Rooftop gardens, also known as green roofs, are vegetative green layer grown on a rooftop of buildings providing greenery, useful products, shade, acoustic benefits as well as remove heat from the air through evapotranspiration, reducing temperatures of the roof surface and the surrounding air. On hot and humid summer days, the surface temperature of a rooftop garden can be cooler than the air temperature, whereas the surface of a conventional rooftop can be up to 50°C warmer.
Rooftop gardens are becoming popular in many developed countries In addition to mitigating urban heat island effect, the benefits of rooftop gardens include reduced energy use as rooftop gardens absorb heat and act as insulators for buildings, reducing energy needed to provide for cooling and heating. Through the daily dew and evaporation cycle, plants on vertical and horizontal surfaces are able to cool cities during hot summer months and reduce the Urban Heat Island (UHI) effect. The light absorbed by vegetation would otherwise be converted into heat energy.
Rooftop gardens can be installed on a wide range of buildings, from industrial facilities to private residences. They can be as simple as a two-inch covering of hardy groundcover or as complex as a fully accessible park complete with trees. The green roof top reduces air pollution and greenhouse gas emissions (GHG) by lowering air conditioning demand. Vegetation can also remove air pollutants and GHG emissions through dry deposition and carbon sequestration and storage. It leads to improved human health and comfort.
The green area also filters pollutants. Rooftop gardens can provide aesthetic value and habitat for many species. Urban greening has long been promoted as an easy and effective strategy for beautifying the built environment and increasing investment opportunity.
With rooftop gardens, water is stored by the substrate and then taken up by the plants from where it is returned to the atmosphere through transpiration and evaporation. In summer, depending on the plants and depth of growing medium, rooftop gardens retain 70-90% of the precipitation that falls on them and in winter they retain between 25-40%.
The plants on rooftop gardens can capture airborne pollutants and atmospheric deposition. They can also filter noxious gases. The temperature moderating effects of rooftop gardens can reduce demand on power plants, and potentially decrease the amount of CO2 and other polluting by-products being released into the air. Rooftop gardens can also help in reducing and dispersing the distribution of dust and particulate matter in the immediate environment and avoid production of smog. This can play a role in reducing GHG emissions and adapting urban areas to improve future weather.
The research published by the National Research Council of Canada found that an extensive green roof can reduce the daily energy demand for air conditioning in the summer by over 75%. The risk posed by electromagnetic radiation (from wireless devices and mobile communication) to human health is still a question for debate. Rooftop gardens are capable of reducing electromagnetic radiation penetration by 99.4%. Rooftop gardens have excellent noise attenuation, especially for low frequency sounds. An extensive green roof can reduce sound from outside by 40 decibels, while an intensive one can reduce sound by 46-50 decibels.
Rooftop gardens can sustain a variety of plants and provide a habitat for various bird species. The reduced pollution and increased water quality that rooftop gardens bring can decrease demands for health care. Rooftop gardens can serve as community hubs, increasing social cohesion, sense of community and public safety.
Rooftop gardens make the most of unused space within the increasing density of our cities. Rooftops can thus be developed into social and recreational spaces and used for urban agriculture. This amenity space can be used for meetings and recreation. Improved aesthetic views are beneficial for area residents and visitors, it also leads to improved worker productivity and creativity and enhance urban food security.