Spotlight on: Investing in a Green Roof

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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