Lightweight Rooftop Vegetable Gardens (Finalist.)
The Food and Water Institute (FWI)
The objective of Rooftop Gardens Inc. is to design and build year-round rooftop vegetable gardens that are lightweight so they don’t exceed the structural capacity of standard flat rooftops. These gardens will serve private citizens, restaurants, community groups and schools by providing local, organic produce on site for kitchens and nutrition programs.
Toronto offers an abundance of flat, open rooftop spaces that are perfect for vegetable gardens. Green roofs are rapidly gaining popularity in Canada, but their potential could be greatly increased if we consider the question: what if we use rooftops for agriculture? In Toronto we have a vast potential to grow tonnes of food on our rooftops, yet large rooftop gardens are rare. This is due to the short growing season, as well as the cost and structural constraints of incorporating a rooftop garden suitable for growing vegetables. Rooftop Gardens Inc. offers solutions to these obstacles, as outlined below:
Rooftop Gardens Inc. solutions to urban agriculture obstacles for Canadian cities:
Obstacle: Limited space, soil may be unsuitable, theft from community or public gardens
Solutions: Acres of flat, open, sunny rooftop spaces; fresh organic soil and compost; privacy
Obstacle: High irrigation needs, access to water, fertilized water runs off into sewer system
Solutions: Collect rainwater; drip irrigation system conserves water; fertilized water is recycled within the system
Obstacle: short May-October season; harsh winters
Solutions: four-season harvest; sustain cool-weather crops in passive-solar, lightweight greenhouse
Obstacle: Intensive; heavy soil requirements; most rooftops cannot structurally support a vegetable garden
Solutions: Lightweight semi-hydroponic or aeroponic containers; organic fertilizers to maximize yield; compost garden waste for use as fertilizer
Obstacle: Lack of awareness with respect to horticulture and where our food comes from
Solutions: Showcase health and environmental benefits of local, organic food; educate community about urban agriculture through our partner organization, The Food and Water Institute (FWI)
Rooftop Gardens Inc. designs are unique because they are lightweight and enable year-round food production:
- Designs incorporate the use of lightweight semi-hydroponic or aeroponic containers;
- In Ontario, the average load capacity for a flat roof is 195 kg/m2 and an intensive green roof generally imparts loads of around 450 kg/m2 and up. A semi-hydroponic garden built by Rooftop Gardens Inc., complete with water, soil, mature vegetation, and a greenhouse will weigh no more than 100 kg/m2;
- Containers are watered via drip irrigation which results in little wasted water, no soil erosion and uniform distribution;
- Organic fertilizers and compost teas are easily added to the irrigation system and distributed to plants;
- Irrigation and fertilization can be automated with a timer to minimize maintenance;
- Gardens can be made more self-sufficient through on-site composting, seed saving, and rainwater capture
- We provide access to organic vegetable seeds and plants that have been grown for generations on a rooftop, and are thus adapted to the environment
- Four-season gardening is achieved using innovative lightweight, inflatable greenhouses that enable the continuous harvest of cool weather crops such as salad greens throughout the winter months.
The SkyGarden, located at the University of Toronto, is our pilot installation, developed in collaboration with the Food and Water Institute and the University. In two growing seasons SkyGarden has produced over 350 kg (800 lbs) of organic vegetables in a surface area of only 50 m2. The SkyGarden is self-sustaining: plants are grown from seed collected from the previous year’s harvest and garden waste is composted and used as a soil amendment. Other features of the garden include a honeybee hive and a solar dehydrator, built through workshops facilitated by FWI. Upcoming additions to the garden include a lightweight greenhouse and experimental cold frames, to continue the harvest year-round.
Individual containers retail for $50. Overall cost depends on the number of containers and whether or not an irrigation system is required. Our system design is approximately $120 per m2 for large installations (>100 containers, or 50 - 100 m2). This cost estimate is based on the design of our pilot garden, and includes an automated drip irrigation system. Our pilot garden yields indicate that based on the value of organic produce grown alone, the payback time of the capital cost for our system is 2-3 years.
Contrast this to a traditional green roof which costs approximately $150 - $375 per m2 for an intensive system, i.e., one with soil deep enough to grow some vegetables. This cost does not include an irrigation system, prices of which vary widely as they are generally customized for the specific design. However, it is not uncommon for the irrigation system to nearly double the cost of the installation.
Rooftop Gardens Inc. offers an affordable and effective solution overcoming obstacles in constructing urban rooftop gardens. In doing so, the community can dramatically reduce its ecological and carbon footprints and experience the many health and environmental benefits of growing and eating local, organic and seasonal food.
Direct Reductions - Heating and Cooling Energy Savings
Direct emission reductions within the city are related to savings on heating and cooling costs. Studies have shown that vegetation on a building exterior (rooftop, walls) can provide a measure of insulation and reduce the energy needed to control the temperature of the interior of the building. Rooftops are particularly important because, for many urban buildings, they have the hottest temperatures in the summer and they are the site of the most heat loss in the winter.
A study by researchers at U of T and Environment Canada demonstrated that green roofs reduce energy consumption in winter by preventing heat loss. For a typical mixed-use (residential and commercial) 3-story space in downtown Toronto, the green roof resulted in a reduction in heating energy of 34% or 6015 kWh for the winter season. That translates to 1125 kg of CO2 for just one building, based on the TAF conversions. For a residential home, the same study estimated heat energy savings of ~5% or 385 kWh, which translates to 72 kg of CO2 emissions per home.
A City of Toronto study noted the benefits of vegetated rooftops in decreasing air conditioning requirements for the buildings. They calculated energy savings on air conditioning at 4.15 kWh/m2 of vegetated rooftop. This translates to 0.77 kg of CO2 per sq. meter of vegetated rooftop.
Reduction of Urban Heat Island:
In large cities, non-vegetated surfaces like rooftops and concrete absorb solar radiation, the surface temperature rises, and with it the surrounding air temperature. This is the urban heat island effect. So, if we increase urban vegetation coverage we can lower the air temperatures and that heat island effect. Adding this vegetation to rooftops turns a liability into an asset. At a large scale, in theory we could reduce the air temperature enough to lower air conditioning requirements in summer.
A 2004 City of Toronto study on the benefits of green roofs concluded the following:
- widespread implementation of vegetated rooftops would reduce air temperatures in Toronto by 0.5 to 2 degrees Celsius
- This translated to energy (cooling) savings of 2.37 kWh/m2 of vegetated rooftop
- The CO2 emission reduction from power generation translated to 32,300 metric tonnes per year
Indirect Reductions - Food Miles
Developing low cost, low energy, four-season rooftop gardens will provide year-round, local and organic produce in the urban environment. In doing so, the potential decreased “food miles” is significant and will act to reduce greenhouse gas and smog causing emissions associated with food transportation.
Local, organic food has a much lower associated carbon footprint than conventionally grown food which has travelled great distances from farm to dinner plate, and that uses excessive chemical fertilizers and pesticides. In Toronto, local food items travel an average of 101 km versus imported food items which travel an average of 5,364 km. A local study on food miles done by Waterloo Public Health concluded that 10 commonly consumed foods in the region had traveled an average of 4,497 km and accounted for 51,709 tonnes of greenhouse gas emissions annually. The study also pointed out that these were foods that were able to be produced within the region. The same study showed that selecting locally-sourced foods, rather than imported foods, would result in an annual reduction of 49,485 tonnes of greenhouse gas emissions in that region alone, the equivalent of taking 16,191 cars off the roads. The potential emission reductions in Toronto with increased rooftop agriculture are significant. In addition, extending the harvest to the entire year using passive energy, lightweight greenhouses increases the availability of local food during winter months when food is imported in even greater quantities and/or produced in energy-intensive, heated greenhouses.
Indirect Reductions - Organic Agriculture
Organic agriculture is more effective at capturing atmospheric CO2 and sequestering it in the soil as beneficial organic matter; in contrast, conventional farming results in a net release of CO2 to the atmosphere. These results are based on a long-term (23 year) study by the Rodale Institute. The same study also estimates that converting one medium-sized farm to organic production stores an amount of carbon in the soil equivalent to taking 117 cars off the road. For 10,000 farms that is equivalent to taking 1,174,400 cars off the road.
In addition, eliminating the use of pesticides, herbicides, and fertilizers made from fossil fuels will further reduce the impact of these rooftop gardens.
Heather Wray is in the final year of her PhD in civil and environmental engineering at the University of Toronto. She also has a MSc in ecology. Prior to beginning her PhD, Heather worked for a green building design firm in Montreal, where she designed and implemented greywater recycling and four-season gardening for local projects.
The Food and Water Institute (FWI) is a Canadian registered charity and believes that it is the right of all Canadians to have access to healthy food and water that is free from unwanted chemicals. The Food and Water Institute shares science-based information about food and water safety, and challenges consumers to make informed choices. FWI maintains SkyGarden at U of T, where educational workshops and seminars are held. Examples of workshop topics include composting, seed saving, building a solar dehydrator and urban beekeeping.
Casey Wong is an industrial designer and a founding partner at Citra design, a Toronto design consultancy specializing in high quality technical solutions for architecture, interactions and living. Citra design is working with Rooftop Gardens Inc. and FWI to develop and install an inflatable greenhouse concept at the University of Toronto SkyGarden.
Erika Richmond is finishing a masters in Landscape Architecture at the University of Toronto. Before beginning her masters degree, Erika operated a small green roof design and construction company in Vancouver. She has also worked at and volunteered at many urban agriculture projects in Toronto and Vancouver.
Rooftop Gardens Inc. will be self-supporting in under five years by charging consulting, installation and equipment fees to the individuals and organizations that purchase our products and services.
Revenues from the installation of three paid projects the size of SkyGarden (100 containers) would enable Rooftop Gardens Inc. to cover minimal operational expenses for a period of 12 months.
Funds from Climate Spark will allow Rooftop Gardens Inc. to 1) design and manufacture new innovative rooftop garden technologies, 2) establish an organizational presence by securing desk space and bringing on a paid intern, 3) establish a strong brand and develop and implement our marketing and communications strategic plan, and 4) support educational workshops and seminars to teach urban gardeners new skills in collaboration with the Food and Water Institute.