Canadian Consulting Engineer

Toronto’s green roof bylaw raises concerns

This spring the city of Toronto became the first municipality in North America to require green roofs on all b...

July 27, 2009   Canadian Consulting Engineer

This spring the city of Toronto became the first municipality in North America to require green roofs on all buildings over 2,000 square metres gross floor area. The bylaw was passed 36-2 by the city council in May and is due to come into effect for residential, commercial and institutional buildings by January next year. For industrial buildings the green roof requirement will apply one year later.

Green or vegetative roofs are seen as a sustainable feature for buildings partly because they act as an insulating barrier and are believed to reduce the heat island effect in large cities. They also absorb rainfall and reduce the problems of extreme stormwater run-off.

However, not everyone is pleased by the bylaw and there are concerns about how it will be implemented.

The bylaw requires between 20-60% of the available roof space to be a green roof, depending on the size of the building. A 2,000-4,999-m2 building, for example, will require 20% coverage, while a 20,000-m2 building will require 60% coverage.

The city has developed a Toronto Green Roof Construction Standard with minimum requirements for construction and maintenance and is currently working on supplementary guidelines to “contain best practices and explanatory material to assist designers and others.”

Developers say the green building roof requirements could add $18-$28 a square foot to the cost of building. As it is, the bylaw will allow developers to pay a financial penalty in lieu of adding a green roof, with the funds allocated to a special account. A special permit will have to be issued for specifically for the green roof. 

Among those who believe that green roofs should be voluntary, not mandated, is Marshall Leslie, author and market researcher for the construction industry. For one thing, Leslie says, it’s not clear which member of the design team will assume responsibility for the green roof.

Before the bylaw was passed, Leslie made a deputation to the city’s Planning & Growth Management Committee, pointing out his reservations in several areas.

In his deputation, Leslie pointed out that researchers at the National Research Council of Canada found that the benefits in terms of reducing building energy use occur only in the summer and that “increased insulation would achieve the same level of performance as the installation of a green roof.”

Leslie’s list of objections also include:

Green roofs impose greater loads than more conventional roofing systems and require stronger supporting structures and foundations.

The amount of green roofing maintenance versus that required for conventional roofing is much greater.  “Even after it is established regular weeding, trimming and irrigation is essential.”

More safety access to the roof is required.

Periods of drought can present an additional fire load and arson target.

Some forms of wildlife that inhabit a green roof could be deemed a nuisance.

Other commentators have said that insurance could become an issue with green roofs, and that to avoid possible problems they need to be designed with irrigation, maintenance, firebreaks and restricted access.

Green Roofs for Health Cities – North America, a not-for-profit advocacy organization, is developing professional training and accreditation courses in green roof design and maintenance.

 


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