The roof garden of the Rockefeller Center in Manhattan.
A roof garden is any garden on the roof of a building. Besides the decorative benefit, roof plantings may provide food, temperature control, hydrological benefits, architectural enhancement, habitats or corridors for wildlife, and recreational opportunities.
Humans have grown plants atop structures since antiquity. The ziggurats of ancient Mesopotamia (4th millennium BC–600 BC) 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. A roof garden has also been discovered around an audience hall in Roman-Byzantine Caesarea. 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.
The roof terrace of the Casa Grande hotel in Santiago de Cuba.
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 insolation, 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, 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). The study was performed by the University of Cardiff.
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. A roof garden can delay run off; reduce the rate and volume of 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.
An extreme example of a roof garden, in Vancouver, British Columbia.
“In an accessible rooftop garden, space becomes available for localized small-scale urban agriculture, a source of local food production. An urban garden can supplement the diets of the community it feeds with fresh produce and provide a tangible tie to food production.”  At Trent University, there is currently a working rooftop garden which provides food to the student café and local citizens.
Available gardening areas in cities are often seriously lacking, which is likely the key impetus for many roof gardens. 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). These also encourage environmentally responsible practices, eliminating tilling, reducing or eliminating pesticides, and weeding, and encouraging the recycling of wastes through composting.
Importance to urban planning
Becoming green is a high priority for urban planners. The environmental and aesthetic benefits to cities is the prime motivation. It was calculated that "the temperature in Tokyo could be lowered by 0.11–0.84 °C (0.2-1.51 °F) if 50% of all available rooftop space were planted with greenery. This would lead to a savings of approximately 100 million yen (~$1.2 million USD) per day in the city's electricity bill."
Singapore is very active in green urban development. "Roof gardens present possibilities for carrying the notions of nature and open space further in tall building development." When surveyed, 80% of Singapore residents voted for more roof gardens to be implemented in the city's plans. Recreational reasons, such as leisure and relaxation, beautifying the environment, and greenery and nature, received the most amount of votes.
Roof garden vs. green roof
A roof garden is actually very different from a green roof, although the two terms are often and incorrectly used interchangeably. A roof garden is an area that is generally used for recreation, entertaining, and as an additional outdoor living space for the building's residents. It may include planters, plants, dining and lounging furniture, outdoor structures such as pergolas and sheds, and automated irrigation and lighting systems. A roof garden reestablishes the relationship between humans and nature that can be lost in urban environments.
It is different from a green roof in that the considerations are primarily of an aesthetic or recreational nature, whereas a green roof is usually constructed to cover a large area in the most economical and efficient means possible with an emphasis towards improving the insulation or improving the overall energy efficiency of cooling and heating costs within a building.
The panels that comprise a green roof are generally no more than a few inches up to a foot in depth, since weight is an important factor when covering an entire roof surface. The plants that go into a green roof are usually sedum or other shallow-rooted plants that will tolerate the hot, dry, windy conditions that prevail on most rooftop gardens. With a green roof, "the plants layer can shield off as much as 87% of solar radiation while a bare roof receives 100% direct exposure".
The planters on a roof garden, on the other hand, can generally range anywhere from 6 in up to 3 ft (0.15 to 0.9 m) in depth, depending on the weight-bearing capacity of the roof, and would be placed more for aesthetic purposes. These planters can hold a range of ornamental plants, anything from trees, shrubs, vines, or an assortment of flowers. Since the planters on a roof garden are placed in random fashion, it would be much less likely to provide the environmental and energy benefits of a green roof.
Science of gardening
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.
7. ^ a b Liu, K. Energy Efficiency and Environmental Benefits of Rooftop Gardens. National Research Council Canada. Retrieved October 30, 2009, from: http://www.ottawa.ca/residents/public_consult/lansdowne_partnership/rooftop_gardens.pdf
8. ^ Rooftop Gardens. The Official Trent University Website. October 30, 2009, from: http://www.trentu.ca/eab/documents/Rooftop_Gardens-Zipple.pdf
9. ^ a b c Yuen, B, & Wong, N. (2005). Resident perceptions and expectations of rooftop gardens in Singapore. Landscape and Urban Planning, 73 (4). Retrieved June 19, 2009, from ScienceDirect database.
10. ^ Wong, N, Tay, S, Wong, R, Ong, C, & Sia, A. (2003). Life cycle cost analysis of rooftop gardens in Singapore. Building and Environment, 38 (3). Retrieved June 19, 2009, from ScienceDirect database.