June 25, 2010

FLYNN CANADA

Workers for Flynn Canada’s green roof division work on an installation above Toronto’s Nathan Phillips Square.

Building Toronto city hall green roof posed host of challenges

SAUL CHERNOS

correspondent

Construction crews building a living roof atop the second-storey podium at Nathan Phillips Square could be forgiven for thinking they were working inside a library. The $2.3 million living roof will stand as one of the city’s quietest projects — at least when politicians and administrative staff didn’t want to be disturbed.

“It wasn’t easy working in a place that’s so public,” says Mary Tremain, a partner with Plant Architect, which designed the space through a joint venture with Shore Tilbe Irwin and Partners (since merged to form Shore Tilbe Perkins+Will).

If it seems like there’s been a greenery up there for at close to a decade, that’s not far off the mark. In fact, municipal green roof proponents had used the space as a test bed for several years.

The real thing — 37,000 square feet of greenery, paved walkways and eco-friendly LED lighting — opened late last month and ranks as the largest living roof in Toronto.

Plant Architect did the planting, lighting, paving and other surface finishes, as well as the design, while Shore Tilbe Irwin oversaw work on the membrane, flashing, insulation and electrical.

Tremain says workers were subject to time restrictions because city hall and the square stayed open.

“There were council sessions going on sometimes, which meant construction stopping and starting and working around scheduled events organized on the square,” Tremain says.

Noise and related restrictions proved challenging in terms of scheduling the work, Tremain says.

And, because City Hall is both a modern building and a heritage site, aesthetic and historic needs raised technical challenges.

Shortly into the project, crews discovered the existing roof was built without a slope, contrary to information designers had been provided.

This led to complications, including drainage.

FLYNN CANADA

An overhead view of the green roof at Toronto City Hall.

“Our construction documents were drawn assuming a slope,” Tremain says, explaining that it turned out the slope was removed during a previous renovation.

“We had to redesign the insulation thickness, and that meant the whole sandwich (membrane, insulation and high-performance bedding) that we ended up putting on the roof brought the level of the paving up higher.”

This became a building code issue, and the team had to heighten an existing guardrail, built around the podium perimeter when regulations were more lax.

That, in turn, had heritage and cost implications, Tremain says. “It was kind of snowball effect.”

Gardens in the Sky, a unit of Flynn Canada that builds living roofs, did much of the installation work. Manager Terry McGlade says the roof’s large size and the fact that adjacent areas remained operative made it tough to co-ordinate the various activities.

“It took 44 days just to get the plants up there. Let alone taking down the old interlocking stone and insulation and bringing all the new materials up there. It was a huge mobilization.”

McGlade says existing buildings are generally more difficult than new sites because they’re often active. “People were working in the building, we had to close off a street to get materials up there, and we could only use the square when people weren’t there.”

A related challenge was public safety. Because the podium wasn’t sealed off, crews had to watch out for curiosity seekers venturing up the outdoor ramp from the square below.

“We had to protect people from our overhead craning and we had to build a scaffold roof to protect people going into the building on the back side,” McGlade says.

Crews often loaded and unloaded at 5 a.m., when the building was easily accessible and popular thoroughfares such as Bay Street were relatively empty.

As is often the case with green roofs, weather proved a powerful factor. While the winter turned out to be unusually mild, the podium seemed under continuous wind attack.

“When the building was built there weren’t that many tall high-rises around, but now there are,” McGlade says. “All the work we did one day would get moved and we’d have to go back and replace it. This happened a few times.”

Crews weighted down a particularly vulnerable drainage board above the roof membrane, but the constant jockeying of weighted materials took time.

“That’s the nature of construction,” McGlade philosophizes. “Nothing ever works as easily as you want it to.”

The project is vying for LEED Gold certification through the Canada Green Building Council. Green attributes include the recycling of old materials, including interlocking stone and standard high-density blue styrofoam insulation.

“The plants are grown locally just outside London, so we should easily achieve LEED Gold,” McGlade says.

With the green roof completed, planners and crews can turn to the project’s second phase.

The square’s revitalization plans include a permanent stage, a two-level restaurant, a landscaped perimeter, an information kiosk and a new skating rink. The overall cost, including the green roof, is projected to be $42.7 million, with completion scheduled for 2012.

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