Spotlight on: Investing in a Green Roof
March 19, 2015 — A green roof isn't a roof garden — access is usually limited to weed-pulling and other maintenance. Many buildings with green roofs keep their roof doors locked and let staff or volunteers go up once or twice a week to pick weeds (and maybe, as a little perk, bring a book and relax for a while).
A green roof — that is, a layer of vegetation that covers your entire roof — can help keep your building cool, reduce rainwater flooding and pollution, and clean the air. It's a costly but effective way to make your building more environmentally friendly. It's not for everyone, but if your board is considering a green roof, here's what you need to know.
Since the cost of the greenery is shared by the corporation, it's important that maintenance be done by people who know what they're doing — professional gardeners or experienced volunteers.
Plantings can range from small plants such as sedums to shrubs or even trees, depending on the type of roof. Most green roofs also include additional components separating the roof membrane and the growing medium, such as a drainage layer to allow excess water to drain from the roof and a root barrier to prevent any aggressive plants from damaging the membrane.
Like a giant sponge, a green roof will absorb rain rather than allow it to flow unimpeded into the sewer system, contributing to flash floods and carrying pollutants into local waterways. The greenery absorbs some of the water, and the rest evaporates. The natural cooling caused by evaporation from a green roof can be comparable to that of a reflective "cool roof," and it will also help rid the air of pollutants.
If a board does opt for a green roof, it's best to call on experienced designers and contractors. The most critical concern is the load-carrying capacity of the roof, because when it's saturated with water, the growth medium can weigh as much as concrete. Inspect your roof's structural condition before going ahead and examine the roof membrane for leaks. It may also prove worthwhile to go the extra mile and install a detection system that will enable any membrane leak to be pinpointed within millimeters.
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