Understanding green roofs
The University of Toronto’s Green Roof Innovation Testing Laboratory (GRIT Lab) was created by the John H. Daniels Faculty of Architecture, Landscape, and Design in 2010, with the goal of investigating the environmental performance of green roofs specifically for the Toronto climate. GRIT Lab brings together researchers and students from many different backgrounds, including landscape architecture, engineering, biology, forestry, and planning. It functions as an educational and research facility. Each year, the lab provides hands-on training for undergraduate and graduate students. Since 2010, it has trained three PhD students, 12 master’s degree students, and numerous undergraduate students. The lab has hosted international students from Brazil, France, and Israel.
GRIT Lab partners with the industry and regulators to produce Ontario-based data on vegetated roof performance, as well as to test innovative and new green roof designs. Data produced by the lab is shared to inform and improve green roof design.
Not all green roofs are equal, and decisions made during the design process have long-term implications on a roof’s as-built performance and function. The lab has investigated the role of four design parameters:
- growing medium type (mineral-based and wood-based compost);
- planting type (sedum plants, as well as grass and herbaceous flowering plants);
- depth (100 and 150 mm [4 and 6 in.]); and
- irrigation practice (none, daily, and on-demand).
The GRIT Lab facility includes 33 green roof modules, each instrumented with sensors to measure temperature, soil moisture, and stormwater discharge. Tipping buckets at the base of each module measures stormwater as it flows out of the green roof, while thermocouples measure ambient temperature conditions above the roof surface. Sensors operate all year long and record measurements every five minutes. Data collected from each green roof module is analyzed to determine the influence of each design parameter (soil, planting, depth, and irrigation) on stormwater and thermal characteristics.
The lab was expanded in 2014 to investigate the performance and benefits of integrating photovoltaic (PV) arrays with a green roof system. Two full-size green roofs and four arrays of solar panels were installed. Temperature sensors were installed on the back side of the solar panels to measure surface temperature, as well as above the green roof to measure ambient temperature. Small and large tipping buckets at the base of each module were also connected in series to measure low and high stormwater flows from the green roof.
Designing for stormwater management
GRIT Lab researchers have found irrigation is a critical operational practice that influences green roof performance, including stormwater management, temperature, and plant growth. Green roofs with irrigation have healthier plants and create cooler local conditions, but have a smaller capacity to store rainwater. Even so, an irrigated green roof intercepts, stores, and evapotranspirates 50 per cent of summer rainfall (May to October). Green roofs with on-demand irrigation or no irrigation retain 70 per cent of summer rainfall. (For more, consult J. Hill, J. Drake, B. Sleep, and L. Margolis’ “Influences of Four Extensive Green Roof Design Variables on Stormwater Hydrology,” in volume 22 of Journal of Hydrologic Engineering.)
Irrigation has proved to be an essential practice for green roofs with grass and herbaceous flowing plants. Test modules with these plant types and no irrigation have, over time, lost up to 100 per cent of their vegetative cover. Sedum plants have an evolutionary adaptation allowing them to regulate transpiration and, as a result, survive water-limited conditions more easily than the grass and herbaceous plants.
The mineral-based growing media used in the test modules follows the German Landscape Research, Development, and Construction Society’s (FLL’s) Green Roof Guidelines. Within the vegetated roof industry, it has been debated by designers and green roof installers whether growing media blends that do not follow FLL guidelines (like the organic-based media tested at GRIT Lab) may be more susceptible to wind erosion, compaction, and/or breakdown. However, GRIT Lab researchers have observed minimal to no loss of media depth on both the FLL-compliant mineral-based and the non-FLL-compliant organic-based vegetated roofs in the beds tested in the lab and roofs surveyed around the Greater Toronto Area (GTA). (See D. El Helow, J. Drake, and L. Margolis’ “Testing the Potential Synergy of Green Roof Integrated Photovoltaics at the University of Toronto Green Roof Innovation Testing [GRIT] Laboratory,” published in 32nd RCI International Convention and Trade Show, pages 229 to 235.) The organic-based media provides additional performance benefits for green roofs—plants were generally healthier and stormwater retention was increased relative to test modules with the mineral media.
Plant type and green roof depth have been found to not affect stormwater processes. Similarly, none of the tested variables affect peak flow reduction values. On average, GRIT Lab test modules reduced peak discharge rates by 88 per cent. (For more information, check out D. Chemisana and C. Lamnatou’s “Photovoltaic Green Roofs: An Experimental Evaluation of System Performance,” in volume 119 of Applied Energy, as well as J. Breuning, K. Tryba, and R. Miller’s “Vegetated Roofs [Green Roofs] Combined with Photovoltaic Panels,” in Green Roof Technology.) This observation is particularly exciting, as it suggests green roofs provide an ‘existence’ value—in other words, the existence or presence of a green roof, regardless of its design, automatically provides a known reduction in stormwater flow rates. These findings give landscape architects and green roof designers a great deal of flexibility. As green roofs provide similar stormwater benefits regardless of the selection of plant type or depth, planting layout and selection can also be governed by other considerations, such as esthetics, biodiversity, or cost.