Green roofs are vegetated layers that sit on top of the conventional waterproofed roof surfaces of a building. Whilst green roofs come in many different forms and types, usually a distinction is made between extensive, intensive and biodiverse or wildlife roofs. These terms refer to the degree of maintenance the roofs require.
Intensive green roofs are composed of relatively deep substrates (20cm+) and can therefore support a wide range of plant types: trees and shrubs as well as perennials, grasses and annuals. As a result they are generally heavy and require specific support from the building. Intensive green roofs (what most people think of as roof gardens) have in the past been rather traditional in their design, simply reproducing landscapes found on the ground, such as lawns, flower beds and water features. However, more contemporary intensive green roofs can be visually and environmentally exciting, integrating water management systems that process waste water from the building as well as storing surplus rainwater in constructed wetlands. Because of their larger plant material and horticultural diversity, intensive green roofs can require substantial input of resources – the usual pruning, clipping, watering and weeding as well as irrigation and fertilization.
Conversely, the green roofs that have received the greatest interest recently are extensive green roofs. They are composed of lightweight layers of free-draining material that support low-growing, hardy, drought-tolerant vegetation. Generally the depth of growing medium is from a few centimeters up to a maximum of around 10-15cm. These roof types have great potential for wide application because, being lightweight, they require little or no additional structural support from the building. Furthermore, because the vegetation is adapted to the extreme roof top environment (high winds, hot sun, drought, and winter cold), extensive green roofs require little in the way of maintenance and resource inputs. Extensive green roofs can be designed into new buildings, or ‘retro-fitted’ onto existing buildings.
Biodiverse or wildlife roof are becoming more popular, as people become more aware of biodiversity issues, and options for conservation. These are designed either to replicate specific habitat needs of a single or small number of species, or to create a range of habitats which can maximise the array of species which inhabit and use the roof.
Because of their very wide range of environmental and economic benefits (in particular their insulation and cooling properties, ability to significantly reduce rainwater runoff from roofs, and their value in promoting biodiversity and habitat in built-up areas), green roofs have become important elements of sustainable and green construction in many countries. Moreover, because they can be highly visible, green roofs clearly signal the intent for sustainable building and can provide a positive and distinctive image to a building or development. (from greenroofcentre.org)
