Published by City Farmer, Canada's Office of Urban Agriculture


Green Roofs, An Icon For Our Urban Future

Alison Empey

The past few years have seen North Americans begin to realize and take advantage of the widespread benefits green roof infrastructure has to offer, benefits the Swiss, Dutch, and Germans have been enjoying since the mid-1980s. Green roofs, spaces of contained greenery located on top of a human-made structure, offer numerous public and private economic, social, and environmental benefits. Among these benefits is the ability to grow food locally through urban agricultural opportunities and the potential to control excessive stormwater runoff - often a serious issue for cities situated on a lake or river.

There are two basic types of green roofs: extensive and intensive. Extensive green roofs range from one to five inches in soil depth, normally consist of mosses and herbs, and are built primarily for their environmental and economic benefits rather than public access. Intensive green roofs require at least a foot of soil depth, an elaborate irrigation and drainage system, and need to be maintained. Typically designed to be publicly accessible, intensive green roof applications feature trees and shrubs and often resemble city parks.

With the ever-increasing population levels in cities today coupled with the concentration of resource use and consumption, environmentalists and urban planners have been required to turn their attention to sustainability and the need to reduce cities' ecological footprint. Green roofs have great potential to help urban areas achieve this through reducing energy consumption and the urban heat island effect, lowering the instance of water pollution through stormwater mitigation, and biodiversity conservation.

In addition to these and the many other benefits green roofs can provide, they present a unique opportunity to help reduce a city's ecological footprint not often considered - food production on top of buildings. Often, there exists the notion that there is a lack of space within urban areas necessary for mass food production. Not so. Rooftops typically comprise at least 30 per cent of a city's total land area - that's a lot of room for growth!

By growing food on green roofs, there are also extended environmental benefits to consider. For example, in Toronto, Ontario, 50 to 60 per cent of consumed food is imported. Growing food locally not only increases food security, creates new employment, training, and creates new green space, it reduces food transportation distance. As a result, the amount of carbon dioxide and other pollutants released into the air is reduced. Recent studies by the National Research Council of Canada have shown that if just six per cent of Toronto's rooftops were greened (equivalent to just one per cent of Toronto's land area), the city would reduce its green house gas emissions by 2.18 mega tonnes per year. Add food production to those rooftops, thereby reducing the amount of transportation in and out of the city, and harmful emissions would be even further reduced, not to mention the astounding $5.5 million worth of locally produced cuisine!

Food Share, a Toronto-based community and food security organization has been involved with rooftop urban agriculture research and experimentation for years. It's through this work that they believe urban rooftops can provide a valuable resource for food production. But for the rooftop agriculture market to develop, there are technical limitations, which must be overcome. Most crops require a high level of nutrients for sustained growth, requiring deep soil levels - something most standard rooftops are not initially equipped to sustain. [1] See Fall 2002 pdf Luke Garnham, Growing Food on the Roof: the Potential of Green Roofs in Urban Agriculture, 2002. To overcome this barrier, lighter, nutrient-rich growing media is being developed and rooftops can be retrofitted to carry the weight. Roofs able to hold the weight of 12 to 18 inches of soil can successfully grow large quantities of produce such as hot and green peppers, cucumbers, sweet potato, tomatoes, and eggplant. With ongoing research and development of lightweight growing media, rooftop greenhouses, and hydroponic techniques, the potential to grow crops on a wider range of rooftops will continue to increase.

Farm To Table ( is a successful program launched in 1995 by Earth Pledge (, a New York-based organization focused on promoting innovative techniques and technologies to help restore the balance between human and natural systems. The program works to raise awareness of the environmental, social, and health benefits of sustainable agriculture and promotes innovative urban agricultural technologies such as the green roof system. Earth Pledge has also launched a program aimed at greening the rooftops of New York City. The green roof initiative promotes and facilitates green roof development as an ecologically sound and economically viable solution to New York's environmental problems such as stormwater runoff pollution.

Green roofs can play a strong leadership role in water pollution prevention. If your particular city is located on a surface body of water, take notice that following a heavy rainfall, there is likely to be a surge in the pollution of that body. In the summer this means the beaches are off limits due to bacterial infestation, or you'll hear of a particular species of wildlife suffering from the increased contaminants in its natural habitat. You may wonder - why does this happen? When rainfall occurs in an urban area, runoff from buildings and streets is typically held in storage basins until it is processed through the city's water treatment system. When heavy rainfall occurs, the excess runoff often overwhelms the city's infrastructure causing it to overflow. The water travels throughout the city's network of gullies and streets picking up toxins and pollutants along the way. Since free flowing water will drain to join the nearest body of surface water, much of this polluted storm water runs straight into the nearest lake or river.

Rooftops containing growing medium and plants have the potential to absorb 50 to 90 per cent of the rainfall it encounters depending on the season. Water is stored in the growing medium and then taken up by the plants from where it is returned to the atmosphere through transpiration and evaporation. Green roofs not only retain the rainwater, but also moderate its temperature and act as natural filters for any of the water that happens to run off. In short, green roofs have enormous potential to prevent subsequent release of untreated water to surface bodies.

Taking their potential for food production and stormwater management into consideration, you may be wondering why green roofs are not everywhere. One of the hurdles for widespread green roof implementation is cost. With installation of a green roof costing approximately twice as much as a standard roof, it is often difficult for builders and developers to make the initial investment. In European countries such as Sweden, Austria, and Germany, governments have developed and put into place policies and incentives for those willing to make the investment in green roof infrastructure. Some of the ways governments are meeting the challenge of offsetting these initial costs are tax incentives and density bonusing, each offering the building owner or developer a way to reap immediate financial benefits. If North American cities want to take advantage of green roof technology and the many benefits it provides, various levels of government need to work with individual markets to determine which policies and incentives make the most sense.

For example, most North American cities make use of a combined stormwater sewage system with flat fees for sewage waste. In a situation such as this, there is no incentive for building owners to take action toward reducing runoff volume. Establishing a split stormwater fee, as many German cities have, creates a situation where building owners and developers are required to pay for excessive runoff. Installing a green roof will then become much more economically attractive offering a quicker payback by eliminating these immediate fees. Cities concerned about their runoff volumes such as Toronto and Portland, Oregon have recognized the potential green roofs offer in the way of water retention, thus mitigating water pollution, and are taking steps to develop such municipal policies. Most recently, Toronto has approved the formation of a green roof task force, which will investigate issues facing the city, such as stormwater management, and will develop the most appropriate policies for implementation.

Although steadily growing, the North American green roof market requires more than promising facts and enthusiasm to reach its full potential and break into the mainstream. The European market is a model to follow and has been greatly enhanced by state grants and municipal planning policies providing incentives for implementing green roofs. Getting North American governments on board, and having them support the green roof movement is essential - our food and water depend on it.

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Revised Friday, October 10, 2003

Published by City Farmer
Canada's Office of Urban Agriculture