Green Roofs

Green roofs are vegetated layers that sit on top of the conventional roof surfaces of a building. Usually a distinction is made between extensive and intensive. These terms refer to the degree of maintenance the roofs require.

Intensive green roofs are composed of relatively deep substrates 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 what tends to be found on the ground, with 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 maintenance resources – the usual pruning, clipping, watering and weeding as well as irrigation and fertilisation.

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, tough drought-resistant vegetation. Generally the depth of growing medium is from a few centimetres up to a maximum of around 10cm. These roof types have great potential for wide application because, being lightweight, they require little or no additional structural support from the building, and because the vegetation is adapted to the extreme roof top environment (high winds, hot sun, drought, and winter cold) they 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. 

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 come to be important elements of sustainable and green construction in many countries. Moreover, because they can be highly visible, they also clearly outwardly signal an intent for sustainable building and can give a very positive and distinctive image to a building or development.

Some benefits include:

Research from around the world indicates that Green Roofs reduce annual run-off from roofs by at least 50%, and more usually by 60-70% - contributing to urban drainage and flood alleviation schemes. Moreover, the rate of release following heavy rainfall is slowed, reducing the problems associated with storm surges. With an increasing need for developments to have limited water run off, the Environment Agency now highlight the use of green roofing and the Agency have been supportive of supplementary planning documents which refer to green roofs.
Green roofs (and other practices such as natural ventilation) reduce the need for air conditioning in the summer and as a result reduce CO2 emissions. Poorly protected and insulated roofs can lead to substantial overheating of spaces beneath them. This can lead to the need for increased air-conditioning. A green roof not only acts as an insulation barrier, but the combination of plant processes and soil processes reduces the amount of solar energy absorbed by the roof membrane, thus leading to cooler temperatures beneath the surface. Research by Nottingham Trent University has shown that at an average temperature of 18.4°C, the roof temperature of a normal roof can reach 32°C whereas the temperature beneath a green roof is only 17.1°C. A study conducted in Chicago, USA, recently estimated that building energy savings to the value of $100,000,000 could be saved each year if all roofs were greened, as the need for air conditioning would be reduced.

Green roofs can help to reduce heat loss from buildings during the winter when root activity of plants, air layers and the totality of the specific system create heat and thereby provide an insulation membrane. However the efficiency of green roofs as thermal barriers is dependent on the amount of water held within the system. Water retention can increase the amount of heat lost through the system and therefore any efficiency gains are dependent on daily conditions. It is therefore difficult to provide accurate figures on the net effect of green roofs on energy efficiency during the winter months. The study at Trent University on the temperatures under membranes of standard roofs and those under green roofs also showed that green roofs appear to have a positive effect in winter. With an average temperature of  0°C the temperature of a standard roof is 0.2°C whereas the temperature under a green roof 4.7°C

New developments lead to a loss of habitats – green roofs can contribute to biodiversity and address local biodiversity action plans. In particular they have been shown to favour many rare invertebrates found on brownfield sites, as well as ground-nesting birds such as skylarks.
Green roofs contribute to a greener urban environment and quality of life for communities in high density developments.

A roof life is at least doubled with the addition of a green roof, thereby reducing resource use in roof replacement and repair.

For more details (including several case studies from Sheffield) please check out the following web sites:

www.livingroofs.org
www.groundwork-sheffield.org.uk