Rooftop Farming

Urban Farm Dublin

Rooftop farming is the practice of cultivating food on the rooftop of buildings. Rooftop farming is usually done using green roof, hydroponics, aeroponics, or air-dynaponics systems or container gardens. Besides using the already present space at the roof itself, additional platforms could possibly be created between high-rise buildings called ‘aero-bridges.’ Besides the decorative benefit, roof plantings may provide food, temperature control, hydrological benefits, and habitats for wildlife.

‘In an accessible rooftop garden, space becomes available for localized small-scale urban agriculture, a source of local food production.’ At Trent University in Ontario, there is currently a working rooftop garden which provides food to the student café and local citizens.

Humans have grown plants atop structures since antiquity. The ziggurats of ancient Mesopotamia had plantings of trees and shrubs on aboveground terraces. An example in Roman times was the Villa of the Mysteries in Pompeii, which had an elevated terrace where plants were grown. The medieval Egyptian city of Fustat had a number of high-rise buildings that Nasir Khusraw in the early 11th century described as rising up to 14 stories, with roof gardens on the top story complete with ox-drawn water wheels for irrigating them.

Roof gardens are most often found in urban environments. Plants have the ability to reduce the overall heat absorption of the building which then reduces energy consumption. ‘The primary cause of heat build-up in cities is insulation, the absorption of solar radiation by roads and buildings in the city and the storage of this heat in the building material and its subsequent re-radiation. Plant surfaces however, as a result of transpiration [evaporation of water], do not rise more than 4–5 °C above the ambient and are sometimes cooler.’ This then translates into a cooling of the environment between 3.6 and 11.3 degrees Celsius (6.5 and 20.3 °F), depending on the area on earth (in hotter areas, the environmental temperature will cool more).

A study at the National Research Council of Canada showed the differences between roofs with gardens and roofs without gardens against temperature. The study shows temperature effects on different layers of each roof at different times of the day. Roof gardens are obviously very beneficial in reducing the effects of temperature against roofs without gardens. ‘If widely adopted, rooftop gardens could reduce the urban heat island, which would decrease smog episodes, problems associated with heat stress and further lower energy consumption.

Aside for rooftop gardens providing resistance to thermal radiation, rooftop gardens are also beneficial in reducing rain run off. ‘As cities grow, permeable substrates are replaced by impervious structures such as buildings and paved roads. Storm water run-off and combined sewage overflow events are now major problems for many cities in North America. A key solution is to reduce peak flow by delaying (e.g., control flow drain on roofs) or retaining run-off (e.g., rain detention basins). Rooftop gardens can delay peak flow and retain the run-off for later use by the plants.’

The garden may be on the roof of an autonomous building which takes care of its own water and waste. Hydroponics and other alternative methods can expand the possibilities of roof top gardening by reducing, for example, the need for soil or its tremendous weight. Plantings in containers are used extensively in roof top gardens. Planting in containers prevents added stress to the roof’s waterproofing. One high-profile example of a building with a roof garden is Chicago City Hall. For those who live in small apartments with little space, square foot gardening, or (when even less space is available) living walls (vertical gardening) can be a solution. These use much less space than traditional gardening (square foot gardening uses 20% of the space of conventional rows; ten times more produce can be generated from vertical gardens).

The related idea of a ‘living machine’ is based on the most basic cycle of gardening: using wastes (organic waste and sewage), appropriately broken down, usually in some specialized container, on the soil, and harvesting food which, when processed, generates biodegradable waste, and when eaten, generates sewage. In most of the world, this kind of very tight closed loop gardening is used, despite certain health risks if necessary precautions are not taken. Composting human or pet waste should achieve thermophilic conditions and age for at least a year before being used.

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