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So What Are Green Roofs?
April 08 2009
 Green roofs are layers of vegetation that sit on top of the conventional roof surfaces of a building. They are either ‘intensive’ or ‘extensive’ referring to the degree of maintenance the roofs require.
Intensive green roofs (what most people think of as roof gardens) are deep substrates and can support a wide range of plant types - trees and shrubs as well as perennials, grasses and annuals. They are generally heavy and require specific support from the building.
Intensive green roofs are usually traditional in design, reproducing what we normally see on the ground with lawns, flower beds and water features. 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. However, because of their larger plant material and horticultural diversity, intensive green roofs can require substantial maintenance - pruning, clipping, watering and weeding as well as irrigation and fertilization.
On the other hand, 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 (like sedum). 
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. They are also usually highly visible, and so send a clear signal of a sustainable building, giving a very positive image to a building or development.
Specific benefits of green roofs :
• 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. 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 in their May 2003 publication “Sustainable Drainage Systems (SUDS) – an introduction” and the Agency have been supportive of supplementary planning documents which refer to green roofs – such as the Leeds Biodiversity and Waterfront Development SPD (2005)
• 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.
• 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.
Green roofs in the UK
In other countries, although they may be supported by national government policy and strategy, they operate at the regional and city level, and focus on the particular needs of that region, with a high degree of interaction and partnership between local authorities, government, local research communities, the green roof industry, and community and regeneration organisations.
In the UK most green roof developments have been one-off projects, often on buildings with an environmental remit. Policies relating to green roofs are advisory, and there is no national standard for green roof implementation or maintenance.
The University of Sheffield is the leading research establishment in this field, with an unrivalled range of expertise across the range of green roof benefits, and has already established an international reputation in green roof studies.
The first national green roof conference, attracting over 200 delegates, was held at the University of Sheffield in September 2003, in partnership with Sheffield City Council, and the green roof at Moorgate Crofts Business Centre, Rotherham, was only installed because the designers were inspired by what they heard at the conference.
The conference spurred the setting up of a national green roof organisation, Livingroofs.org. In 2004 the Green Roof Forum was established in Sheffield. A significant number of demonstration green roofs in the South East and London have proved vital in gaining confidence with private developers. As a result, this area has pioneered green roof implementation in the UK.
Information courtesy of the Green Roof Centre, founded by the University of Sheffield
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