More Green Roofs Promised

In the recent municipal election, Jim Watson borrowed a plank from Miller’s Toronto. Watson called for more “green” roofs, and city staff to be knowledgeable about green roofs. Toronto did the same thing a few years ago. There the original impetus was that private sector owners would be obliged to install green roofs. This was later expanded to include some government sector buildings.

As a policy, it is a delight for the Mayor. He looks green and “with it”. The regulation cost is minimal. The real cost is borne by others. OBrien criticized Watson, and I was surprised when The Citizen criticized him for pointing out what should be obvious: it’s a feel good promise with high external costs.

Ottawa already has a number of green roofs, some west side examples can be found by entering “green roof” in the search button on this blog.

To me, green roofs are most useful at the ground or podium level where they can be seen and enjoyed. Atop a 24 storey office tower … not so visible. Of course, there they might still have some benefits in heat and rain absorption. But the area of a roof in built up urban areas is often small in relation to the wall area.

Rather than prescribe green roofs, I would rather see more effort put into green walls. We should identify what we want to achieve and then let the market find the means, rather than adopting the solution first and demanding it be implemented.

Toronto re-did the roof of the terrace around City Hall to be a green roof demonstration. Rows of pavers were removed and replaced with sedum in trays. The weight of the sedum trays is not an issue here as they simply replaced the concrete pavers.

The sedums and grasses are planted in patterns to make an interesting artwork on the roof. The grasses make eye-catching movements.

The green plantings made the roof terrace, about two stories above Nathan Phillips plaza, pleasant to visit. I image the previous all-concrete pavers version was incredibly hot or cold depending on the season.

Green roofs are not planted right up to the edge of the roof. About 12′ is left for workers and equipment along the perimeter of the building as access is required to the exterior skin of the structure.

In a rather dramatic contrast to the shallow roof plantings retrofitted onto City Hall, the private commercial complex directly across Queen Street, built in the same era as City Hall, had a green roof about 40 years before Mayor Miller:

There appear to be several acres of landscaped roof on the Sheraton Centre, most of it planted with trees that are thriving. The trees are over 30′ high and provide a lush view as well as usable gardens.

I also noticed in Toronto that new building roof tops all had enclosed mechanical penthouses to mask the rooftop equipment. In Ottawa, giant air conditioners and similar equipment are just stuck on the roof for all to admire.

2 thoughts on “More Green Roofs Promised

  1. More green walls would be great, but I don’t see where the extra cost for green roof construction comes from. If the green roof is lighter than the alternative, should they not be cheaper to build?

  2. A great question. The toronto city hall roof is not heavier than the old roof (I guess…) but it is probably more expensive to plant and maintain (a sprinkler system is visible in the photos) than concrete or gravel. If the green roof was cheaper than the conventional roof, more developer/owners would do it. They are not blind to making a buck. That they build conventional roofs most of the time answers the question.

    Note that on a shallow roof with plants in baskets, the plants can be trimmed back and lifted out while repairs are made. For deep plantings, such as around the Claridge roof plantings between the towers on Lebreton Flats, it will be very very expensive to repair the roof or redo it in 30 years time … note that the developer is doing the deep green roof option in circumstances where he does not have to maintain it. For buildings he has a long term interest in …

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