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Intensive Green Roofs

Intensive green roofs are a type of green roof with deeper substrate and shrubby vegetation or even trees. They are usually accessible and can often take the shape of a garden, which also means they require more maintenance than extensive roofs. They can also include blue roof elements (e.g. rainwater irrigation or water storage features). Due to their deeper substrate, they put higher loads onto roof structures than extensive green roofs, however this also means that they have higher capacities to store water. They are often termed roof gardens.

Their effectiveness increases as plants mature and the roof ages.

 

Access

Accessible intensive green roofs can provide stress relief, space for exercise, and improve mental health.  However, access may be restricted. (4,17)

Air Quality

IGR provide high potential for removing pollutants from the air. Studies in Chicago have estimated removal of 50% O3, 27% NO2 and 7% SO2.Through their mix of different vegetation types, IGR have the potential to remove 3x as many pollutants in total compared to those with only grass. Removal depends on season, species, and local factors. (13, 15, 16, 19)

Pluvial Flooding

Intensive green roofs are considered to have an attenuation capacity of 90-100%, capturing 70+% of rainfall volume and delaying peak flows. (2, 4, 5, 6, 7, 10, 16,24)

Fluvial Flooding

Green roofs are unlikely to contribute to reducing fluvial flooding.

Climate Regulation

The carbon sequestration/storage potential depends on the vegetation used. Additionally, IGR regulate air temperature – green surface areas can reduce temperatures by up to 3 degrees. (1, 4, 8, 9, 13)

Habitat Provision

Green roofs can provide important ecological stepping stones and habitats to a number of even endangered invertebrates. Intensive green roofs face more disturbance through maintenance and use. Ecological potential can be maximised through the selection of suitable vegetation. (4, 6, 11)

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Low Flows

IGR could even need irrigation and so increase demand on water resources.

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Water Quality

Overall, they have a positive impact on water quality. Pollution reduction can exceed 90% for various metals and phosphorus up to 64%. First flush effects may occur. (2,4, 5, 13, 16)

Aesthetics

Green roofs can provide the same high aesthetic benefits as public parks or gardens, however there is little literature analysing this benefit. (6,23)

Cultural Activities

Where access is given, these places can provide settings for social bonding, strengthen communities and potentially allow cultural activities like gardening and farming. (6,17)

Property Values

Studies have mentioned increases in property value through installation of green roofs but have not quantified them.

Flood Damage

By reducing the impermeability of an urban area, green roofs can help to reduce severity of floods.

Considering the Bigger Picture

Intensive 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 effectively reduce the impermeable surface of an urban area and act to reduce runoff.

They are able to provide storage to an extent, but need further connection to drainage systems.

Green roofs can be combined with rainwater harvest systems or feature blue spaces – like ponds – that can use the collected runoff. As they cannot receive runoff from adjoining areas, their effect is on a limited scale, but cumulative effects on a wider area should not be underestimated.

Additionally, green roofs can improve wellbeing by reducing air temperature and improving air quality in urban areas.

On the left, you can find an example of how different interventions can be incorporated into the urban landscape.

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.

Costs

£100-140/m2 (high). But can increase the lifetime of roofing compared to conventional roofs by up to three times. May be higher for retrofit. However, no additional land take is required.

 

Feasibility

Domestic, Industrial, Retrofit possible. Only on flat roofs. Plants should be carefully selected to minimise irrigation and fertilisation needs.  Intensive green roofs need strong roof structures due to their higher weight.


Maintenance

Low to High. Regular inspection needed. May need irrigation and drainage systems. Due to the importance of their appearance, maintenance similar to that of parks or gardens can be required.

 

Additional Benefits

 

 

Trade-offs and Potential Dis-services

(Mental) Health

Green spaces have positive effects on physical and mental health that are related to exercise and the ability to view green/natural areas. Green roofs, can therefore be a contribution to raising quality of life especially in highly urbanised areas.

Economic /Energy savings

Green roofs can reduce temperatures in buildings (up to 75% reduction in cooling demand shown in extensive roofs, and higher for intensive). A case study in Bridgewater, Somerset (see below) estimated a fuel saving of GBP 5.20/m² per year.

Property Value

Potential negative impact on properties (shading, roots, litter), unhealthy trees can pose safety risk. Trees can also obscure views, leading to less aesthetic value and in some cases even higher perceptions of unsafety. %

Health

Allergy attacks due to pollen are possible, additionally some trees can produce VOCs and increase ozone generation. Selection of species is important as well as their placing in the urban ladscape to avoid trapping of pollutants.

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Noise Reduction

Green roofs have been shown to reduce noise. One study has shown a reduction of 8dB compared to a conventional roof.

Education

In highly dense urban environments, accessible green roofs can provide a safe and convenient outdoor learning environment that not only gives access to natural habitats but can also increase focus and wellbeing of pupils/students.

Climate Regulation

Release of VOC can have negative impacts on GHG emissions, as can fuel-intense maintenance. It is therefore important to select the right species and keep maintenance as low carbon as possible (coordinating maintenance activities, minimising them (e.g. by selecting suitable species for certain areas and so minimising their potential need for maintenance or replacement) and resorting to manual labour where possible).

References

  1. Coutts, A.M. et al., Assessing practical measures to reduce urban heat: Green and cool roofs. Building and Environment, 70, pp.266–276.
  2. Czemiel Berndtsson, J., 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.
  3. Environment Agency (2015) Cost estimation for SUDS – summary of evidence. Bristol.
  4. Forest Research, 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.
  5. Glass, C.C., Green Roof Water Quality and Quantity Monitoring,
  6. Kellagher, R., Martin, P., Jefferies, C., Bray, R., Shaffer, P., Wallingford, H. R., Woods-Ballard, B., Woods Ballard, B. (2015) The SUDS manual, CIRIA. London. Intensive green roofs require maintenance and are usually accessible. They provide good contribution to thermal performance of buildings, as well as good water retention capacity. Pollution removal is variable. Can provide great amenity benefits.
  7. Lamera, C. et al., Green roof impact on the hydrological cycle components. In EGU 10th General Assembly. p. 8038.
  8. Lehmann, S., Low carbon districts: Mitigating the urban heat island with green roof infrastructure. City, Culture and Society, 5(1), pp.1–8
  9. Liu, K.K.Y. & Baskaran, B., Thermal performance of green roofs through field evaluation, Ottawa.
  10. Mentens, J., Raes, D. & Hermy, M., 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.
  11. Oberndorfer, E., Lundholm, J., Bass, B., Coffmann, R. R., Doshi, H., Dunnett, N., Gaffin, S., Koehler, M., Liu, K. K. Y. and Rowe, B. (2007) ‘Green Roofs as Urban Ecosystems: Ecological Structures, Functions, and Services’, BioScience. Oxford University Press, 57(10), p. 823.
  12. Red Rose Forest, 2014. University of Manchester Green Roof – Green Wall Policy and Guidance, Manchester.
  13. Rowe, D.B., 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.
  14. Royal Haskoning DHV, 2012. Costs and Benefits of Sustainable Drainage Systems,
  15. Speak, A.F. et al., 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.
  16. S. Environmental Protection Agency, 2008. Green Roofs. In Reducing Urban Heat Islands: Compendium of Strategies. Wasington D.C.: U.S. Environmental Protection Agency. 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%.
  17. Wolch, J.R., Byrne, J. & Newell, J.P., Urban green space, public health, and environmental justice: The challenge of making cities “just green enough.” Landscape and Urban Planning, 125, pp.234–244.
  18. Wong, N. H., Tay, S. F., Wong, R., Ong, C. L. and Sia, A. (2003) ‘Life cycle cost analysis of rooftop gardens in Singapore’, Building and Environment, 38(3), pp. 499–509.
  19. Yang, J., Yu, Q. & Gong, P., Quantifying air pollution removal by green roofs in Chicago. Atmospheric Environment, 42(31), pp.7266–7273.
  20. http://www.thegreenroofcentre.co.uk/green_roofs/faq
  21. http://livingroofs.org/
  22. http://www.greenroofguide.co.uk/
  23. Lee, K. E., Williams, K. J. H., Sargent, L. D., Williams, N. S. G. and Johnson, K. A. (2015) ‘40-second green roof views sustain attention: The role of micro-breaks in attention restoration’, Journal of Environmental Psychology, 42, pp. 182–189.
  24. Mentens, J., Raes, D. & Hermy, M., 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.