Extensive Green Roofs
Green roofs are distinguished into two main categories: intensive and extensive. Extensive green roofs usually feature a thin layer of soil medium and plants like succulents, grasses or other low maintenance, low growing vegetation. They require little to no maintenance and are usually not accessible. By intercepting precipitation and allowing infiltration in the soil media as well as evaporation and transpiration from plants, extensive green roofs reduce the impermeable surface of an area. They are most effective in small to medium rainfall events with low intensities and longer durations.
Extensive green roofs are usually not accessible but can provide mental health benefits if they are visible from other places. (4,17)
Sedum covered green roofs can remove up to 200g PM/a/m2 from the atmosphere and provide benefits through the improvement of air quality. Different types of vegetation can account for even higher reductions. Studies have shown that 19m2 of extensive green roof can reduce pollution by the same amount as a medium sized tree. (13, 15, 16, 19)
About 50% (27-81%) of runoff can be retained in small to medium rainfall events by extensive green roofs, depending on soil thickness and vegetation characteristics. A study in Brussels has shown that greening only 10% of possible roofs would lead to overall runoff reduction of 2.7%. (2, 4, 5, 6, 7, 10, 16)
Amount of water discharging into rivers can be reduced.
Green roofs can reduce temperatures (up to 75% reduction in cooling demand shown). They impact positively on the UHI effect by lowering the air temperature (vegetated areas can decrease air temperatures by up to 3 degrees). Depending on their vegetation, they can store and sequester carbon. (1, 4, 8, 9, 13)
Green roofs can provide important ecological stepping stones for wildlife and habitats to a number of even endangered invertebrates. This depends on their design and species selection as well as maintenance. (4, 6, 11)
The capacity of green roofs to reduce pollutants is linked to their age (more mature roofs capture more pollutants), design, season (removal rates are higher in summer) and species. Overall, they have a positive impact on water quality. Studies have shown retention of PO4 of up to 80%, and retention of heavy metals of 80-99%. Sedum roofs are less effective at reducing pollution than herbaceous perennials. (2,4, 5, 13, 16)
Ext. green roofs can be designed to be aesthetically pleasing.
As they are usually not accessible, ext. green roofs have little potential to provide cultural benefits.
Studies have mentioned increases in property value through installation of green roofs but have not quantified them.
By reducing the impermeability of an urban area, green roofs can help to reduce severity of floods.
Position in the Catchment
Green roofs have the same function as any open, permeable surface: they provide interception and source control, and are therefore part of the first stages of treatment. They may be able to provide storage to an extent, but will need further connection to drainage systems.
They can be combined with rainwater harvest systems. They only receive water from the area of the roof.
To provide a comprehensive treatment and management of surface water, trees should be seen within the wider landscape. While they are able to intercept rainfall before it becomes runoff, it is important to understand that their ability to take up existing runoff and infiltrate it is limited. In close proximity to watercourses, they can slow the flow and so influence the extent of flooding.
Especially where large quantities of surface runoff are expected, for example where large impermeable areas are located upstream, trees should be complemented with additional interventions. They can be combined with raingardens that are planted in their pits or amenity lawns or detention basins that can take up additional water (options for realising these combinations can be found not only in private gardens and parks but also roadside verges or other small areas. Water can further be lead into retention ponds or wetlands to undergo additional treatment or to be stored over a longer period.
Below, you see some of the multitude of additional benefits that trees provide in the context of the urban landscape.
£55-130/m2 (medium to high). Depends on type – may be higher for retrofit. Longer life expectancy than conventional roofs (up to 3 times). Relative costs depend on area, location (and with it the accessibility of the site). Benefit of not using any additional land. (3, 6, 14, 18)
Residential and Industrial, Retrofit possible. Flat and sloping roofs are possible. Slopes however influence drainage and will lead to less water holding capacity. (6, 12)
Maintenance requirements are minimal if at all. Usually no requirement of artificial irrigation or fertilization. Invasive species removal may be required, as well as clearing of drains. (6)
Trade-offs and Potential Dis-services
The view of green roofs can provide relaxation and restoration and so have beneficial effects on the mental health of those in vicinity. Noise reduction. Green roofs can impact on acoustic transfer into and out of a building.
Economic /Energy savings
Depending on temperature, green roofs can provide substantial energy savings by cooling a building in summer (up to 75%0 and providing isolation in winter (up to 10%). Electricity savings could amount to £5.20/m2/yr. They could play an important role in adapting cities to climate change.
Runoff can include high pollution loads from green roofs than can either be a symptom of the “first flush” effect after longer dry periods, due to the vegetation or – in some cases – fertilization. Care needs to be taken to avoid this through informed design.
- Coutts, A.M. et al., 2013. Assessing practical measures to reduce urban heat: Green and cool roofs. Building and Environment, 70, pp.266–276.
- Czemiel Berndtsson, J., 2010. Green roof performance towards management of runoff water quantity and quality: A review. Ecological Engineering, 36(4), pp.351–360. Numerous studies show a runoff reduction of 27-81% for extensive roofs. Exact amount depends on rainfall intensity, substrate and drainage. Runoff water quality varies greatly but they can contribute significantly to pollutant reduction. Green roofs can be an effective tool to manage small storms in urbanised areas, but additional measures need to be taken for larger storms.
- Environmental Agency, 2015. Cost estimation for SUDS – summary of evidence, Bristol: Environmental Agency.
- Forest Research, 2010. Benefits of Green Infrastructure, Farnham: Forest Research. Extensive green roofs can reduce pollution compared to convetional roofs. They can reduce runoff by 45%, and also provide ecological services, being used by birds and invertebrates.
- Glass, C.C., 2007. Green Roof Water Quality and Quantity Monitoring,
- Kellagher, R. et al., 2015. The SUDS manual, London: CIRIA. Extensive green roofs can provide highly valuable habitats. They are mostly useful to reduce runoff in small events or the initial stages of more intense events.
- Lamera, C. et al., 2013. Green roof impact on the hydrological cycle components. In EGU 10th General Assembly. p. 8038.
- Lehmann, S., 2014. Low carbon districts: Mitigating the urban heat island with green roof infrastructure. City, Culture and Society, 5(1), pp.1–8
- Liu, K.K.Y. & Baskaran, B., 2003. Thermal performance of green roofs through field evaluation, Ottawa.
- Mentens, J., Raes, D. & Hermy, M., 2006. Green roofs as a tool for solving the rainwater runoff problem in the urbanized 21st century? Landscape and Urban Planning, 77(3), pp.217–226.
- Oberndorfer, E. et al., 2007. Green Roofs as Urban Ecosystems: Ecological Structures, Functions, and Services. BioScience, 57(10), p.823.
- Red Rose Forest, 2014. University of Manchester Green Roof – Green Wall Policy and Guidance, Manchester.
- Rowe, D.B., 2011. Green roofs as a means of pollution abatement. Environmental pollution, 159(8-9), pp.2100–10. Comprehensive literature review of peer reviewed English language literature. Up to 0.5kg of PM/m2 are removed by grassed green roofs. Intensive roofs reduce even more – vegetation plays a key role. They can also sequester carbon, however their construction is often more carbon intensive than those of conventional roofs. Green roofs can effectively retain pollutants like heavy metals by up to 99%, however this depends on their age, time of year and magnitude of rainfall.
- Royal Haskoning DHV, 2012. Costs and Benefits of Sustainable Drainage Systems,
- Speak, A.F. et al., 2012. Urban particulate pollution reduction by four species of green roof vegetation in a UK city. Atmospheric Environment, 61, pp.283–293. Green roofs can remove 0.425 (sedum roof) to 3.21g (grass) PM10/m2/a. Intensive roofs have higher impacts than extensive roofs.
- S. Environmental Protection Agency, 2008. Green Roofs. In Reducing Urban Heat Islands: Compendium of Strategies. Wasington D.C.: U.S. Environmental Protection Agency. Extensive green roofs reduce runoff by 50-75%. Studies have shown up to 75% reduction in demand for cooling, and 10% for heating (both studies carried out in Canada). They improve air quality by removing pollutants, studies having shown a removal of 0.2kg of PM/m2/a. They can also reduce heavy metals in runoff by up to 95% and reduce peak runoff as well as total runoff by 50-100%.
- Wolch, J.R., Byrne, J. & Newell, J.P., 2014. Urban green space, public health, and environmental justice: The challenge of making cities “just green enough.” Landscape and Urban Planning, 125, pp.234–244.
- Wong, N.H. et al., 2003. Life cycle cost analysis of rooftop gardens in Singapore. Building and Environment, 38(3), pp.499–509.
- Yang, J., Yu, Q. & Gong, P., 2008. Quantifying air pollution removal by green roofs in Chicago. Atmospheric Environment, 42(31), pp.7266–7273.