Improving green roof performance

Improving green roof performance

As more buildings feature green roofs, experts say that diversity in plants and style is key
Monday, March 10, 2014
By Leah Wong

The introduction of Toronto’s green roof bylaw in 2010 has changed the fabric of the city by requiring green roofs on certain developments. Today, there is more than 245,900 square metres of rooftop green space across the municipality. But while the quantity of dedicated green roof space has noticeably increased, experts now say that building owners and managers should start thinking of improving green roof performance and quality by focusing on a diversity of green roofs styles and plant life.

Speaking at Green Roof Gurus Panel on March 6 at the University of Toronto were Scott Torrance of Scott Torrance Landscape Architect Inc., Joe D’Abramo, acting director of zoning and environmental planning for the City of Toronto, and Rick Buist, principal and president of Bioroof Systems Inc. While the three talked about how building owners can work with landscape architects and designers to install higher performing green roofs, Torrance succinctly summed up the issue: “all green roofs are not created equal.”

The panelists agreed that designers should not all use the same methodology when creating green roofs, as different styles and plant life all have distinct benefits. This is a point that Toronto emphasized in its Guidelines for Biodiverse Green Roofs report, which was prepared in part by Torrance’s firm.

The guidelines consist of strategies that identify and describe best practices for “creating habitat and promoting biodiversity on green roofs in Toronto.”

Torrance referred to the spread of the invasive emerald ash borer throughout Ontario, which has caused mass destruction to ash trees across the province. Seeing how one species can cause negative effects on a certain type of plant life, he explained that diversity in green roof plants could prevent disasters.

And, as Rick Buist added, a diversity of green roof types and plants can lead to maximized benefits for urban areas and buildings.

He said that two of green roof’s most important functions are stormwater management and reducing heat energy transfer, thereby increasing a building’s energy efficiency. Buist explained that the most common types of plants used in green roofs, succulents, are chosen for their ability to thrive. But while they are survivors, they may not provide the best return to building owners.

“Sedum (a type of succulent) is hardy, but it is not the best for stormwater and heat reduction,” he said.

The type of roof and structure of a building are major factors in the decisions landscape architects and designers make in planning a green roof system. Torrance agreed that while sedum is a tough performer, it is not necessarily the right plant for every project. Rather, plants should be chosen on a case-by-case basis. Sometimes, such as when the structure of a building can only withstand a thin green roof system, sedum is the best choice.

Landscape architects and building owners also have to think about where the green roof is located. An alternative, higher-performing system that Buist suggested is a meadow-style green roof, named for its use of grass meadow plantings. This is the same system that his company installed on the Williams Engineering Building at the Northern Alberta Institute of Technology in Edmonton.

This particular green roof, which consists of native perennials, has been thriving in an area that, due to its cool climate, has a notably low survival rate. And as an added bonus, the native plants chosen offer higher cooling benefits and increased water retention, as the depth of growing medium is greater than a typical green roof. Greater moisture retention increases the cooling potential of the roof, and reduces the burden on the municipal stormwater system.

Buist said that an urban meadow-style green roof with a greater depth of growing media could hold 300 per cent more water than a shallower sedum-style roof. When a six-inch growing medium is installed with a meadow roof, it has the potential to hold water from even incredibly heavy storms.

While Toronto is a green roof leader due to its bylaw, the city is not resting on its laurels.

Joe D’Abramo said that city staff are continuously studying the benefits of green roofs, and looking at how to best act as policy maker and regulator for the systems. The program and bylaw is getting more and more popular over time, despite some initial push back from the industry. Since the bylaw first came into effect, there have been more than 450 development applications for new green roofs.

However, Buist said that Toronto’s bylaw should only be the beginning. “The (green roof) bylaw was a great start,” Buist said, before adding that the city should now go a step further to bring the quality of green roofs up to a standard that will not only benefit the individual buildings, but the area as a whole.

Leah Wong is the online editor of Building Strategies & Sustainability magazine.

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