Just a thought:
http://www.ecogreenroofs.co.uk/benefits/
"Green roofs provide many benefits, not only to the environment, but also in an aesthetic and ecomomical way. Here are some of the major benefits having a green roof installed onto your property or building.
Storm Water Run Off
Vegetated roofs are able to retain on average 82 % of rainfall compared to 27 % on pure gravel rooftops; and this is even better on roofs with a 2 % slope (up to 87 % retention). (Nicholaus et al. 2005).
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Habitat for rare wildlife
Green roofs provide a stepping stone for many rare birds endangered by the lack of suitable habitats. They also provide a home for many rare insects, beetles, spiders and plant species.
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Sound Insulation
University of Sheffield research shows that a 20 - 100mm dry growing medium could achieve an extra SPL (Sound Pressure Level) attenuation of 10 - 40dB, depending on frequency.
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Air quality
Studies have suggested a 37% reduction of sulphur dioxide (that causes acid rain) and a 21% reduction in nitrous acid can be achieved in the air above a green roof, when compared to other air samples taken nearby (Yok Tan and Sia 2005.)
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Water Quality
Water is cleaned when passing through a green roof system. Kohler and Schmidt (1990) found that 60% of cadmium, copper and lead, and approximately 16% of zinc, can be removed from rainwater on passing through a green roof.
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Air conditioning bills
Publications have indicated that the need for air conditioning in summer months can actually be reduced by 6 % and, combined with a 1-2 % reduction of the urban heat island effect; greened cities could see electrical demand come down by 5 % (Booth 2006).
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Heating Bills
Heating bills can also reduced by a green roof, with an average saving of around 4 %.
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Roof life Span
Studies have proven that a green roof can increase a roof’s lifespan by up to 3 times. There are a couple of examples where the green roof has not needed to be replaced for over 100 years! The green roof protects the waterproofing from UV rays, frost and ice.
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Thermal Benefits
Investigations into the thermal properties of green roofs have revealed that plants themselves significantly reduce air temperatures both inside the building and in the immediate environment (Niachou et al. 2001). Experiments in Toronto, Canada, comparing common flat roofs, painted white roofs and green roofs found that, on average, roof temperatures on dry summer days were 65˚C, 42˚C and 35˚C respectively.
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Planning Permission
In some cases a green roof factored into a new building’s design can help you gain planning permission. If green roofs are for an existing building then planning permission for the green roof itself is very rarely needed."