Extensive planting within cities is now widely recognised as a means of improving air quality. Therefore, green roofs contribute to the reduction of a number of polluting air particles and compounds not only through the plants themselves, but also by deposition in the growing medium itself.
Plants reduce carbon dioxide in the atmosphere and produce oxygen Green roofs reduce the heat island effect, which is the main cause of ozone production Plant roofs remove heavy metals, airborne particles and volatile organic compounds Being absorbed into the green roof system these polluting particles do not enter the water system through surface run off leading to improvement in water quality Although green roofs are recognised as playing a positive role in improving air quality, this is linked to the positive effect they have on the urban heat island effect. Individual roofs in themselves will not have a great effect. However a large areas of green roofs in specific areas of large cities or in Air Quality Management Zones would have a noticeable effect. Livingroofs.org understands that in the UK, the Greater London Authority's Climate Change Adaptability Team are modeling the urban heat island effect in London to establish areas where action will be required in the future to adapt to climate change. Green roofs will certainly be a pragmatic and in some cases the only means of adapting the urban fabric in London, especially central London, to climate change. As 28 times the size of Richmond Park has the potential to be retro fitted with green roofs in London, such an area of green roofs would have a significant positive impact on the urban heat island effect and resulting in improved air quality.
Green roofs are intrinsically of greater benefit to biodiversity than more traditional roofing methods. Many green roof manufacturers promote green roofs as benefiting wildlife, but with little evidence to demonstrate this. Of course 'off the shelf' green roof systems do provide benefits for wildlife compared to non-green roofs, but research in Switzerland and in London shows that green roofs need to be designed to meet specific local biodiversity conservation objectives.
Swiss research Detailed research into biodiversity and green roofs has been undertaken since 1997. This research was specifically driven by concern over the impacts of new developments on brownfield land in the city. Such land has been recognised as important for a number of national scarce beetles and rare spiders. These species were originally associated with Rhineland alluvial gravel habitat, little of which remains. These species had found refuge on brownfields in Northern Switzerland. Could green roofs be better designed to provide refuge for such species as new developments encroached on their habitats?
A number of design principles were arrived at:
The placing of objects associated with natural habitats such as dead wood and old branches increased the biodiversity of the roofs. Around the same time the term 'brown' roof was coined to ensure that where green roofs are to be placed in developments as mitigation for brownfield biodiversity issues, these roofs would not use 'off the shelf' solutions but be designed specifically for the biodiversity that is to be mitigated for.
Green roof systems are recognised as providing greater thermal performance and roof insulation for the buildings they are laid on. This can vary depending on the time of the year, and the amount of water held within the system.
Cooling [summer] 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 [photosynthesis and evapotranspiration] and soil processes [evapo-transmission] 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 the following:
Mean daily temperature 18.4C
Temperature beneath membrane of normal roof 32C
Temperature beneath membrane of green roof 17.1C
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.
Thermal Insulation [winter] 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.
Mean Temperature 0C
Temperature under membrane standard roof 0.2C
Temperature under membrane green roof 4.7C
This shows that green roofs do have the ability to affect the temperature and insulation properties of roofs, though this is variable due to the daily conditions of the green roof. The potential for the cooling and thermal insulation properties of green roofs can have costs benefits for building owners/managers.
The value of green spaces to people living and working in towns and cities has increasingly been recognised by Government. The work of the Urban Green Spaces Taskforce (Green Spaces, Better Places, 2002) demonstrated the various benefits that green space provide, such as ecological function, visually softening the built environment, supporting biodiversity, aiding people's mental and physical health, and providing a communal focus and sense of place. Government has subsequently launched a raft of new policies, initiatives and funding to promote the good design and management of green spaces.
English Nature has published research that suggests that an accessible natural green space should be no more than 300 metres from where anyone lives in order to meet people's needs for contact with nature. Evidence suggests that regular direct contact with natural green space (and elements of the natural world such as birdsong and seasonal colour change) is good for people's health (see below).
There is a need for increased densities in urban residential development (>30 dwellings per hectare), which could result in terrestrial green space being reduced or lost. In the urban core the provision of green space is usually already severely limited, partly through historical circumstances, and more recently very high land values; this makes the creation of new green space both important and difficult. Given the nature and pressures of urban regeneration, the creation of new spaces has to meet a number of interests; these generally result in highly formal spaces with little ecological benefit. Creating low-maintenance, terrestrial, naturalistic green spaces in the urban core is not popular; green roofs may provide one solution. Green roofs can provide both visually accessible and physically accessible green space. Roofs are largely visually 'dead' and unappealing and their appearance to those overlooking them can be softened by vegetation. There are instances where the sole justification of a green roof installation is for visual aesthetics. Areas of green roofs can also provide accessible space for people to enjoy, and some can be landscaped to extend existing green space, for example at Canary Wharf station on the Isle of Dogs, London. Roof gardens and terraces are options for smaller buildings and have some historical ancestry. The Berlin roof gardens of the 19th century, have been adopted on similar housing blocks in Britain (for example, Peabody Trust's Balderton Flats in Mayfair) and were one of the inspirations for the first modern green roof in the UK, at Derry & Toms, Kensington, 1938, which still serves as a garden, albeit with limited public access. Roof gardens are increasingly being proposed for new office and housing developments. Large areas of accessible green roof space can be created if the building is large enough, for example above Cannon Street Station in the City of London [intensive], and at Chicago City Hall [extensive]. More 'extreme' examples include a golf course on a roof in the USA. The key issues that need to be considered in providing accessible open space are health & safety (the need for a external rail or fence), over-looking neighbouring properties (a material planning consideration), access to and from the roof-space, load-bearing (if the proposed numbers of people are to be more than a few), and management. The existence of green roofs that provide this function suggest that these issues can usually be easily addressed.
There is a growing body of evidence that the visual and physical contact with natural greenery provides a range of benefits to people. These include both mental benefits (such as reduction of stress) and physical benefits (including the provision of cleaner air). Access to green space can bring about direct reductions in a person's heart rate and blood-pressure, and can aid general well-being. A Texan study of post-surgery recovery in hospitals demonstrated that recovery was quicker and with less chance of relapse if patients could look out onto green space. A number of American hospitals have subsequently been redesigned to bring these benefits to patients, and have been rewarded with greater patient 'through-put'. A roof on the Kanton Hospital in Basel was redesigned 20 years ago by vegetating it, because it was felt that patients in intensive care would benefit from looking out onto this rather than the grey-space of before. A few community hospitals in the UK are now being designed with a greater consideration of green-space provision, and the good-practice work on hospital design being developed by Commission for Architecture and the Built Environment [CABE] is likely to further this.
The thermal benefits that green roofs provide may also have indirect benefits for people living or working within the buildings. This has not been researched, but anecdotal evidence from Germany in the late 1990s is of interest. In a survey of staff absence from sickness at the Bundepost offices in Stuttgart, it was shown that staff in one building demonstrated significantly lower absences than those in others. The only change in the 4-year period that could be identified was that one of the buildings was given a green roof; this building supported lower staff sickness levels. It is possible that the green roof reduced the fluctuation of daily mean temperatures within the upper levels of the building, and/or the vegetation helped cool and moisturise in-going air near ventilation ducts.
In the past 2-3 years, possibly picking up on the increasing interest in green roofs and Government's interest in green space, developers are increasingly showing green roof space as a component of their new commercial development proposals. The provision of specific accessible green roof space for future workers appears to be gaining currency, and could help off-set the likely constraints of green space provision on the ground.
The urban heat island effect is the difference in temperature between urban areas and the surrounding countryside. In large cities this can as much as a 5oC difference between the city centre and the rural environs. Urban areas have large areas of hard reflective surfaces. This is referred to as the albedo effect. These surfaces absorb solar radiation and reflect this heat back into the atmosphere. Any reduction in this effect can have a positive effect on smog and airborne particles in the atmosphere.
Roof areas are a significant part of urban hard surfaces. Plants on green surfaces absorb heat and then use it through evapotranspiration. Green roofs therefore would play an important role in reducing urban temperatures, and subsequent improvements in air pollution/smog, as associated with the albedo effect.
In Tokyo the albedo effect increases the humidity within the city and this, with increased air pollution, is one of the main reasons for the growing tendency for very complex intensive green roofs on many buildings in that City.
In many parts of the USA there is growing interest in this benefit of green roofs. Research by NASA in Atlanta has compared temperatures of different surfaces. On a typical Atlanta day with maximum air temperature of 25C (77F) the following temperatures were recorded:
Tree shaded grass 28C
Tree canopy 21C
Asphalt in full sun 50C
Membrane roof surface 52C
Research at Trent University has found on a typical day with a temperature of 18.4oC a normal roof surface temperature was 32oC while that of a green roof was 15oC.
The reduction in the protection of photochemical smog and subsequent improvements in air quality needs to be recognised as a powerful planning 'tool' and potential mitigation for polluting developments. Local Authorities may include green roof plans as part of their commitment to Air Quality Management Areas [AQMA].
The combination of soil, plants and trapped layers of air within green roof systems can act as a sound insulation barrier. Sound waves are absorbed, reflected or deflected. The growing medium tends to block lower sound frequencies whilst the plants block higher frequencies.The amount of sound insulation is dependent on the system used and the substrate depth. A green roof with a 12 cm substrate layer can reduce sound by 40dB and one of 20 cm by 46-50dB. A study by Kalzip [www.kalzip.co.uk] compared sound insulation of their standard unvegetated roof system with that of the Kalzip vegetated 'NatureRoof':
Standard Unvegetated 33dB
Vegetated [dry] 41dB
Vegetated [wet] 51dB
100mm Concrete Wall 43dB
This suggests that a green roof can reduce sound by 8dB compared with a conventional roof system. This could be particularly important in areas of high noise pollution such in the approaches to airports, as these levels are sufficient to provide noise insulation to buildings under aircraft flight paths.
A number of materials used in green roofs are from recycled sources, such as the membranes and growing mediums, such as crushed porous brick, which is used by some suppliers. In London, uniquely, there has been a move to use recycled secondary aggregate as the growing medium, preferably from the original site.
This reduces the need for waste disposal to landfill and reduces the transport miles/distances for used for disposal of waste. This meets UK government targets for the reuse of secondary aggregates and where reuse from site can reduce the impact of lorries in terms of importation and exportation of materials.
Green roofs store rainwater in the plants and growing mediums and evaporate water into the atmosphere. The amount of water that is stored on a green roof and evaporated back is dependent on the growing medium, its depth and the type of plants used. In summer green roofs can retain 70-80% of rainfall and in winter they retain between 25-40%.
In Germany, the world leader in green roofs, 25 million m2 of green roofs were installed between 2000 and 2001. This area is primarily down to legal requirements in certain 'landers' for roofs to be installed for their benefits in alleviating storm water run off. In Portland, Oregon one of the leading cities in USA for installing green roofs green roof policies are being driven over concerns of storm water run off and the consequences of it on water quality in rivers, and therefore the continued health of rivers for salmon [a key cultural indicator].
Green roofs also reduce and delay run off during times of heavy and prolonged precipitation. A study in Germany has shown that during a 10mm rainstorm, 200 litres of rainwater fell on an 18m2 extensive green roof and only 15 litres actually passed from the roof to the ground.
Green roofs, therefore, reduce the impact of run off on the storm water drainage system, and reduce the likelihood of local flooding.