What is green roof supposed to be ? Basically, a green roof is a roof of a building that is partially or completely covered with vegetation and soil, or a growing medium, planted over a waterproofing membrane. This does not refer to roofs which are merely colored green, as with green roof shingles. It may also include additional layers such as a root barrier and drainage and irrigation systems.
Green roofs are used to:
- Grow fruits, vegetables, and flowers
- Reduce heating (by adding mass and thermal resistance value) and cooling (by evaporative cooling) loads on a building, especially if it is glassed in so as to act as a terrarium and passive solar heat reservoir, a concentration of green roofs in an urban area can even reduce the city's average temperatures during the summer
- Increase roof life span
- Reduce stormwater run off, see water-wise gardening
- Filter pollutants and carbon dioxide out of the air, see living wall
- The soil and plants on green roofs help to insulate a building for sound; the soil helps to block lower frequencies and the plants block higher frequencies.
- Filter pollutants and heavy metals out of rainwater
- Increase wildlife habitat in built-up areas, see urban wilderness
A green roof is often a key component of an autonomous building. A 2005 study by Brad Bass of the University of Toronto showed that green roofs can also reduce heat loss and energy consumption in winter conditions. In a recent study on the impacts of green infrastructure and in particular green roofs in the Greater Manchester area, researchers found that adding green roofs will help keep temperatures down, particularly in urban areas: “adding green roofs to all buildings can have a dramatic effect on maximum surface temperatures, keeping temperatures below the 1961-1990 current form case for all time periods and emissions scenarios. Roof greening makes the biggest difference, where the building proportion is high and the evaporative fraction is low.
A Green roofs can be categorized as semi-intensive, intensive, or extensive. It is depending on the depth of planting medium and the amount of maintenance they need. Traditional roof gardens, which require a reasonable depth of soil to grow large plants or conventional lawns, are considered "intensive" because they are labour-intensive, requiring irrigation, feeding and other maintenance. Intensive roofs are more park-like with easy access and may include anything from kitchen herbs to shrubs and small trees.
"Extensive" green roofs, by contrast, are designed to be virtually self-sustaining and should require only a minimum of maintenance, perhaps a once-yearly weeding or an application of slow-release fertiliser to boost growth. Extensive roofs are usually only accessed for maintenance. They can be established on a very thin layer of "soil" (most use specially formulated composts): even a thin layer of rockwool laid directly onto a watertight roof can support a planting of Sedum species and mosses.
The disadvantegs
Some disadvantage of green roofs, it's have more demanding structural standards. Some existing buildings cannot be retrofitted with a green roof because of the weight load of the soil and vegetation. Depending on what kind of roof it is, the maintenance costs could be higher. The green roofs also place higher demands on the waterproofing system of the structure both because water is retained on the roof and due to the possibility of roots penetrating the waterproof membrane."However a sedum covering doesn't need water to be retained on the roof as these plants can tolerate long periods without rainfall, so a drainage layer will combat this particular problem"(Chris Sorrell). Installing adequate waterproofing systems and root barriers can increase the cost of the roof.
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