Green Roofs in the Sustainable Built Environment

Recent and ongoing urban regeneration programs are leading to increased population densities in the UK's major towns and cities. The associated reduction in soft landscaped areas creates issues that require considered, sustainable design, notably in respect of:

  1. Energy consumption
  2. Climate change
  3. Sustainable drainage and water quality
  4. Biological diversity
  5. Air quality and urban heat islands
  6. Amenity space

Green roof installations can redress this imbalance, to the benefit of the building owner and the wider community. Accordingly, cities worldwide have implemented policies to promote, incentivize or enforce green roof installations on new developments.

In the UK, cities such as London and Sheffield are at the forefront of innovative policy development. Whilst there is no UK national policy directly covering green roofs, they are beneficial in delivering important government policy /strategy objectives:

Greenroofs _sustainability _1

Energy consumption

ObjectivesPolicy / Strategy DocumentTargets
Reduced energy consumption ODPM (2006) Approved Document L2a & L2b To reduce energy consumption in buildings by 25% (vs ADL 2002)
Increased use of renewable energy The Energy White Paper (2007) 10% of UK Electricity to be from renewable sources by 2010 & 20% by 2020

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.

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Climate Change Mitigation

ObjectivesPolicy / Strategy DocumentTargets
CO2 Emissions Reductions Kyoto Protocol 60% reduction by 2050 (vs 1990)
BERR (2008) Strategy for Sustainable Construction
  • 26% reduction by 2020 (vs 1990)
  • Zero Carbon Homes by 2016
  • Zero Carbon Schools by 2016
  • Zero Carbon Public Non-domestic buildings by 2018
  • Zero Carbon Other Non-domestic buildings by 2019
UK Framework Indicators Indicator 1 - Reduced Greenhouse gas emissions
Carbon Emissions Reduction Target To achieve annual net savings of 4.2Mto CO2

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.

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Sustainable Drainage & Water Quality

ObjectivesPolicy / Strategy DocumentTargets
Flood Risk Mitigation DEFRA (2008) Government Strategy for Water - Vision 2030 Land increasingly flexibly managed for flood storage
UK Framework Indicators Indicator 31 - reduce no. of properties at risk of flooding
Good water quality EU Water Framework Directive 2000/60/EC All surface water and groundwater to be classified as "good status" by 2015
DEFRA (2002) A Biodiversity Strategy for England Biological quality of rivers in England (Indicator H7)
Nutrient levels in rivers & lakes (W5)
DEFRA (2008) Government Strategy for Water - Vision 2030 Large majority of water bodies having good ecological & chemical status
Healthy rivers, lakes, estuaries, coasts & groundwaters
UK Framework Indicators Indicator 30 - River quality

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.

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Biological Diversity

ObjectivesPolicy / Strategy DocumentTargets
To protect habitats and species UN World Summit on Sustainable Development (2002) Significant reduction in rate of biodiversity loss by 2010
UK Framework Indicators Indicator 17 - Priority species & habitat priorities
Convention on the Conservation of European Wildlife & Natural Habitats - 82/72/EEC To protect 500+ plants and 1,000+ animal species
Habitats Directive - 92/43/EEC To ensure the restoration / maintenance of natural habitats
Convention on the Conservation of Migratory Species of Wild Animals (Bonn Convention / CMS) To conserve / restore habitats where migratory wild species live
DEFRA (2002) A Biodiversity Strategy for England Develop Action Plans relevant to urban areas that facilitate:
  • HABITATS: Lowland heaths; Wood pastures; Parklands; &
  • SPECIES: stag beetle, song thrush & bats
To protect bird population Council Directive 79/409/EEC on conservation of wild birds ("Birds Directive") To maintain sufficient diversity of habitat for all European bird species
DEFRA (2002) A Biodiversity Strategy for England Populations of birds in towns & gardens (T3)
UK Framework Indicators Indicator 20 - Bird Populations
DEFRA PSA 3 To reverse long-term decline of farmland birds by 2020

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.

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Air Quality and Heat Islands

ObjectivesPolicy / Strategy DocumentTargets
To deliver better air quality EC Directive 96/62;
National Emissions Ceiling Directive (2001/81)
DEFRA (2007) The Air Quality Strategy
Sets UK national emissions ceilings for:
  • Sulphur dioxide (SO2) - 585 kilotonnes
  • Nitrogen oxides (NOx) - 1167 kilotonnes
  • Volatile Organic Compounds (VOC) - 1200 kilotonnes
  • Ammonia (NH3) - 297 kilotonnes
UK Framework Indicators Indicator 29 - Emissions of air pollutants (NH3, Nox, PM10, SO2)

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.

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Amenity Space

ObjectivesPolicy / Strategy DocumentTargets
Access to Green Space DEFRA (2002) A Biodiversity Strategy for England Ease of access to local green space & countryside (T4)
UK Framework Indicators Indicator 68 - access to green space

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.

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