Evaluation of an Extensive Green Roof – York University, Toronto, Ontario


YorkuGRpage_smallpicThis study was initiated in 2002 in an effort to address the growing need for research on the stormwater management and biodiversity benefits of greenroof technology within cold weather climates. The specific objectives of this study were to:

  • evaluate the potential of rooftop gardens to reduce the quantity and improve the quality of stormwater runoff;
  • quantify the stormwater management benefits of greenroofs at a watershed scale through scenario modelling;
  • assess the ability of greenroofs to improve urban biodiversity and recommend flora species for use on greenroofs in southern Ontario; and
  • provide recommendations on the design and maintenance of greenroofs to maximize benefits related to stormwater management and biodiversity.

Analysis of the chemistry of several commercially available greenroof growing media was also conducted as part of this study.


The greenroof selected for the study was constructed in 2003 on the York University Computer Science and Engineering building and is one of several sustainable features of the building design.  The roof itself is sloped with a 140 mm (5.5 inch) substrate planted with wildflowers.  The monitoring program included continuous measurements of rainfall, surface runoff quantity and quality, air temperature, relative humidity, soil temperature, and soil moisture levels.  Data collected at the study site were used to model the water quantity and quality benefits or different levels of greenroof implementation in an urbanized watershed in the Greater Toronto Area.  Inventories of flora, fauna and insect species were also conducted in 2004 and 2005 as part of a biodiversity assessment.

The garden was found to be highly effective in storing and evaporating runoff, resulting in over 60% less runoff than a traditional roof top adjacent to the green roof.  The following chart illustrates this point by comparing runoff from the garden (red line) to runoff from conventional roof (blue line) during a 16 mm storm event on August 10, 2004.  Both runoff volumes and peak flows are significantly lower on the garden (Click image to enlarge).  Most of this benefit occurs during the summer when soil and plant evapotranspiration rates are greatest.

For more detailed results from this study, download the fact sheet and  report from the side bar.