Green Roofs - Did the Teletubbies have it right?
by Stefan Harris
Green roofs have been around for centuries. From the traditional Scandinavian sod roofs to the UK utilising green roofs to camouflage military buildings during the second world war. Green roofs are far more than just an adornment to an eco-house.
How do green roofs benefit the environment? They increase biodiversity, help manage storm water, improve air quality, and reduce the urban heat island effect. They can help manage the energy use within buildings, reducing the building carbon footprint and help mitigate the effects of climate change.
What does the research say
Rain water Retention
Worldwide, urban spaces are dominated by non-porous surfaces. Urban space is defined as cities, towns and wider conurbations. Of this urban space only 30% is classified as natural land cover . The remaining space is typically non-porous roads, pavements and roofs. Roofs account for nearly 20–25% of the overall urban surface area  and play a significant role in contributing surface water run-off. High rates of surface water run-off during storms increase the chances of flooding and sewage overflowing into UK waterways. 
Green roofs on the other hand substantially reduce surface water run-off. They retain on average 62% of the rain which falls on the roof  which results in an important delay to the peak storm water flow which, if left unmanaged, causes flooding.
Additionally, green roofs work as a multi-functional sustainable drainage solution. Typically, urban rainwater attenuation solutions consist simply of underground storage tanks. Green roofs offer not just rain water retention, but biodiversity and other benefits. 
Green roofs have a great potential value for biodiversity, especially for supporting pollinators and other invertebrates. A study of 5 green roofs in London found species of spiders and beetles, some of which are nationally rare, as well as finding snails, aphids and ladybirds.  Ground nesting birds are also known to nest on green roofs. However, it is not fully understood whether or not green roofs can provide sufficient long-term prey for successful breeding. 
Green roofs do not typically carry a U-value in the same way other building products would. Moisture content, soil density, vegetation type etc all affect the performance. These variables in performance means it is incredibly difficult to accurately determine thermal conductivity.
However, studies have shown that green roofs can help the thermal performance of a building.  Researchers compared the heat loss during the heating period of two sections of a building’s roof; one, a green roof and the other a traditional bitumen roof. Between October to March the green roof lost on average 26% less heat than the control roof.
In a separate study, during the heating months, the green roof was found to lose 8% less heat than the control roof and gain 75% less heat than the control roof during the cooling months. 
The performance of the green roof is not down to the thermal conductivity but other factors; The vegetation and soil keeps solar radiation from affecting the surface of the roof; the soil creates a buffer against heat fluctuations; and evapotranspiration of the moisture within the green roof cools the building internally. These factors both help the building stay warm or stay cool.
Cools the city
Large urban areas create a phenomenon called the Urban Heat Island. Typically, urban areas are warmer than surrounding rural areas due to the absorption of solar radiation by man-made surfaces such as roads, pavements, walls and roofs. This leads to higher cooling energy consumption, greater thermal discomfort, in extreme cases heat related illnesses and can cause an increase in air pollution . Green roofs can help lessen this heat island effect, with land surface temperatures dropping by almost 1°C. 
Green roofs have also been found to cool the buildings beneath them. Research found that the temperatures could be drastically different between inside and outside. 12°C of cooling was observed by a study in Italy and a huge 27°C in Texas compared to conventional roofs . The use of green roofs in these cases could drastically improve the thermal comfort of the internal environment and negate the need for energy intensive mechanical cooling.
Who’s taking green roofs seriously?
Germany has been utilising green roof technology in some form since the turn of the 20th century . Modern green roofs had their renaissance in Germany in the 1970s when a substantial amount of research was conducted. This research looked into root-repelling agents, waterproof membranes, drainage, light-weight growing media and plants. The development of the green roof market expanded quickly in the 1980s, with average annual growth of 15-20%. Much of this growth was down to government support . Fast forward to today and the support continues.
Hamburg’s green roof strategy
Hamburg is the first German city to develop a comprehensive Green Roof Strategy . Starting in 2014 the plan was to install 100 hectares of green roofs in the metropolitan area over the next 10 years. The Hamburg Ministry for Environment and Energy is providing financial support of €3 million for the creation of green roofs.
In 2014 there was already 80 hectares of green roof in Hamburg and they were on track to have 168 hectares in 2020.
The €3 million funding covers up to 40% of the installation costs for a green roof to a maximum value of €100 000. To qualify for the funding the roof must be larger than 20m², the soil layer must be thicker than 12 cm and the green roof must be a voluntary decision of the house owner.
“The combination of regulation, dialogue, financial incentives and science advice, and evaluation is key to successful urban nature-based solution (NBS) implementation.” 
Hamburg’s Green Roof Strategy is still awaiting further evidence to support the success of the scheme but given the evidence from other projects and research, the benefits of the Hamburg scheme are likely to be positive.
Department for Education output specification
The benefits of green roofs have been accepted by the Department for Education. The November 2021 Output specification lists Green Bio-Solar roofs as a requirement on all secondary but only on primary schools, where there is a lack of green infrastructure. 
The output specification calls for the green roofs to be installed in accordance with the German FFL, recognising the expertise Germany holds when it comes to green roofs. It also references the UKs GRO Code of best practice. The Green Roof Organisation, GRO is an independent organisation set up to promote green roofs within the UK.
There are many benefits now being recognised by government departments. An initiative from the other government departments would go a long way to helping install more green roofs on more buildings across the UK.
Given the on-going effects climate change is having on the planet, every route must be explored to help mitigate these effects. Other than the DfE there are no specific policies aimed at adding green roofs to buildings in the UK.
On the 23rd May 2022, the government urged the UK public to help bees, butterflies and other pollinators  and set out an action plan to address the needs of pollinators . This action plan does not mention green roofs specifically but shows the government commitment to tackling the problem. Green roofs could easily be a part of the solution.
Given the role green roofs can play and how simple they are to implement, creating specific green roof policies would be an easy win, and offer significant potential for positive improvements.
Nature-based solutions in UK climate adaptation policy  is a report commissioned by the RSPB and WWF. It summarises evidence on the ways in which Nature-based solutions (NBS) can address help mitigate the challenges faced by climate change.
The report goes into extensive detail around many different nature-based solutions including the important role green roofs can play, especially with regards to flood reduction, urban cooling and biodiversity.
The report concludes with a set of recommendations. These include introducing a wider set of national adaptation plans, developing more coherent policy, funding high quality NBS and setting standards for high quality NBS.
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- DEFRA, “Evidence review of factors contributing to surface water flooding from Section 19 LLFA reports,” 21 10 2015. [Online]. Available: https://assets.publishing.serv....
- X. Zhenga, “Green roofs for stormwater runoff retention: A global quantitative synthesis of the performance,” 07 04 2021. [Online]. Available: https://www.sciencedirect.com/... .
- CIWEM, L. Grant, A. Chisholm and R. Benwell, “A PLACE FOR SuDS,” 03 2017. [Online]. Available: https://www.ciwem.org/assets/p....
- Buglife – The Invertebrate Conservation Trust, “Creating Green Roofs for Invertebrates A Best Practice Guide,” [Online]. Available: https://cdn.buglife.org.uk/201....
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- J. Dong, M. Lin, J. Zuo, T. Lin and J. Liu, “Quantitative study on the cooling effect of green roofs in a high-density urban Area—A case study of Xiamen, China,” 30 01 2020. [Online]. Available: https://www.sciencedirect.com/....
- European Federation of Green Roof and Green Wall Association, “Living Roofs and Walls from policy to practice,” 2019. [Online]. Available: https://livingroofs.org/wp-con... .
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- D. H. Bornholdt, “On your roofs, get set, green! – Hamburgs green roof strategy implement NBS at different level,” [Online]. Available: https://oppla.eu/casestudy/212.....
- D. Rizzi, S. Utkarsh and R. R. Vallejo, “GREEN ROOF STRATEGY OF HAMBURG,” [Online]. Available: https://www.regreen-project.eu....
- DfE, “School Output Specification Tecnhical Annex 2C: External Fabric,” 11 2021. [Online]. Available: https://assets.publishing.serv....
- Department for Environment, Food & Rural Affairs , “Public urged to help bees, butterflies and other pollinators,” 23 23 2022. [Online]. Available: https://www.gov.uk/government/....
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