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A worker helps assemble the 9,500-square-foot Liza’s Garden on the roof of the Philosopher’s Walk wing of the Royal Ontario Museum in Toronto. The garden is inaccessible but can be seen from the museum restaurant C5.

When Toronto's city council passed a bylaw May 27 mandating green roofs on new commercial, industrial, institutional and high-rise residential construction, its proponents hailed it as a major step toward preserving the environment.

The city would be the first in North America to make roofs planted with greenery a must on almost everything except the smallest structures.

The development industry, however, warns that the laws of unanticipated consequences may start kicking in and that a city-ordered focus on green roofs may actually slow the steady march toward energy conservation and environmental preservation.

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"What the bylaw does is limit our options for future buildings," says Stephen Upton, vice-president for development at Tridel Corp., one of the city's major high-rise condominium developers. "Council has decided on this path without investigating any other options or even creating pilot projects to work out details and measure results."

The mandate for green roofs may impede acceptance of other technologies, says Keith Major, senior vice-president for property management at Bentall LP, which manages office towers such as 200 King St. West. As he explains, budgets for new developments may include room for only eight items directed toward energy conservation and environmental protection.

"Now that green roofs are mandated, that will bring the number of line items we can include down to seven," he says. "Decisions have to be made on cost-benefits analysis - getting the biggest bang for the buck. We may not be able to go with another technology which would deliver greater benefits because we must build green roofs."

The new bylaw says that new commercial and institutional structures larger than 2,000 square metres in total floor space and built after Jan. 31, 2010, must devote a portion of their roofs to green plantings with the size of that green space escalating from 20 per cent to 60 per cent depending on the size of the roof. Industrial buildings of the same size need devote only 10 per cent of roof space to greenery and they will get an extra year's grace to start incorporating it into designs.

Residential structures are affected if they are more than 20 metres or about six storeys tall. There is also a clause, however, that allows developers to opt out by making a payment to a new fund that will be used to promote retrofitting existing structures with green roofs.

"I think one of the main concerns about the bylaw is that it is just one more thing the development industry has to deal with, one more cost, one more demand to face at a time when they are already being squeezed by recessionary pressures," says Scott Addison, executive managing director, Toronto, for Colliers International, a major real estate broker.

"On the office and industrial side you already have tenants pressing for lower rents; construction costs are rising; there is fierce competition for development in surrounding municipalities."

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Industrial developers are especially concerned about health problems, he adds. A large chunk of Toronto's industrial buildings are devoted to the food industry, and green roofs would seem a natural habitat for rodents and insects.

"It was an easy win for the city because it is hard to argue against anything that helps preserve the environment, but green roofs have all kinds of implications that don't seem to have been considered and worked out," Mr. Addison says.

One of those implications is the towering heights new residential and commercial towers are reaching in Toronto, says Lyle Scott, director of sustainable design at Cohos Evamy Integrated Design Inc., which has created green roofs on such Toronto projects as Minto Midtown condominiums at Yonge and Eglinton.

"What has been forgotten in the process is the height Toronto buildings are reaching," he says. "Many recent structures top 70 storeys. I can't think of another city where green roofs have been tried on buildings that reach so high in the air."

While champions such as Steven Peck, president of the Toronto-based group Green Roofs for Healthy Cities, cite Chicago as a glowing example of their success, Mr. Scott points out two major differences from Toronto.

First, green roofs in Chicago can be found mainly on lower and mid-rise structures, and second, Chicago achieved its leading status through a program of incentives to developers and not by city mandate.

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Mr. Peck admits he cannot give an accurate picture of the overall benefits green roofs are likely to yield to the city.

"We need an idea of just how much roof area will be involved," he says. "The city does not have those numbers yet. What we do know is that it has been significant in places like Chicago."


Up on the roof



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  • Juniperus communis: common juniper
  • Prunus pumila var. susquehanae: sand cherry
  • Spiraea alba: meadowsweet

Grasses, grass-like plants

  • Agrostis scabra: ticklegrass
  • Danthonia spicata: poverty oat grass
  • Panicum acuminatum: hairy panic grass

Flowering plants

  • Aster oolentangiensis: sky-blue aster
  • Liatris spicata: spike blazing-star
  • Rudbeckia hirta: black-eyed Susan

Source: York University Greenroof project



A green roof is required for all new development in Toronto above 2,000 square metres of gross floor area. Here are the required percentages of greenery:

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20 per cent: Portion needed for 2,000 to 4,999 square metres

30 per cent: For 5,000 to 9,999 square metres

40 per cent: For 10,000 to 14,999 square metres

50 per cent: For 15,000 to 19,999 square metres

60 per cent: For 20,000 square metres or greater

Source: City of Toronto

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If every roof in Toronto larger than 350 square metres had 75 per cent of its area in greenery it would reduce the heat-island effect caused by sunlight reflecting off pavement and roofs by 26 per cent. It would also slash storm-water runoff by 38 per cent, save 22 per cent on the energy required to heat and air condition buildings and ease the burden on combined storm and sanitary sewers by 16 per cent, says a 2005 report commissioned by the City of Toronto and conducted by Ryerson University.

But ask Jane Welsh, acting manager, environment, in the city's planning department, just how much impact the new Toronto bylaw will bring and she admits it is difficult to pin down.

"If this was before the current [construction]slowdown, we could have projected the impact," she says. "But right now since none of us has a crystal ball, we can't say. What we do know is that green roofs offer a great package of benefits."

Green roofs come in three versions, the report explains: A complete system, whose design integrates the underlying roof structure; a modular system, slipped into place on conventional roofs, and pre-seeded blankets that are rolled over drainage mats and root barriers on conventional roofs.

The types of plants they carry depend on access and view. The Manulife Centre at Bloor and Bay Streets has had a green roof over its parking garage for 25 years; its mature trees now reach three storeys high.

The green-roof effect at 410 Richmond St. West, a converted warehouse, was achieved by putting the growing medium and plants in moveable containers.

The one at York University's computer sciences building is inaccessible, so it relies on hardy plants such as alpine varieties that require little care.

Special to The Globe and Mail

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