A roof garden is a 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, recreational opportunities, and in large scale it may even have ecological benefits. The practice of cultivating food on the rooftop of buildings is sometimes referred to as rooftop farming.Rooftop farming is usually done using green roof, hydroponics, aeroponics or air-dynaponicssystems or container gardens.
The environmental impact
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 from 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.”
Roof garden vs. green roof
A roof garden can be distinguished from a green roof, although the two terms are often used interchangeably. The term roof garden is well suited to roof spaces that incorporate recreation, entertaining, and provide 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.
Although they may provide aesthetic and recreational benefits a green roof is not necessarily designed for this purpose. A green roof may not provide any recreational space and be constructed with an emphasis towards improving the insulation or improving the overall energy efficiency and reducing the cooling and heating costs within a building.
Green roofs may be extensive or intensive. The terms are used to describe the type of planting required. 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 may be designed for a variety of functions and vary greatly in depth to satisfy aesthetic and recreational purposes. These planters can hold a range of ornamental plants: anything from trees, shrubs, vines, or an assortment of flowers. As aesthetics and recreation are the priority they may not provide the environmental and energy benefits of a green roof.Planting on roof tops can make urban living more self-sufficient and make fresh vegetables more accessible to urban people.