, as well as improving the quality of stormwater runoff while decreasing the amount of runoff during storm events. Green roofs also slow precipitation that rooftops capture so that when it drains to the public sewer system, or some other receiving body, it happens over a period of hours and sometimes days rather than minutes or seconds.  This reduction in rainwater release dampens the pulse, or volume of water, that must be transported off-site and treated in wastewater plants. In cities or areas with combined rainwater and sewer systems, such as Seattle and Portland, this pulse sometimes overwhelms the wastewater treatment plant, resulting in released sewage and rainwater. In our region, preventing combined sewer overflows is an important action for improving the ecological health of Puget Sound and other large bodies of water.

Over the past several decades, green roof technology has become a popular sustainable building component in the U.S. and across the globe. While the concept and basic technologies of this type of infrastructure are not new, it is only in the past few years that these techniques have become a mainstream roofing option in North America, with new companies patenting systems and promoting products on a regular basis. Green roofs have also been adopted and encouraged as a new building practice by sustainable building certification programs like the Leadership in Energy and Environmental Design (LEED) certification process for new construction, as well as municipal incentive programs such as Seattle’s Green Factor.

At the building scale, the installation of a green roof has several advantages to a traditional roofing system including, but not limited to, economic, ecological, and aesthetic concerns. Economically, green roofs allow for an increased lifespan of the waterproof membrane, dramatically reducing the amount of roofing material entering the landfill during the life of the building. They have also been found to provide savings on energy requirements for heating and cooling a building.² From an ecological perspective, green roofs have the potential to significantly increase the available habitat for insects and birds in an area. In Germany, research has shown that green roofs, due to a relative lack of disturbance, can support a much more diverse community of insects than is found on the ground in surrounding areas. If a deep enough growing media is used (greater than 4”), the roof may be designed as a therapeutic garden or for food production such as the case with Toronto’s City Hall demonstration project. Such a diverse set of uses dramatically increases the aesthetic appreciation of these places and activates building areas that were traditionally ignored by building designers and users.

What are we doing?

In partnership with Snyder Roofing of Washington, LLC and the Green Futures Lab from the College of Built Environments at the University of Washington we are in the initial stages of a long-term study examining the effectiveness of green roof techniques and strategies within our region. Research coming out of Vancouver, British Columbia³ and Portland, Oregon⁴ has made significant strides in providing preliminary regional data on stormwater and thermal performance of green roofs, however, there is little available information examining the regional potential for green roof infrastructure to 1) improve on-site water quality and detention, 2) assess durability of green roof materials, and 3) fortify ecological connectivity and function in our regional urban environments. The primary goal of our project is to comparatively examine five differently designed structured green roof plots over time to specifically assess the volume and rate-of-release of rainwater, product durability, and plant establishment and survival.

Thanks to a recent grant received from the Roof Consultants Institute Foundation (RCIF) we are beginning the initial phase of this study. Over the coming years, we hope to expand our research agenda to, in short, remove the ‘mystique’ surrounding green roof technologies and applications in this region by developing a base of knowledge that is specific to the climatic conditions we experience in the Puget lowlands.

Plant Palette: Questions of Survivability

A primary research agenda for our project is to assess the establishment and survivability of select plant communities endemic to our region. Green roofs and the plants that grow on them are exposed to the harshest of weather and the extremes of our climate patterns in the Puget lowlands. Often over-saturated during the winter months and dry in the summer months, these plants are also fully exposed to extreme wind. Plant survival and species establishment is a distinct challenge for most green roof projects within the region. While these projects often use a mixture of regionally native and nonnative sedums, we are interested in exploring the potential of other plant species and community types within the region. The plant palette for one of the research plots in our project is composed entirely of native species specifically chosen for their hardiness and ability to withstand harsh growing conditions. The plants for this plot were donated by Fourth Corner Nursery and are listed in the table below. Over the course of the next several years we will be monitoring how each species and combination of species survive and establish in these harsh conditions, so that we can begin to determine an appropriate template of plant species for green roof projects that are endemic to this region.

It is our hope that initial studies will generate findings that guide the protocols for establishing a long-term inquiry into the effectiveness of sustainable green roof strategies within the Puget Sound|Georgia Basin region. As with this initial (year-long) phase, data generated and lessons learned during long-term studies will serve as a basis for a protocol outlining regionally specific guidelines for green roof performance. It is our intention that this project also serve as a case study for the development of protocols for green roof performance evaluation transferable to similar regions in the US and around the world.

Who we are.

Currently, our core research team consists of two faculty members from the Department of Landscape Architecture at the University of Washington, and two research scientists. Several additional faculty members act as consultants and advisors to this project. In addition to our core research team, professionals from the fields of engineering, horticulture, landscape architecture and architecture, as well as the manufactured soil and roofing industries, are participating with their enthusiasm, resources, and leadership.

The Green Futures Lab (http://greenfutures.washington.edu/) in the College of Built Environments at the University of Washington is dedicated to developing innovative approaches to ecologically focused planning and design of built environments through interdisciplinary research, design and education. We have assembled a diverse project team from the University and both the private and public sectors to establish methodologies for this research and to collect, analyze, and report on the data.

 

 

Table 1: Plant list

Known Project Plants	Native to PS|GB
Achilea millefolium	Yes
Allium acuminatum	Yes
Allium cernuum	Yes
Anaphalis margaritacea	Yes
Aster subspicatus	Yes
Camassia  quamash	Yes
Carex inops	Yes
Cerastium arvense	Yes
Danthonia intermedia	Yes
Delphinium  nuttalllii	Yes
Delphinium menziesii	Yes
Dodecatheon pulchellum	Yes
Festuca roemeri	Yes
Fragaria chiloensis	Yes
Fragaria virginiana	Yes
Iris chrysophylla	No
Juncus tenuls	Yes
Lathyrus littoralis	Yes
Lomatium utriculatum	Yes
Lupinus littoralis	Yes
Mimulus guttatus	Yes
Penstemon davidsonii	Yes
Plectritis congesta	Yes
Sedum acre ‘Aurea’	No
Sedum album ‘Coral Carpet’, ‘Orange Ice’	No
Sedum floriferum ‘Weighenstephaner Gold’	No
Sedum hybridum ‘Immergrunchen’	No
Sedum kamtschaticum ‘Variegatum’	No
Sedum middendorffianum diffusum	No
Sedum oreganum	Yes
Sedum reflexum ‘Blue Spruce’, ‘Green Spruce’	No
Sedum rupestre ‘Angelina’	No
Sedum spathulifolium ‘Cape Blanco’	Yes
Sedum spurium ‘Coccineum’, ‘Fuldaglut’, ‘Green Mantle’, ‘John Creech’, ‘Red Carpet’, ‘Roseum’, ‘Summer Glory’, ‘Tricolor’	No
Sedum stefco	No
Sedum takesimensis ‘Golden Carpet’	No
Sedum tetractinum ‘Coral Reef’	No
Sedum Weighenstephaner Gold	No
Sisyrinchium douglasii	Yes
Total Plant Species	Total PS|GB Species
39	25

* Plant species are known for four out of five plots. It is assumed that the plot with unknown species is planted with non-native species to the PS|GB region.

Acknowledgments

The authors wish to acknowledge the very helpful comments provided by local green roof experts: Brian Taylor and Drew Gagnes, MKA engineers; and the following companies for material support: George Pfeiffer and David Walter, Carlisle & Coastal Specified Products; Vikki Henry, Pacific Topsoils Inc.; Dr. Richard Haard and Veronica Wisniewski, Fourth Corner Nurseries; Derbigum; Don Carter and Vanessa Keitges, Columbia Green Technologies; Brian Partridge, Derbigum; Brian Mosley, Xero Flor America LLC; Hydrotech; Mike Brandvold and Ron Branham, Pacific Earth Works Inc. Lastly the authors would like to acknowledge Snyder Roofing of Washington, for providing the location as well as incalculable support for this project.

Resources

The Georgia Basin. Georgia Basin Ecosystem Initiative, 2005.

City of Seattle Department of Planning and Development. Seattle Green Factor. 2009. Available:http://www.seattle.gov/dpd/Permits/GreenFactor/Overview/. January 18 2010.

Getter, K. L., and D. B. Rowe. “The Role of Extensive Green Roofs in Sustainable Development.” Hortscience 41.5 (2006): 1276.

United States Environmental Protection Agency. Greening EPA Glossary. 2009. Available:http://www.epa.gov/greeningepa/glossary.htm. January 15 2010.

United States Green Building Council. LEED for New Construction. 2010. Available:http://www.usgbc.org/DisplayPage.aspx?CMSPageID=220 January 18 2010.

Footnotes

¹ Green Roofs for Healthy Cities (Green Roof Infrastructure Industry Association) Accessed at:www.greenroofs.org

² Getter & Rowe (2006) The Role of Extensive Green Roofs in Sustainable Development.” Hortscience 41.5: 1276

³ Greenskins Lab, University of British Columbia: accessible at: www.greenskinslab.sala.ubc.ca/

⁴ Spolek, G (2008) Performance monitoring of three ecoroofs in Portland, Oregon Urban Ecosystems 11:349-359