Why Green Roofs?
The idea of greening a roof dates back to thousands of years. Civilizations of Mesopotamia (hanging gardens of Babylon), Greeks, Romans, Persians and other cultures had roof gardens to green and cool their hot landscapes. At the other climate (cold and wet Scandinavian countries) sod roofs provided extra warmth and insulation. Very often nature is the inspiration for green roofs, as self-seeds germinate in leaf, dirt, in gutters and often flat roof.
CLIMATE Temperatures are higher in high-density cities than in surrounding suburbs due to the concentration of heat-radiating surfaces and the lack of plants. Asphalt and concrete rooftops, huge parking lots and roads cover up to seventy percent of land area in dense cities.
Heat islands are created by dark surfaces that absorb heat energy and radiate it back into the air. Tar and concrete absorb and radiate the most heat, that's why asphalt and concrete rooftops play major role in the heat island effect.
GREEN ROOF BENEFITS Green roofs cool and filter air, reducing the urban heat island effect. Planted roofs increase the oxygen output, soften urban streetscapes, reduce impervious surfaces and absorb the storm water rand, reduce runoff volume and slow the rate at which it enters the sewage system. Green roofs play critical role in purifying the water and air by absorbing nitrogen and remediating the acidity of the water running off the roofs. Extensive green roofs can retain fifty percent of one-inch rainfall and as much as fifty to seventy percent of rainfall annually. Black rooftops can become new green spaces without compromising development.
CHALLENGES Green roofs present a number of challenges, they are not like regular gardens. Green roofs have no equivalent in nature. Green roof installed many stories above the ground has limited accessibility -- all materials, medium, plants, as well as laborers must be transported to the site. This adds cost usually not associated with landscaping. Selecting the right plants is one of the biggest challenges. Extensive roofs are traditionally non-irrigated and consist mostly lightweight, inorganic medium, a plant specification list for a green roof differs a lot from typical ground-level garden.
Green roofs are lightweight, layered systems that cover conventional roof surfaces with growing medium and plants. Extensive green roofs ( also known as eco-roofs or roof meadows) are shallow; 2 to 6 inches of growing medium (weighing 15-30 pounds per square foot) planted with drought-tolerant succulents or grasses requiring minimal maintenance. Roof meadows cover the vast majority of the roof and are generally not built for human occupancy. Most extensive roof gardens are strikingly different from ground-level gardens, both visually and functionally. Intensive roofs usually have deeper soil and and can accommodate a wide range of plants, edibles, shrubs and even trees, which require regular maintenance and irrigation.
Bronx, New York