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


Evaluating the Potential of Green Roof Agriculture:

Robin Kortright
for Professor Tom Hutchinson
Trent University
October 2001

"I am a recent graduate of Trent University's Environmental Studies program. Last summer, under the supervision of Professor Tom Hutchinson and funded by an undergraduate NSERC grant, I established an experimental vegetable garden on the green roof of the Trent University Environmental Sciences building. I was looking at the suitability of green roofs for urban agriculture, comparing different crops and growing conditions."

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Short Summary of Project

Evaluating the potential of green roof agriculture NSERC undergraduate research summary

As the world's population becomes increasingly urbanized, issues of urban sustainability are becoming more important. Cities, particularly in North America, are at present quite unsustainable, using far more land and resources than they physically encompass. One proposed alternative to the current system is the nurturing of a more localized urban agriculture. Unfortunately, land which has traditionally been used for agriculture within our urban areas is now in high demand and vulnerable to development. As a result, rooftop agriculture, combined with green roof systems, has become an attractive possibility. However, the efficiency of green rooftop agriculture has not been extensively tested.

One of the most common barriers to the adoption of sustainable alternatives is lack of confidence and easily accessible straightforward and tested experience to depend on. Previous work on rooftop agriculture, and particularly the use of green roofs for agriculture, has been largely theoretical. Therefore, a demonstration project was conducted to evaluate perceived barriers to rooftop vegetable production in the green roof context. The project goal was to evaluate the feasibility of green roof technology for urban agriculture for Peterborough and elsewhere, testing the hypothesis that green rooftop agriculture is viable as an urban agricultural alternative. Soil temperature, soil moisture, crop health, quality, and productivity were monitored. The results for different soil treatments, of mulch, shadecloth, and bare soil, were compared. In this way the suitability of various crops, varieties of crops, and soil treatments to rooftop conditions were evaluated. Background research was conducted to provide context for the project, and the benefits and barriers to green roofs and urban food production were detailed. Available literature on the special conditions of rooftop agriculture was also examined. All of these were useful in framing the demonstration project results.

The demonstration project was conducted on the green roof located on the Trent University Environmental Sciences building, Peterborough, Ontario. 14 common vegetable crops were planted in a 7.5 x 30 m plot, subdivided into four equal beds. 2 different varieties of each crop were planted, chosen for their possible suitability to rooftop conditions. Each of these was planted in two beds, one covered with mulch or shadecloth, depending on the crop, the other uncovered. The productivity of individual crops and beds was monitored separately. In addition, wind speed, soil temperature and moisture were monitored on the rooftop and at ground level. The results were then compared to each other, to the ground level results, and also to official Ontario Ministry of Agriculture, Food, and Rural Affairs crop productivity statistics.

From my results it was not possible to conclude that green rooftop food production can serve as a viable agricultural alternative in any broad commercial sense. However, it can be concluded that, on a green roof such as the one considered here, rooftop growing conditions are not substantially different from those on the ground. Therefore, it is possible to conclude that such a development is possible on a small scale, given experience and a broader incorporation of green roofs such as this one into the urban landscape.

Evaluating the Potential of Green Roof Agriculture:

Note: The "Review of Literature and Appendices" report follows below in HTML format.

The "Demonstration Project" report [6,700 words] can be downloaded here as a MS Word file [240K] (Three charts are not included.)

Download here A Demonstration Project - project write-up

A review of the literature


Recently, a major milestone in human history was reached: urban dwellers became the majority of the world's population (Baird 1999). The steady expansion of cities has had innumerable consequences for the quality of human daily life. Arguably one of the most central is the distancing that city living allows from the environment which ultimately sustains it. This is visible not only in the lack of urban green space, and its diverse environmental consequences, such as higher temperatures and the loss of biological diversity. Urban society's environmental isolation is also visible in the distancing of the urban relationship to food. Food is what Winson (1993) calls 'the intimate commodity.' It sustains people and communities unlike any simple commodity, and has many complex spiritual, cultural and social meanings worldwide. The distancing of consumer from producer, as a result, has consequences not only environmentally but also socially and economically. Attempts have been made to address this through numerous urban greening and urban agriculture initiatives. Some of these have set their sights high, upon acres of flat urban rooftop. While encroaching economic demands make preserving green space in most city centres difficult, open space is plentifully available a few stories higher. As a result, the idea of the 'green roof' is gaining proponents. This paper will explore the current state of knowledge on green roofs and, specifically, the possibilities for their use in urban food production.

What is a green roof?

Any planted open space that is separated from the earth by a building or other structure can be considered a roof garden. These are most obvious well above ground level, but are also commonly found at or just above ground level, on top of structures such as underground parking garages (Osmundson 1999). 'Green roof,' as the term is used in this paper, refers to a specific type of roof garden. Like any other roof garden, a green roof is made up of contained green space on top of a human-made structure, but in this case the container is essentially a few more layers to the roofing system. Far more common in Europe than in North America, green roof technology, as described by Peck et al. (1999), typically includes a number of layers, as illustrated below in Figure 1: (Image not include here.)

Figure 1: A typical green roof system, with the individual layers illustrated and described (ZinCo GmbH 2001).

There are various kinds of green roofs, the primary division being between extensive and intensive roofs. Extensive green roofs are low in weight, capital, and maintenance costs in comparison to intensive roofs, but the range of plant species which they can support is restricted and they are often less accessible than intensive roofs (Peck et al 1999). Often, they are planted with low-growing drought resistant species, such as sedums. More demanding plants, such as agricultural crops, require more accessible intensive roofs with deeper and richer soils. These green roofs are more costly but allow for a far wider range of uses than an extensive roof can support.


Green roofs:

Green roofs themselves have a number of benefits apart from their potential use for food production. The best publicized of these is their usefulness in retaining stormwater, which can translate into a significant cost savings in management infrastructure due to stormwater quantity reductions as well as quality improvements. (Peck et al 1999). The ability of planted roofs to detain and slow runoff is especially useful in industrial areas covered in impervious surfaces, where green roofs may be one of the few means to reduce runoff peaks on-site. Large flat roofs, which can cover from 10,000 - 100,000 ft2 , are perfect for green roof systems (Pedersen 1999: 86). If greened, these roofs could retain significant amounts of stormwater, as well as providing excellent habitat for birds, butterflies and bees.

Another important environmental benefit of green roof systems is their potential to moderate the urban heat island effect. Two important contributors to the artificially high temperature of our cities are the storage and reradiation of solar energy as heat by surfaces such as asphalt and concrete and the rapid runoff of precipitation, which reduces the cooling effect of evaporation (Draper 1998: 422). Both of these effects could be mitigated by increasing urban green space. It is uncertain how significant this mitigation could potentially be, but a number of municipalities are gambling on green roofs regardless. Tokyo, where temperatures have been rising steeply over recent decades, recently passed a law stating that all new buildings with roofs over 1000 m2 must be greened, in an effort to moderate the city's climate (Geographical Magazine 2001).

The climactic benefits of green roof systems are not limited to temperature moderation. Urban plantings have also been shown to improve urban air quality, by trapping and absorbing nitrous oxides, volatile organic compounds, and airborne particulate matter. (Peck et al 1999). The insulation the green roof provides results in increased energy efficiency in heating and cooling systems which not only reduces costs, but can also improve the climate on a larger level, by reducing energy use and therefore frequently greenhouse gas emissions (Peck et al 1999).

Green roof systems are not only environmentally beneficial. They are also economically sound, lasting much longer than conventional roofs, since the green layer protects the roof membrane from temperature fluctuations, puncture, and UV damage. Peck et al. (1999) report the case of a London department store whose roof membrane, after 50 years under a green roof planting, was still in excellent condition, far surpassing the 10 - 15 year life span of its contemporaries.

This protecting layer also insulates against sound, slows winds, and is aesthetically and philosophically appealing. Often, rooftop greening has been incorporated into architecture for reasons far from such practicalities as urban food sustainability or climactic moderation. Rooftop gardens have been mainstays of a number of modern architects. These include two acknowledged masters of modern architecture, Le Corbusier and Frank Lloyd Wright. Both have incorporated green roofs into their work, though they tended to view them more as outdoor rooms than traditional gardens (Osmundson 1999). The toit-jardin, or roof-garden, was one of the five points of Le Corbusier's new architecture, which he advocated in order to restore the green space lost under the building and to create a private arcadia for residents away from the clamour of the street (Curtis 1986). Aesthetic appeal may seem less critical than some of the other benefits listed here, but its benefits are also to some degree quantifiable. Access to, or simply views of areas of green roof can improve property values and increase worker productivity and creativity, as well as being useful for health and horticultural therapy (Osmundson 1999; Peck et al 1999). These are important considerations, especially for hotels, hospitals, and prisons (Osmundson 1999; Gavrilov 2001).

Food production:

The sustainability of urban systems can be significantly bolstered by fostering a more urban agriculture. The average American meal travels 1500 miles from field to table (Norberg-Hodge et al 2000), using 10 times more energy than the caloric value of the food itself (TFPC 1999). This represents an incredible environmental cost in fossil fuel emissions, pollution associated with extraction, and loss and division of natural habitat by asphalt, to name a few of the more direct costs. Less direct are the costs of the industrial agricultural system required to maintain the artificially low cost of that well-travelled meal. These include environmental costs associated with high-input monocultural growing methods as well as social and health costs for the local rural community and farm workers. Unless alternatives are sought to this global sourcing of food without regard for externalized costs, rising urbanization will continue to be accompanied by increased environmental and social costs.

Modern cities, in ecological terms, have become parasitic energy and resource 'sinks,' consuming 75 % of the world's resources on only 2 % of the global land area (TFPC 1999). Natural systems are fairly local, retaining a dynamic balance of nutrients throughout the ecosystem. Instead of building on that model, urban areas today deplete areas worldwide of nutrients and dispose of them in extremely concentrated amounts, far from the original source (Nelson 1996). In this way cities 'short-circuit' the natural ecological cycle, harming both the nutrient source and sink.

Rooftop agriculture is one way in which urban areas could attempt to be more balanced and sustainable in their resource consumption. It is possible to produce a variety of fruit, grain, and vegetable crops on rooftops, either in containers or as field crops (TFPC 1999). However at this point there has been little practical experience in Canada with commercial rooftop food production, though small container gardens are relatively common. There is a significant and growing interest in rooftop agriculture. This is particularly true since it is the one form of urban agriculture most likely to be resistant to negative political pressures, as the space it occupies is not currently under demand. Having rooftop agriculture as an option can even resolve existing conflicts, as in the case of the On Lok Senior Center in San Francisco, where the resident's community garden was simply moved up eight stories to the roof when a high-rise displaced it (Ableman 2000).

Apart from the convenience of exploiting an underused resource, urban agriculture in general, from small backyard plots to commercial production, has a number of further benefits. From an ecological perspective, urban agriculture of any stripe is an excellent way of capturing nutrients in urban compostable wastes. Food wastes make up 15 - 20% of Toronto's solid wastes, but currently these are a pollution problem, rather than a resource (TFPC 1999). These nutrients, properly composted, could potentially be used to provide for a sizable proportion of urban food requirements. In many Chinese cities, intensive urban agricultural production is enabled in part through continual recycling of urban organic wastes, though the practice is declining due to the increased availability of chemical fertilizers (Pepall 1997). Many cities worldwide continue to source a large percentage of their food from within city boundaries, though as the global food system grows, that percentage is declining. Urban agriculture is strongest in developing countries, but even in the Netherlands, 33% of total agricultural production is within urban lands (Ableman 2000). This potential is, however, decidedly underutilised in Canada. At the moment, only a few days supply of many essential foods are available in Toronto at any one time. This is wasteful on many levels, but it is also a concern in terms of planning for potential emergencies, due to disruptions of long supply lines by factors such as crop failure and transport strikes (TFPC 1999).

Also from an environmental perspective, it is important to point out that food grown in urban areas is more likely to be organic than that produced by the conventional industrial food system. This is due to a number of factors, including the fairly small scale of urban agriculture, the large labour pool, and cost reductions due to lowered transport costs. In addition, urban farms, since they are worked more intensely, can produce up to 15 times more per acre than their rural counterparts (Ableman 2000). Economically and socially, localized farming returns income to the community as well as providing it with healthier, fresher food. And finally, interaction with growing things soothes and heals the mind and body while providing a reminder to urban dwellers divorced from nature of the ultimate source of human support, and of the ecological limits of that support (Draper 1998; Nelson 1996).


Nothing is perfect, and green roofs and rooftop agriculture are no exception. Green roof technology has yet to be adopted widely, and if not all the reasons are good, they are at least comprehensible. The most significant barrier, naturally, is the up-front capital cost of green roof systems. Though the adoption of green roof systems can lead to significant savings, these are not usually visible to the average building owner (Peck et al 1999). Savings by the health system as a result of lowered airborne particulate concentrations, or by the municipal water authority due to stormwater containment are easily overridden by the immediate felt costs to the individual. The increased cost is not overly high, but it is sufficient to create a barrier to green roof construction. It is estimated that the green roof section of the Trent University Environmental and Resource Science building in Peterborough, Ontario cost $35.20/m2 more than the rest of the roof, which was built in a more traditional style (Johnstone 2001; see appendix for detailed breakdown of costs).

The need for maintenance is another economic barrier to the installation of green roof systems. Like any other garden, a green roof needs upkeep to maintain many of its benefits, especially if it is to be amenity for people, and that costs money and labour (Osmundson 1999). This is a problem in conventional community gardens as well, but the difficulty is compounded if access to the garden is awkward. Ease of watering and equipment transport are other key barriers for any roof garden, particularly those with more intensive plantings. This is one reason why low maintenance sedum-planted roofs may be more successful, even though the potential benefits from intensively planted roofs are greater. Even a low maintenance green roof, such as Trent University's, which is planted with wildflowers and grasses, requires some cropping to maintain visibility from adjacent windows, increasing its maintenance cost over the non-green roof by approximately $1/m2 (Johnstone 2001).

Both of these barriers are the more real because of another compounding factor, the lack of regulatory incentives. In North America there are virtually no government incentives which support the diffusion of green roof technology (Peck et al 1999). The German green roof industry benefits from both direct financial investment and changes in regulation to encourage green roofs by over seventy local governments. This has enabled the industry to grow by 10 - 15 % annually over the past decade. As of 1999, 50 million m2 of German roofs, or more than 10 % of all the country's flat-roofed buildings, were planted with green roof systems (Green Roofs for Healthy Cities 2000).

The lack of regulatory incentives in North America is in part due to a simple lack of knowledge and awareness. Although there are many benefits to these technologies, these are not well known to most members of the conventional construction sector, municipal officials or the general public (Peck et al 1999). The technology is widely used and well proven abroad, but here in North America there are still many technical issues and risks associated with uncertainty, which officials, contractors, architects and building owners are all reluctant to address. These types of barriers cover a wide spectrum. They include the lack of specialized products on the market, few built examples of green roof installations, no industry technical standards for green roofs and therefore also no building code standards (Peck et al 1999).

From the perspective of the plants, there are also a few drawbacks which must be considered. Rooftops in general are fairly exposed locations in terms of both sun and wind. Plants can either be chosen to withstand high soil temperatures, full sun, and strong winds, or these effects can be moderated with the help of shadecloths, mulches, more frequent watering, soil additives, interplanting and/or windbreaks (Kuhn 1995). Another concern, about which little is known, is the possibility that food crops grown in the city may be contaminated by airborne pollutants. However, David McLaughlin, the Phytotoxicology Investigator Coordinator at the Ontario Ministry of Environment and Energy, Standards Development Branch, is cited by St. Lawrence (1996) as saying that air pollution is unlikely to have any influence on the safety of urban crops. This is due to the fact that most toxins which are collected by plants come from the soil rather than the air. Air borne contamination should only be a serious concern if there is a industry point-source nearby. Soil, on the other hand, can be a concern depending on where it was originally obtained, and should be tested for heavy metals, particularly lead, before any cultivation is planned (St. Lawrence 1996).

Special conditions of rooftop agriculture:

In order to set this project in its proper context, it is important not only to review the benefits and drawbacks of green roof agriculture, but also to delineate what special conditions differentiate rooftop growing conditions from those of other urban community gardens. First among these when the adaptation of an existing structure is being considered is the allowable weight and therefore the depth of the soil. If the roof garden is built into the original building plan, structural allowance can be made for several feet of soil, accommodating not only vegetable crops but also trees and shrubs. However, if this is not possible weight is a serious concern. When saturated, one cubic foot of soil weighs approximately 100 lb (Fairholm 1999). Most Canadian buildings are built to support only the roof structure, known as the dead load, as well as a minimum live load to accommodate snow and occasional maintenance (St. Lawrence 1996). In Toronto, the minimum live load for flat roofs is 30 pounds per square foot (p.s.f.), with extra loading allowances where the roof abuts a wall, to allow for snowdrifts. Unless either the snow or the soil is removed in winter, the live load-bearing capacity of the roof may potentially be exceeded, even if only 3 inches of compost are used to grow crops such as lettuce greens. St. Lawrence (1996) suggests that a tarp might be erected over the garden as an alternative to removing the soil. However, high winds might make this a difficult. FoodShare, a Toronto-based non-profit organization, grow vegetables on their rooftop each summer for their Good Food Box program. Since the building is not structurally adapted to this use, they must move all their planters off the roof each winter. The amount of work this entails begs the question whether the benefits continue to outweigh the drawbacks of rooftop agriculture, at least in this particular case (Baker 2001).

Using straight compost is most likely the best way to maximize the depth/weight ratio, since it is lightweight and high in nutrients (IDRC 1998). It can even be mixed 1:1 with another lightweight material to give added depth and aerate the soil bed. The International Development Research Centre (1998) suggests using crushed pop cans with holes punched in them. In implementing this suggestion it would most likely be wise to be cautious, since leachates from the aluminum may potentially contaminate the soil. It also recommends an 8 - 20 cm depth for vegetable planters, which should be able to support a somewhat larger range of crops than the 3 inches cited above. According to Osmundson (1999), most plants do not necessarily have to send down deep roots. Even in natural conditions, trees rarely have more than 76.2 cm (30 inches) to grow in. In some parts of Norway, the soil can be as shallow as 30.5 cm (1 foot) deep and still support dense forests of birch trees (Osmundson 1999).

Temperature is another important factor in successful rooftop agriculture. Most large rooftops receive full sun all day long, with little of the natural shade relief from trees or other structures which would normally be present at ground level. As a result, the heat of the soil is a serious concern. This is particularly true if the soil used is shallow. The greater the soil depth, the better the plants are insulated from the high surface temperatures by the cooler soil below. If the soil temperatures rise too high, plant growth is limited through reductions in water and nutrient uptake (McLelland 2000).

In addition, earthworms, which enrich and aerate the soil, require fairly cool and moist conditions to survive. At ground level, they can simply go deeper underground to find suitable habitat, but this is not possible on a rooftop if the soil is too shallow. Earthworms may be purchased and incorporated into rooftop soils. Generally the species used is Lumbricus terrestris; worms used for vermiculture are also available for purchase, but they are adapted to different conditions (University of Saskatchewan 1997).

However, the higher soil temperatures can also be beneficial. FoodShare experiences longer seasons in their rooftop garden than at ground level, most likely due to the heat which reflects off of the concrete roof and is absorbed by their containers (Baker 2001). However, this benefit may not translate to green roof agricultural systems.

High temperatures relate to another crucial aspect of rooftop agriculture, namely the provision of water. Particularly if the soil is shallow, plants dry out quickly and more frequent watering is needed than might be necessary at grade (IDRC 1998). This can be a serious problem, especially if an irrigation system is not in place or breaks down. From a sustainability perspective the most appropriate as well as convenient and inexpensive thing would be a rainwater collection system. However, in times of drought this may not be adequate, and in some cases stormwater drains are internal, making redirecting the water flow difficult (St. Lawrence 1996).

Exposure to wind is also a possible problem for rooftop agriculture, though this varies with the types of crops to be grown. Some kind of windbreak is especially important in naturally windy climates, such as coastal regions or areas where surrounding high rises artificially exaggerate breezes (Osmundson 1999). If wind proves to be a serious problem, options include flexible or dwarf plant varieties as well as windbreaks of glass, fencing, or dense plantings (Osmundson 1999).

Choosing crop varieties is an important way of dealing with other aspects of the rooftop environment. Plants suitable for container gardening are usually appropriate for rooftops as well, since the environment is similar. Typically, the plants best suited to rooftop conditions are those adapted to drier climates with compact, sturdy shapes. To grow cooler season crops (radishes, spinach, etc.) it may be necessary to interplant them with taller crops in order to retain moisture and provide shade. Herbs like rosemary, which tolerate hot sun, should do well with less care (Taylor 1994). Depending on the depth of soil and the size of the rooftop, vines and root crops may not be suitable for cultivation. Carrots grow to between 5 and 10 inches long, and vines need space and a substantial amount of water to support healthy growth (Veseys 2001; IDRC 1998).

Last but certainly not least are accessibility concerns. If a rooftop is to be used for agricultural purposes, it must be easily accessible to both people and equipment. This is often one of the greatest barriers to rooftop agriculture. With a green roof or even just planters there are any number of times that an elevator and ramp would prove invaluable, but they are rarely present. Without them, the difficulty in accessing adequate quantities of soil amendments and mulches as well as heavy equipment such as rototillers becomes a major barrier. In addition, the absence of easy access bars the rooftop garden from some members of the community. For this reason the Trent University roof was rejected as a possible community garden site (Dance 2001). After the roof's installation it was left largely to weeds, despite the best efforts of the university grounds crew. The only visitors, for the most part, were curious students on late night explorations, to the point where the garden became more of a safety hazard than a community asset in the minds of many.


While green roof agriculture has substantial potential to increase the sustainability of North American cities, much research and practical experience is needed before it can feasibly play a real part in supporting their populations. There exist continuing knowledge and confidence gaps which must be addressed before most municipal agencies will be willing to establish the necessary regulatory supports for widespread adoption of green roof systems. The benefits of green rooftop agriculture are substantial, but many of these are currently masked or externalized. Drawbacks such as higher costs are much more directly borne by individual implementing agencies. Without governmental intervention these costs generally outweigh larger societal long-term benefits.


Ableman, Michael. 2000. "Agriculture's Next Frontier: How urban farms could feed the world," Utne Reader, (102): 60 - 65.

Baker, Lauren. 2001. Personal communication. (FoodShare Urban agriculture co- ordinator).

Baird, Vanessa. 1999. "Green cities," New Internationalist, (313): 7 - 10.

Curtis, William J. R. 1986. Le Corbusier: ideas and forms, (Oxford, Phaidon Press).

Dance, Christina. 2001. Personal communication, (OPIRG community garden researcher).

Draper, Diane. 1998. Our environment: a Canadian perspective, (Scarborough, ITP Nelson).

Fairholm, Jacinda. 1999. Urban Agriculture and Food Security Initiatives in Canada: A Survey of Canadian Non_Governmental Organizations, (International Development Research Centre (IDRC),

Gavrilov, Alexander (Sasha). 2001. Rooftop gardening in St. Petersburg, Russia, (, last modified March 28).

Geographical magazine. 2001. "Tokyo keeps its cool with roof gardens," Geographical Magazine, 73 (3): 12.

Green Roofs for Healthy Cities. 2000. "Outstanding growth of the German green roof infrastructure industry," Green roof infrastructure monitor, 2 (1): 5.

IDRC (International Development Research Centre). 1998. GARDEN ON YOUR ROOFTOP. (Developing Country Farm Radio Network script, ON YOUR).

Johnstone, Stephen. 2001. Estimated incremental cost to install grassed roof. (Peterborough, Trent University Physical Resources).

Kuhn, Monica. 1995. Rooftop resource, (

McLelland, Murray. 2000. "Soil temperature and plant growth," Soil moisture and temperature consideration, (Alberta Agriculture, food and rural development,

Nelson, Toni. 1996. "Closing the nutrient loop," World Watch, 9 (6): 10 - 17.

Norberg-Hodge, Helena, Todd Merrifield and Steven Gorelick. 2000. Bringing the food economy home: the social, ecological and economic benefits of local food, (Foxhole, Devon, International Society for Ecology and Culture).

Osmundson, Theodore. 1999. Roof gardens: history, design, and construction, (New York, W. W. Norton & Company, Inc.). Peck, Steven, Chris Callaghan, Monica E. Kuhn, and Brad Bass. 1999. Greenbacks from green roofs: forging a new industry in Canada, (Ottawa, Canadian Mortgage and Housing Corporation).

Pedersen, Kimberly. 2001. Meadows in the sky: contemporary applications for eco-roofs in the Vancouver region (Master's thesis), (Vancouver, University of British Columbia School of Architecture,

Pepall, Jennifer. 1993. "New Challenges for China's Urban Farmers," IDRC Reports, 21 (3) (

St. Lawrence, Joseph. 1996. Urban agriculture: the potential of rooftop gardening, (Toronto, York University Environmental Studies Master's thesis,

Taylor, Julie. 1994. Plant list - edible plants, (Toronto, Rooftop gardens resource group).

TFPC (Toronto Food Policy Council). 1999. Feeding the city from the back forty: a commercial food production plan for the city of Toronto, (Toronto, Toronto Board of Health).

University of Saskatchewan. 1997. Earthworms: Friend or Foe? (College of Agriculture Yard and Garden factsheet,, last modified April 3).

Veseys. 2001. "Carrots," The Veseys Store, (

Winson, Anthony. 1993. The intimate commodity: food and the development of the agro- industrial complex in Canada, (Toronto, Garamond Press).

ZinCo GmbH. 2001. Green roof systems, (, last updated April 17).

Title page image: Friedensriech Hundertwasser's 'The houses are hanging underneath the meadows.' (Image not included here.)

Appendix 1:

Estimated incremental cost to install grassed roof: $/m2
Increased structure size to carry additional load 20
300mm of soil (supply and place) 7.5
Increase in roof flashings, etc. 0.5
Increased depth of insulation 2
Geotextile fabric 0.3
Increased design fees 2.4
Parapet (estimate) 2.5
Total $35.20/m2

(Johnstone 2001)

Appendix 2: A selection of rooftop agriculture and green roof examples.

Rooftop agriculture examples:


Royal York Hotel - planters, filled with herbs for the hotel's restaurant (Fairholm 1999).

FoodShare Urban Agriculture Program - planters, growing tomatoes, green beans, peppers, eggplant, herbs and greens largely for the Good Food Box (a FoodShare program). 3 bee hives are also maintained (Baker 2001).

Brock School - planters, which furnish a working garden as well as an outdoor classroom, funded and built by the children, parents, and teachers (Kuhn 1995).

Mary Lambert-Swale housing project - 4250 sq feet of cedar planters, which provide each of the 75 tenants with a 5 x 5 foot roofgarden plot, growing vegetables, herbs and even fruit trees (Peck et al 1999).

Toronto City Hall demonstration project - 6 different plots have been established, some extensive others semi-intensive. Two semi-intensive plots will grow vegetables and herbs, such as peppers, tomatoes, corn, beans, squash, chives, and sage. Other semi-intensive plots are planted with black oak savanna species and native prairie species planted to attract birds and butterflies, while the extensive plots host alpine and dryland species (Green roofs for healthy cities 2001).


VanCity Place for Youth (Vancouver) - planters, filled with indigenous edible plants and culinary herbs (EYA 1997).

Josiah Quincy School (Boston) - Rooftop garden established as a classroom extension for science investigations. Children, along with neighboring seniors, harvest strawberries, tomatoes, beans, radishes, and watermelons (Sheung 2001).

Bartol Intergenerational Garden at the On Lok Senior Centre (San Francisco) - Chinese vegetables, structure unknown (Ableman 2000).

Mt Gravatt Central Main Street Program (Brisbane, Australia) - Experimantal urban rooftop integrated microfarm, which proposes the use of vermiculture and hydroponics as the basis for a microfarm enterprise to provide food for and utilize wastes the wastes of the local area (Wilson 1999).

Ecohouse (St. Petersburg) - 4 to 8 cm beds built on existing apartment building rooftop. Soil obtained from vermicomposting of the building resident's wastes is used for planting of vegetables (zucchini, lettuce, broccoli, tomatoes, cucumbers), herbs (parsley, dill), and soft fruit (strawberries, gooseberries, currants), and grass in thin beds of 4-8 cm on the roof-top. Produce from the rooftop garden is consumed by residents. Any excess is sold on the market or exchanged for services with maintenance companies (St Petersburg Sustainable Urban Community Development Project 1999).

Green roof examples:

In Canada:

York University Computer Sciences building (Toronto) largely inaccessible green roof, built mainly to help retain stormwater (McMinn 2001).

Merchandise lofts (Toronto) - large accessible green roof which includes a 150' prairie meadow, a wetland garden, and birch trees, all growing on Soprema's 'Sopraflor' growing medium, nourished by an in-ground irrigation system (Green Roofs for Healthy Cities 2000).

YMCA Environmental Learning Centre (Kitchener-Waterloo) - 8 inches of dirt and natural grasses, which cover two partly earth-sheltered buildings (Canadian Architect 1996).

North West Territories Legislative Building (Kuhn 1996) - green roof planted with native species.

See Peck et al. (1999), available on the web or in hard copy from the Canadian Mortgage and Housing Corporation, for 12 additional Canadian examples.


Chicago City Hall (Chicago) - 38, 800 square foot inacsessible green roof, installed on existing structure. The roof is a mix of intensive and extensive plantings, with soil depths ranging from 3.5", planted with sedums, to 24", over supoorting coloumns, which can support trees and shrubs (Beaudry 2001; Roofscapes, Inc. 2001).

City of Portland green roof demonstration projects (Portland, OR) - Two demonstration sites have been established in connection with a new program to reduce stormwater management lot level fees for buildings with green roof systems. One site is comparing 3 and 5 inch growing mediums, monitored for energy benefits and level of flow. The other is planted with species native to the west coast. It is not being irrigated in order to determine which of the plants are most successful in the green roof environment (Lipton 2001).

Patricia Neal Rehabilitation Centre (Knoxville) - A rooftop therapy park, constructed in 1994 to provide a rejuvanative environment for recovering patients. Intensive green roof design which includes small trees (Patricia Neal Rehabilitation Centre 2001).

The Company Group Gegenbauer Golf Course (Berlin) - The entire 1400 m2 intensive green roof is a miniature golf course. It was constructed on the existing building using ZinCo International's Floradrain FD 25 + FD 60 (Velazquez, 2001).

Note: Many more green roof examples exist, particularly in Germany, where 43% of municipalities provide incentives for green roof construction (Lenart 2001). The descriptions provided here are intended simply to give an idea of the different reasons green roofs are installed and different ways they may be used.


Ableman, Michael. 2000. "Agriculture's Next Frontier: How urban farms could feed the world," Utne Reader, (102): 60 - 65.

Baker, Lauren. 2001. Personal communication. (FoodShare Urban agriculture co- ordinator).

Beaudry, John. 1999. "Chicago Green Roof," Green Roofs for Healthy Cities, (, revised April 23). Canadian Architect. 1996. "Green buildings 1: natural harmony," Canadian Architect, 41 (7): 14-15.

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Revised November 4, 2006

Published by City Farmer
Canada's Office of Urban Agriculture