Sustainability

The importance of thermal benefits from green roofs is greatest in the summer months, reducing solar gain and the need for air conditioning (and associated energy use). Their value associated with winter heating reductions is less clear, and depends upon the climatic conditions. Whilst such benefits should not be included in 'U' value calculations, numerous studies have highlighted energy reductions, with a conservative estimate being 11.25kWh/m²/year.

Studies have also shown that, when installed in the vicinity of photovoltaic panels, green roofs can increase the efficiency of the PV units, potentially encouraging increased uptake of this renewable energy source.

The aforementioned 11.25kWh/m²/year energy reduction would equate to savings of 4.84kg CO₂/m²/year. In addition, the planting on green roofs actively consumes CO₂ during the process of photosynthesis. Wider uptake of green roofs would also mitigate climate change through localized reductions in urban air temperatures.

Green roofs can retain stormwater through storage in and evapotranspiration from the green roof's plant leaves, substrate and drainage layers; typically reducing runoff volumes by between 30 - 100%, depending upon the incidence of rainfall, local climate and green roof configuration.

The detention of stormwater - associated with infiltration - also results in reduced peak rates of runoff. Once saturated, these attenuation benefits are lessened (but still important), however they will continue to offer benefits to the quality of runoff - filtering contaminants (e.g. nitrogen) and metals (e.g. zinc, copper) from the rainwater and restricting the transmission of pollutants into ground waters.

The variety of flora and fauna of green roofs can provide nesting and resting places, habitat and food for creatures displaced by urban development (i.e. spiders, beetles, butterflies, birds etc). With appropriate design, studies have shown green roof substrates to contain collembolan densities similar to those expected in ground level soils.

Certain species of bird (i.e. swift, housemartin, black redstart, wagtail) are known to seek urban habitat and green roofs are acknowledged to have great potential to provide habitat for such birds - 9 species being listed in biodiversity action plans.

Green roofs filter polluted air; trapping particulate matter and capturing gases (e.g. nitrogen oxide, ozone, sulphur dioxide and carbon monoxide) to improve urban air quality, such that green roofs could form part of Local Authorities' commitments in Air Quality Management Areas (AQMA).

Equally, the urban heat island effect - whereby urban temperatures are higher than surrounding rural areas due to sensible heat storage in hard building surfaces - can be reduced by the absorptive properties of green roofs, providing cooler and healthier urban environments.

Green roofs are a sustainable design solution to counter the loss of green space at ground level. Particularly in urban areas, where recreation space is scarce, green roofs can provide valuable outdoor space for business, pleasure or relaxation. Providing such contact with nature is believed to have positive health effects.