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urban living

The award-winning green roof designed by Scott Torrance Landscaping for Esri Canada in Toronto. It gives Esri workers a view over 53 types of trees, grasses, sedums, shrubs, herbs and flowers.Margaret Mulligan

"Look, there's a red-tailed hawk flying over," Alex Miller says, interrupting his description of Esri Canada's green roof and its benefits.

"We wanted to have our boardroom outside," laughs Mr. Miller, adding that his original objective was to lease new offices in Toronto with space for an accessible green roof.

"We got the big table. But there's quite a wind up here and so far we haven't figured out a way to keep the paper from blowing around."

The award-winning model roof opened three years ago, giving Esri workers a view over 53 types of trees, grasses, sedums, shrubs, herbs and flowers – all part of the company's commitment to sustainability.

Esri provides geographic information system (GIS) software to corporate and public-sector clients. The City of Toronto used Esri's technology to identify commercial and industrial properties with green potential before North America's first green roof bylaw took effect in Toronto two years ago.

"I'd love to have more building managers and property owners come up here to see the roof," Mr. Miller says. "If people saw it, I think there'd be even more being built. We're on the eighth floor but when you look out the windows you don't feel isolated in a sterile high-rise tower, you feel like you're near nature."

Toronto introduced the bylaw to reduce costs associated with processing storm water runoff, and to gain energy savings from the cooling potential of vegetation cover and improve air quality from the reduction in carbon dioxide and increased production of ozone. There are about 135 existing green roofs in Toronto and a similar number under construction.

"If even just five per cent of the roofs in the city were green, you'd get a one-per-cent drop in city temperature," Mr. Miller says.

Esri Canada has offices across the country, but its first green roof is on a nine-storey office building in the Toronto suburb of Don Mills, overlooking the Don Valley Parkway.

Esri's roof cost $200,000, or about $270 a square foot, to build. "Some law firms spend that much on their lobby," Mr. Miller says.

But the biggest benefits for Esri come from the enriched and calming experience employees enjoy by having access to the living natural world from their work stations.

"There are some cost savings but the main benefit is the aesthetics," Mr. Miller says. "It makes a difference in the way people work together."

Green roofs have existed in Toronto for decades, but two years ago the new bylaw made them mandatory on all new commercial, industrial and public buildings with roofs larger than 2,000 square metres in size, says Jane Welsh, City of Toronto's acting project manager for zoning bylaw and environmental planning.

Sprawling low-rise industrial buildings have lower requirements, and tall slender condo towers, of the type the city is encouraging, do not have to meet green roof requirements.

"That's because above a certain height, it would be technically difficult to implement," says Leona Savoie, co-chair of the Toronto chapter of the Building Industry and Land Development (BILD) Association. "The wind blows all the sedums off."

Sedums are low-growing succulents able to withstand extreme heat and sun because of their special water retention capabilities. Sedums are the plant of choice for the majority of green roofs, especially so-called "extensive" green roofs that have shallow growing mediums and are designed to be maintenance-free. Often these roofs are not visible to, or accessible by, the public.

By contrast, Esri's roof is an "intensive" green roof with a greater depth of growing medium and diverse vegetation that requires maintenance and irrigation from on-roof water storage systems during dry periods.

It is the water contained in the growing medium, not the vegetation, that provides the main cooling effect of a green roof, says Steven Peck, president of Toronto-based Green Roofs for Healthy Cities.

While developers initially were reluctant to take on new responsibilities for living roofs, acceptance is growing.

"I understand there was quite a bit of resistance from the industry at first," Ms. Savoie says. "They wanted to let the market figure it out and then everyone would follow the best practices that emerged. But the city consulted with the industry before the bylaw was introduced and the members I've spoken with say it's working well."

"Most of the buildings that are being built as a result of the bylaw are still under construction," Ms. Savoie says. "So it's too early to say whether it's been successful. We're not going to know for five to 10 years how they hold up and whether they're doing what they are supposed to."

In the meantime, however, there is the sheer joy of watching a red-tailed hawk soar past one's office desk.

Depth and diversity

Despite the romantic idea of lush gardens in the sky, most green roofs are modest monocultures with low-growing sedum plants that are drought and heat resistant.

They offer environmental benefits in the urban heat islands that cover cities and they require minimal maintenance, but there are not many additional benefits.

Scott Torrance, the landscape architect whose many green roof projects include Esri Canada's building, the Victoria Park Subway Station and the Native Child and Family Services building, would like more variety.

"All green roofs are not created equal," Mr. Torrance says. "We have a very diverse city system and that's what makes the city so interesting. We shouldn't use the same ecosystem on every roof."

Diversity requires:

Growing medium to a depth of at least 10 centimetres or more

Even though seven centimetres is the accepted minimum for plant life, the City of Toronto's green roof bylaw calls for a minimum depth of 10 centimetres. Deeper growing medium allows more types of plants to be grown and it significantly increases the cooling effect. But it adds disproportionately more weight that must be supported by the building's structure, which adds to the cost. "When you get to six inches [15 centimetres] you can increase the plant diversity and at 12 inches [30 centimetres] you can plant small shrubs," Mr. Torrance said.

A three-week source of water supply with irrigation

A source of water is necessary to help establish and sustain plants during extended dry periods.

An ecosystem that helps control the growth of weeds

By providing a variety of plant types that will vie with each other to adapt to each roof's microclimates.

Biodiversity to create a sustainable garden

A rare bee has been discovered on the York University computer science building's large green roof. Mr. Torrance is pleased that spiders have established themselves in some of his gardens.

Good design to overcome challenges

On the Esri roof, Mr. Torrance used growing medium in different depths. Over the main support pillars of the building, a greater depth allowed trees such as Scotch pines to be planted.