Benefits of Green Roofs

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Green roofs have numerous social, economic and environmental benefits and can contribute positively to issues surrounding climate change, flooding, biodiversity and declining green space in urban areas.


Reducing storm-water runoff as part of a sustainable drainage systems (SuDS) strategy

Any built-up area needs to be drained of excess rain water to prevent localised flooding. Traditionally this has been achieved by a series of underground pipes connected to the sewage system. Up to 95% of the ground surface in cities is now sealed due to urban development and this is ground space through which rainwater cannot be lost by permeation. This leads to up to 75% of rainwater becoming run-off in urban areas (Ferguson Introduction to storm water: concept, purpose, design, 1998).

Traditional drainage systems are not built to cope with such instant changes in flow rate and volume, leading to the flooding of the drainage system or areas further down stream. In addition, this runoff carries nutrients, silts and hydrocarbons, chlorinated organics and heavy metals from surfaces of buildings directly into watercourses.

Global warming is leading to climate change that is predicted to cause an increase in the frequency and intensity of rainfall (Atkins et al 1999, DOE 1996, UKCIP 2001). Buildings must be designed to meet these new challenges and introducing ways to mitigate storm water runoff is increasingly becoming a requirement in new developments.  The Pitt Report by Sir Michael Pitt into the floods of 2007 states that “new developments should not expect to be automatically connected to the public drainage system” ( Learning Lessons from the 2007 Floods, Sir Michael Pitt25 June 2008) The Flood and Water Management Bill is also nearing implementation. This legislation will provide better, more comprehensive management of flood risk for people, homes and businesses. A heavy focus of the Bill is to encourage the uptake of sustainable drainage systems by removing the automatic right to connect to sewers and providing for unitary and county councils to adopt SuDS for new developments and redevelopments.
SuDS are engineered solutions that aim to mimic natural drainage systems and processes. They use permeable surfaces –soil and vegetation –to filter, absorb and moderate flows of runoff. SuDS help to reduce pollution of watercourses and localised flooding as well as providing amenity and biodiversity benefits. Green roofs are one method of controlling storm-water at source (i.e. closest to the source of the precipitation) under a SuDS strategy. Green roofs are much easier to retrofit in the urban environment than many other SuDS components, so their potential for reducing storm-water problems in the UK’s cities is significant.

Once established a green roof can significantly reduce both peak flow rates and total runoff volume of rainwater from the roof compared to a conventional roof. Green roofs store rainwater in the plants and substrate and release water back into the atmosphere through evapotranspiration.

 The amount of water that is stored on a green roof, and then evapotranspired into the atmosphere, is dependent on the depth and type of growing medium, type of drainage layer, vegetation used and regional weather. The FLL Guidelines should be followed to ensure that actual runoff will be in accordance with calculated runoff.

A green roof can easily be designed to prevent runoff from all rainfall events of up to 5 mm and as part of a SuDS strategy, should reduce the volume of surface or underground attenuation required at the site boundary. In summer, green roofs can retain 70–80% of rainfall and in winter they retain 10–35% depending on their build-up (Green roofs benefits and cost implications, Livingroofs.org In association with ecologyconsultancy, March 2004). The difference is due to a combination of more winter rainfall and less evapotranspiration by the plants because growth is not as vigorous during the winter months. 

Generally, the deeper the substrate the greater the average annual water retention. Intensive green roofs with deeper substrates can hold up to 20% of the rainfall absorbed  for up to 2 months (Osmundson 1999),

In Germany, it is recognised that a green roof will have a positive effect on storm-water runoff, and figures are provided for various substrate depths at various rainfall rates as. In the UK the Environment Agency recognises the same positive effects, however, there is no commonly agreed method for measuring the amount of attenuation volume that can be offset.
 

Roof lifespan increase

Roof surfaces are constantly under attack from ultra-violet light and temperature change. A roof can suffer from huge thermal fluctuations on its upper surface throughout the day and year, in extreme cases these can range over 100°C. (Papadopoulos and Axarli, 1992). The original green roofs in Germany were created in 1880s when it was typical to cover bitumen with 6cm of sand to protect the bitumen from fire. The sand was also found to extend the life of the waterproof layer and was colonised naturally with vegetation. Green roofs have now been shown to double if not triple the life of the waterproofing membranes contained underneath the green roof by creating a barrier which protects the waterproofing from harm.
 

Reducing energy use

Green roofs have been shown to impact positively on a building’s energy consumption by improving the roofs thermal performance, although the level of difference this makes depends on daily and seasonal weather conditions. Poorly insulated roofs lead to overheating of spaces beneath them during the summer, increasing the need for artificial cooling and excessive heating demand during the winter. By retrofitting green roofs, both air conditioning and heating usage is decreased. Flat un-vegetated gravel roofs may be up to 21ºc hotter than vegetated roofs (Kaiser 1981).  Studies carried out at Trent University under British climatic conditions have proved that planted roofs can have markedly lower temperatures throughout the roof layers compared to the unplanted roof.

During the summer, solar energy is utilised by plants for evapotranspiration, reducing the temperature of the green roof and the surrounding microclimate. During the winter months, a green roof can add to the insulating qualities of the roof.  However thermal performance is extremely dependent on the amount of water held within the green roof substrate. Water has a negative effect on thermal conductance. So in damp winter climate, such as the UK, a green roof will add little to the overall thermal performance of the roof.  Green roofs are not assigned a fixed U-Value as they assumed to hold water.
 

Climate change mitigation

In the UK, buildings are responsible for 44 percent of CO2 emissions: 26% of the UK’s emissions come from homes, 18% from non-residential buildings (UKQBC).  A high proportion of these emissions are from heating and cooling the internal environment. Reducing the energy consumption of the UK’s buildings will reduce their contribution to climate change. The IPCC (Intergovernmental Panel on Climate Change) have said that buildings provide some of the greatest, most cost effective and fastest opportunities to tackle climate change. Green roofs can significantly reduce the cooling load of a building, resulting in reduced air cooling requirements and therefore reduced energy consumption and associated output of atmospheric carbon dioxide.

Climate change adaptation

Even in the most optimistic of scenarios, whereby the emission of greenhouse gases ceased immediately, the associated climatic effects would continue due to the long lifespan of the gases residing in the atmosphere and the thermal inertia of the oceans. This means that summer temperatures and associated urban heat islands are expected to worsen. For this reason buildings must now be ‘future proofed’ so they are able to cope with these changing conditions.  Green roofs are one of the most effective ways of combating the urban heat island effect and will therefore be part of the raft of future measures designed to help cities adapt.
 

Lessening the Urban Heat Island Effect (UHIE)

The urban heat island effect is the temperature disparity between urbanised areas and surrounding rural areas. Urban landscapes have a much higher proportion of dense, dark impermeable surfaces which have a low albedo (reflectivity)  This means they absorb heat unlike plants which reflect it. This stored heat is re-radiated at night when warming the city more than the surrounding countryside.  This can make city centres up to 7ºc higher than the surrounding countryside due to the heat island effect (USEPA 1992).

The urban heat island effect will increase as summer temperatures increase and will therefore become even more of a problem in the future. During the hot summer of 2003 night-time temperatures in London reached 8–9 °C higher than outlying rural areas on a number of occasions.

Increasing biodiversity and wildlife

As urbanisation increases, ensuring that biodiversity is retained is a key requirement for local councils and public bodies under the Biodiversity Duty which is a requirement of The Natural Environment and Rural Communities (NERC) Act. It requires all public bodies to have regard to biodiversity conservation when carrying out their functions. This is commonly referred to as the ‘biodiversity duty’. (Natural England)

Whilst green roofs do not directly replace ground-based habitats and are not part of a ground level ‘green corridor’, they can be thought of as green ‘stepping stones’ for wildlife, and, if well planned, can cater for a variety of flora and fauna unattainable on traditional roofs.

Different types of green roofs support different habitats and species according to the type of vegetation and substrate they contain.  Roofs designed to either replicate the habitat for a single or limited number of plant or animal species are often referred to as Biodiverse roofs.

They can be especially important as a tool to recreate the pioneer (wasteland) communities that are sometimes lost to redevelopment. It is often neglected brownfield sites in the urban areas which are most biodiverse. The best biodiverse roofs support a range of habitats for wildlife through a range of substrates, depths and microhabitats.

Green roof designs should be varied regionally to meet the objectives of Local Biodiversity Action Plans. Green roofs can also be used to recreate habitat for some endangered species. For example in Deptford Creek in London, when habitat which was known to support the rare Black Redstart, was threatened by redevelopment, suitable habitat was created on the roofs of new buildings to compensate. Extensive green roofs have similar characteristics to brownfield land as they have well drained and nutrient poor substrate (Gedge and Kadas, 2004).

The skylark, a species listed on the UK Biodiversity Action Plan, has bred successfully on the green roof of Rolls Royce factory near Chichester.  Brenneisen studied  birds, beetles and spiders associated with green roofs in the Basel area, Switzerland. A sample of 11 roofs were found to support a total of 172 species of beetle with 10% listed in the Swiss red data book (BRENNEISEN, S., 2001. There were a total of 1844 bird sightings on the roof including Wheatear, Skylark, Lapwing, Common Tern and Mallard.

An excellent example of the biodiversity of plant life which can be achieved on a green roof is Sharrow primary School, Sheffield which has recently been declared a Local Nature reserve.  Green roof covers 2,000m2 and incorporates a variety of wildlife habitats including limestone grassland, pioneer woodland, urban brownfield meadows and a wetland area with a shallow pond.

Improving air and water quality

In the UK an estimated 24,000 people die every year from air pollution (GLA, 2001b) and air quality is substantially worse in urban areas (GLA (2001b) Clean air for London – the Mayor’s Draft Air Quality Strategy. (Greater London Authority Publications.)

The urban heat island effect exacerbates ground-level ozone production, which is formed by a reaction between volatile organic compounds and nitrous oxides catalysed by heat and sunlight. Ozone is classified as a pollutant and is the principal component of smog. During the heat wave of 2006 the European Environment Agency reported that safe levels of ozone were surpassed often in many places throughout Europe, including the UK. Green roofs can improve local air quality through mitigating the urban heat island effect. Having a living roof can also help to remove airborne particles, heavy metals and volatile organic compounds from the local atmosphere. These contaminants are retained by the green roof itself and so their infiltration into the water system through surface runoff is reduced,  improving local water quality. 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.

Another study reported a 37% reduction of sulfur dioxide and a 21% reduction in nitrous acid in the air above a green roof when compared to other air samples taken nearby (Yok Tan and Sia 2005). Other studies have estimated that green roofs can remove 0.2 kg of dust particles per year per square meter of vegetated roof (Peck, Kuhn et al. 2003). Doernach (1979) found that climbing plants can filter out dust, pollutants and even viruses.
 

Reduced sound transfer

Rain hammer on some deck roofed buildings, in particular schools have caused concern for building designers for some years.  Sound escaping from industrial procedure and other  processes can also cause issues within local communities. The combination of growing medium, plants and trapped layers of air within green roof systems can act as a sound insulation barrier.

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. The effects of adding water in the drainage layer and adding the vegetation layer seem to be insignificant.  This compares with a typical reduction of 43dB for a 100mm concrete wall. These figures suggest that a green roof can reduce sound within a building by 8dB or more when compared with a conventional roof. (Experimental study of the sound insulation of semi-extensive green roofs – Kang & Huang 2009)

Green Roofs have been employed successfully as a means of sound abatement along new runway approaches at Frankfurt International airport and Schiphol airport in Amsterdam.
 

Amenity space

Greened roof areas can add a great deal of value to buildings, with improved views making buildings easier to let. Accessible roofs designed to allow people to relax, attend events or participate in gardening can make a real difference to how people use and enjoy buildings.
 



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