What are green roofs?

What are 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 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, tough drought-resistant vegetation. Generally the depth of growing medium is from a few centimeters 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.
Figure 1. A typical extensive green roof		Figure 2. A typical green roof ‘build-up’ from a roof surface.
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.
Specific benefits of green roofs 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 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 Europe Currently the main centres of green roof activity are Germany, Austria and Switzerland, where, particularly in Germany, it is a legal requirement in many large cities to include green roofs on new flat-roofed buildings. Alongside this regulatory framework is a system of grants, administered by local authorities to reflect their regional context, that typically pay up to 50% of the additional cost of green roof installation. The original driver for the widespread take-up of green roofs in these countries was state legislation that required mitigation for loss of open space at ground level as a result of development, but now green roofs are primarily used as part of integrated Sustainable Urban Drainage Schemes to minimise the amount of storm-water runoff from a development, and thereby to reduce the risk of urban flooding after heavy rain.
Green roofs in North America In North America, a number of city authorities have pioneered green roof development in a very high profile manner. Notably, Chicago has a major green roof programme (including a show-case example retro-fitted on top of the municipal City Hall) that is spear-heading the strategy of the city to become the greenest city in the US. This high-profile greening is aimed at changing the image of the city from its heavy industrial past to a high technology future that attracts new investment partly through quality of life and environment. Portland, Oregon, and Seattle, Washington have dynamic city programmes that centre around green roofs, with planning incentives that allow greater floor space for developments that incorporate green roofs. In this case, water quality has been the major driver: in the high rainfall north-west coast climate, run-off from built development causes significant flooding problems, but also pollution of rivers and water courses. Green roofs and other measures are used routinely to reduce the amount of run-off leaving built areas. Toronto City Government is heavily involved in green roof research and implementation as a means of combating the city’s urban heat island, which sees summer temperatures in the city centre soar to dangerous and uncomfortable levels. Green roofs significantly reduce the amount of heat reflected off a roof surface, and also reduce the amount of heat leaving a building. Similarly in Japan, green roofs have been given high priority at national and city level as a primary means of reducing building energy use. As is the case now in the UK, electricity demand peaks in the summer, not the winter, as a result of air conditioning requirements. Tokyo is one of the major global centres of green roof implementation, and green roofs are seen to deliver cost-savings to building operators, but also to reduce green house gas emissions.
Green roofs in the UK The initiatives within other countries have a high international profile and share one feature in common: 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. They are also all characterised by a high degree of interaction and partnership between local authorities and city governments, local research communities, the green roof industry, and community and regeneration organisations, again delivering solutions to meet regional needs. To date, there is no such example in the UK of such a coherent and integrated regional initiative to bring forward green roof infrastructure and associated sustainable building techniques. To date, most green roof developments have been on one-off projects rather than as part of wholesale strategies, 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. There is a major opportunity to establish a centre of excellence in green roof and associated technologies in the sub-region. 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. This conference cemented the working partnership between UoS and SCC, and led directly to the implementation of green roof projects in the region. For example, 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. Moreover, the close partnership that has developed with Sheffield City Council, and Groundwork Sheffield is unique in the UK in this context, and has great potential, through the contact networks of those two organisations, for rolling out throughout the sub-region to maximise knowledge exchange and technology transfer. In 2004 the Green Roof Forum was established in Sheffield, comprising Sheffield City Council, Groundwork Sheffield and the University of Sheffield and is striving to establish green roof sites in the sub region, in order to demonstrate their benefits to mainstream developers. A significant number of demonstration green roofs in the South East and London (over 60) have proved vital in gaining confidence with private developers. As a result, this area has pioneered green roof implementation in the UK.