December 1, 2009

Building Envelope Council launches review of Toronto green roof bylaw

PETER KENTER

correspondent

As with most complex regulations, the devil is in the details. That’s why a technical committee of the Ontario Building Envelope Council (OBEC) is conducting a thorough review of Toronto By-Law 583-2009, the “Green Roof By-Law.”

“While the idea of green roofs may be a sound one, at this point we see two major concerns with the by-law,” says Gerry Genge, president of OBEC and principle of GRG Building Consultants in Newmarket. “The by-law may not be looking at the structural requirements of operating a green roof over time, or at the full life cycle costs of a green roof.”

While design guides specify the amount of weight a green roof has to sustain in order to support plants and a soil medium, the numbers only represent the initial load on the roof.

“The load of the soil will increase over time as the drainable soil becomes contaminated and as the organic content goes up,” he says. “Structural engineers need to design for actual loads and not some arbitrary number.”

While a green roof won’t necessarily cover the entire upper surface of a building, Genge says that even a roof with 50 per cent green coverage will require the entire roof to be structurally capable of supporting the soil medium.

“If I have a building downtown and need to perform roof maintenance, where would I put all that soil,” he asks. “It’s not going to go down the elevator or be pulled off by a crane.

“The soil will need to be piled on top of the open section of roof, or on top of other soil and that will increase the load requirements of the roof structure. This needs to be worked into the roof equation.”

Green roofs are an old idea, even in Toronto, says Genge. In his work as a building envelope consultant, he notes rental building stock of the 1960s and 1970s routinely included rooftop terraces and gardens.

“It wasn’t to the same degree as current green roof designs, but clearly they were intended to be used by the tenants,” he says. “They were never used and in almost every case I’ve seen, were abandoned.

“It was another level of maintenance required for the building owners who eventually couldn’t justify the cost of something that was never used. They simply hadn’t worked the long-term costs of maintenance into the equation.” While Genge notes that some optimistic assessments of green roofing suggest that the structures and their underlying membrane will never require maintenance, such an idea is unreasonable.

“From the moment you lay in that roof, the clock starts ticking toward repair and replacement,” he says. “We need to work in the cost of maintaining the foliage and soil and the eventual cost of replacing the roof 25 to 30 years from now.”

Genge notes that green roofs do have a de facto price inherent in the Toronto Green Roof bylaw. “You can currently buy your way out of the bylaw,” he says.

Building owners can opt to apply for a variance and pay a current price of $200 per square metre in lieu of installing a green roof, with the funds directed to the Eco-Roof Incentive Program for the provision of green roofs on existing buildings.

While OBEC doesn’t normally release opinion papers, Genge says the organization will decide whether to submit a public statement after the technical review of the bylaw has been completed.

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