Rooftop Gardening

Many urban environmental problems can be linked to the displacement of vegetation by roads and buildings. Without sufficient space set aside for vegetation, cities are more vulnerable to air and water pollution. However, a solution to solve this environmental dilemma does exist: using the plentiful supply of unused urban space on the rooftops of most city buildings. A workshop on June 25, 2001 in Ottawa will be devoted to discussing the research on this emerging technology.

Green roofs, or rooftop gardens, are now a thriving industry in several European countries and are receiving increasing attention in North America and Australia. Toronto City Hall and the public library in Vancouver are two places in Canada where rooftop gardens have been established. These "gardens" include plants, a light-weight growing medium as an alternative to soil, a drainage layer to collect water and channel it off the roof and a membrane to prevent leaks and root penetration through the roof.

The growing popularity of this technology is not purely aesthetic, although green roofs can be quite attractive. It lies in the need to find low-cost solutions to urban environmental problems, such as stormwater runoff with its impacts on water quality, air pollution and the urban heat island.

Plants intercept rainfall and allow it to be evaporated, infiltrate the soil or delay its conversion to runoff until after the peak flow has passed. In addition to its flooding potential, stormwater runoff can also cause pollutants from roads to be absorbed in the groundwater or it can flush pollutants out of the sewage system before they can be removed at treatment plants.

Vegetation can improve air quality by filtering out gaseous pollutants and particles. Gardens also help to reduce the urban heat island, a phenomenon that causes cities to be up to 8° C warmer than the surrounding countryside. This results from replacing vegetation with surfaces such as pavement and dark-roofed buildings that absorb solar energy and reradiate it as heat. Higher urban temperatures generate greater demand for energy in the summer and increase the rate of smog formation, a urban health hazard.

A major study on the benefits of rooftop gardens was completed in 1999 for the Canada Mortgage and Housing Corporation by Environment Canada and several private sector partners. The report, entitled Greenbacks for Green Roofs: Forging a new industry in Canada, includes several case studies on successful rooftop gardens in Canada.

One recommendation led to the creation of a demonstration project, currently underway in Ottawa, which compares two similar buildings, one with and one without a rooftop garden. This five-year monitoring program, at the National Research Council facilities, is entering its second year. The preliminary results will be part of discussions at the workshop being held at the Council's Ottawa Campus on June 25.

This workshop is part of the International Conference on Building Envelopes and Systems and Technologies. It will provide an opportunity to learn about this technology, its benefits and to see the first field site constructed for green roof research. Details on the Green Roof Infrastructure Workshop are available at http://www.peck.ca/grhcc/june25workshop.pdf.

For those interested in getting started, a flat or moderately sloped roof is best suited to construct a green roof. The standard technology is available in Canada through some of the major roofing companies. These companies or the Coalition for Green Roofs for Healthy Cities (see related sites) can provide a referral to an engineer, architect or landscape architect who can determine if a roof is suitable and if the cost is affordable.

Greening our cities takes on a whole new meaning when plants, grasses and trees are added to building rooftops to help cool down the city and provide cleaner air and water. But even greater benefits will be achieved when a municipal greening rooftop strategy is applied throughout our cities.