Roofs with gardens and greenery on top could become a lot more common in Vancouver if a motion is successfully passed at city hall Tuesday — but developers are concerned that they weren't consulted first.

Coun. Adriane Carr wants to make having a green roof mandatory for new large buildings, including commercial, industrial, institutional and multi-family residential developments. Single-family homes wouldn't be impacted.

Carr said there are a host of reasons behind the motion, the most immediate being coping with excess rainfall in the city.

"People probably around the city have seen the bubbling up of the water from our storm sewer, drains that come out and flood their streets. We have an excess of water and we need to deal with it and green roofs are a great way to do it," she said.

A study from BCIT's centre for architectural ecology found that a green roof reduced rainfall runoff by 28 per cent when averaged between the wet and dry seasons.

The convention centre's green roof helps prevent runoff from entering the city sewers. (Denis Dossman/CBC)

Carr said Toronto introduced a similar bylaw and found green roofs reduced the amount of rainwater hitting the streets by about 25 per cent because it gets absorbed by the roofs.

That could have cost-saving measures for the City of Vancouver, according to Carr.

"It would reduce our costs a lot, because we're having to replace pipes, and it increases the capacity because primarily we are getting more rain and we are expected, under climate-change predictions, to get even more rain in the future," she said.

Concern from developers

But the Urban Development Institute, which represents 870 members in the development and planning industry, voiced concerns about the proposal.

"As a key city stakeholder, we are extremely disappointed UDI was not consulted in the development of the Mandatory Green Roofs Motion, and that there does not appear to be any plans to engage the industry," the institute said in a press release.

Of particular concern were added construction costs and the potential for conflict with urban design policies for peaked roofs or green objectives like solar panels.

The Vancouver Public Library's $15.5-million revitalization project includes plans for a rooftop garden. (VPL)

Carr said she expected there would be some pushback from developers, but said in the long term, green roofs save on heating and cooling costs and therefore contribute to overall savings.

She said many developers in Vancouver are already choosing to incorporate green spaces on rooftops into their designs, for play space for children, garden space and patio areas.

Green roofs are part of the plan for several major projects in Vancouver, including the redevelopment of the Plaza of Nations and the Oakridge Centre.

The Vancouver Park Board tweeted out this concept image of what the green space at the redeveloped Oakridge Centre could look like. (@ParkBoard/Twitter)

If the motion is passed successfully, staff will be instructed to draft a green roofs policy for council to vote on at a later date.

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