Facts and Values Green-Blue Roofs

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On this page, we present an overview of the various sources that were used to compile the ‘Facts and Values Green-Blue Roofs’ fact sheet, produced by the Facts and Values working group for the Green Deal on Green Roofs.

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In addition to general websites and reports, you can find the sources used for each green roof benefit presented in the infographic above.

General publications about the performance of green roofs

Websites / portals

Reports

Sources for the performance of each type of green roof from the infographic

  Technical and financial

 

  Energy

Cooling and insulation

  • up to 4°C cooler inside the building
    Depending on the insulation of the roof itself, the cooling provided by a green roof can be up to 4°C. This cooling occurs due to the evaporation of water from the plants, which extracts heat from the roof. On older roofs (in the Netherlands, construction of before 1987) this can save up to 75% energy on air conditioning costs. In short: if you want to stay cool during the next heat wave, choose a green roof!
    source: Technical properties of green roofs and facades (in Dutch), N. Damen et al., (2012)
  • 75% energy saving for air conditioning
    See explanation above regarding cooling. On older roofs, green roofs also keep the heat inside the building, so less energy is needed for heating in the winter (year of construction before 1987).
    source: Technical properties of green roofs and facades (in Dutch), N. Damen et al., (2012)
  • up to 2°C cooler in the surrounding area
    Green roofs also cool the surrounding area through  evaporation and because less heat from the sun is retained than with a black roof. If half of all roofs were green, the urban heat island effect would be reduced by 2°C.
    source: Green Roofs as Urban Ecosystems: Ecological Structures, Functions, and Services, E. Oberndorfer et al., BioScience (2007)

Energy yield

  • bonus: more biodiversity underneath the solar panels
    Solar panels create shade and shelter. The diversity of microclimates on the roof ensures a wider variety in plants and attracts butterflies, bees and other insects.
    source: Interaction between PV-systems and extensive green roofs, M. Köhler et al. (2007), p14

 

  Water

Storing Water

  • store 60 – 150mm of rainwater, or 60-150 litres of water/m2 
    This storage capacity can be achieved with a green and blue roof, over 100 litres of water per m2, especially for new buildings
    source: Water storage capacity (in Dutch), SBRCURnet

Evaporating Water

  • 45 to 70% rainfall evaporates
    The plants on the roof help water to evaporate, providing cooling indoors and outdoors, and ensuring that less water runs off the roof.
    source: Green roofs further considered (in Dutch), K. Broks, et al., STOWA en RIONED (2015)
  • Savings of €5/m2 on sewage treatment in 40 years
    Every m3 of rainwater that falls on the roof and evaporates (50% on average) means €0.25 spent on water treatment costs for water purification or €5 per m2 over 40 years.
    source: Handbook Environmental prices: Methodical underpinning of indicators used for the valuation of emissions and environmental impacts. CE Delft, (2017)
    source: Symbaal zuivering, (in Dutch) STOWA, 2013-10
  • up to 50% less combined sewer overflow
    If all roofs in a neighbourhood are green, this reduction can be achieved, even when only Sedum roofs are used. The reduction is probably even greater if more storage capacity is included, such as with a green and blue roof.
    source: How blue are green roofs? (in Dutch), K. Broks et al., H2O (2015)
  • be prepared for severe rainfall, drought or heat with a micro-water management system
    With a micro-water management system, you can drain your roof in 24 to 48 hours or, for example, the day before heavy rainfall is expected.

Using Water

  • store water for garden or toilet
    Rainwater that does not evaporate can be stored on the roof in case of a drought, for use in the garden or as domestic grey water (for flushing toilets etc.). If you collect all the water that drains from a sedum roof (evaporate 45% rainfall) of 30 m2, you can flush the toilet approximately 2,100 times per year using the water collected. Starting from 6 litres per flush. During the summer, a shortage will occasionally occur, depending on how dry conditions are, and additional mains water may be required. An average family flushes the toilet about 16 times a day, which means that they can rely on rainwater for 130 days of the year.
    Rainproof shows the  possibilities and systems (in Dutch)

 

  Biodiversity

  • cultivate up to 2kg fruit and vegetables per m2
    On a green roof, it is possible to grow up to 2 kilos of fruit and vegetables per m2.
    source: Rooftop Impact Model, Rovers et al. (2016)
  • bed & breakfast for butterflies, bees and birds
    Butterflies, bees and other insects find more food when indigenous plants are planted. In turn, these insects attract birds and bats. See the nature roofs guide for tips on plant choice and providing nesting sites (available from January 2019).
    source: Making City Nature (in Dutch), J. Vink et al. (2017) p162-163
    source: Rotterdam green from above (in Dutch), (2009) p25
  • habitat as compensation for construction projects
    source: Handreiking natuurdaken (in Dutch), W. Van Heerewaarden, available from January 2019.
  • contribution to the regional ecological network
    By tailoring a nature roof for vulnerable species (insects, birds and amphibians) and ensuring a link with the ecological characteristics of the local ecosystem.
    source: Handreiking natuurdaken (in Dutch), W. Van Heerewaarden, available from January 2019.
    source: Creating green roofs for biodiversity 

 

  Health

Sound

  • 3 dB less sound reflected
    The vegetation and the substrate provide sound insulation. The thicker the layer, the greater the effect. This makes green roofs an interesting option in the vicinity of traffic noise, railways or other noise sources.
    source: Sound reduction by vegetated roof tops (green roofs): a measurement campaign, T. Van Renterghem, D. Botteldooren, (2018)
  • 10 dB less noise from outside
    The vegetation and substrate provide sound insulation. The thicker the layer, the greater the effect. A 10dB reduction is achieved with a ‘average’ sedum roof, and a reduction of up to 46dB is possible with a base layer of 200mm.
    source:  Sound reduction by vegetated roof tops (green roofs): a measurement campaign, T. Van Renterghem, D. Botteldooren, (2018)

Welfare

  • Enjoy the view for 40 seconds to improve concentration
    A study shows that after a 40-second break spent looking out onto a green roof, versus a traditional roof, those who looked out onto the green roof made fewer mistakes in their computer tasks.
    source:  40-second green roof views sustain attention, Lee e.a. (2015)
  • more user space
    There is more social cohesion from even just 1% more green space in a neighbourhood. For example, we see that a garden committee is being built on the roof of a multi-company building, and that children play and learn together in a roof park.
    source:  Not with red only; Neighbourhood green and social cohesion, nr. 2070, Alterra, (2010)
  • up to 30% fewer painkillers
    Patients use up to 30% fewer painkillers in a greener environment.
    source: Natuur en Gezondheid,Gezondheidsraad, (2004)
  • 1 day earlier out of hospital
    Patients stay on average one day less in hospital if they have a view of a green roof.
    source: Natuur en Gezondheid,Gezondheidsraad, (2004)

Air purification

  • 200 grams particles suspended in the air absorbed per m2 per year
  • Plants filter fine dust particles from the air. This effect is particularly strong for roofs with grass, but even a sedum roof will have some filtering effect. Roof systems are being developed where the plants will also play a role in filtering indoor air.
    source: Green roofs and living walls, M. Trepanier, e.a., Chronica Horticulturae, 2009. 49(2): p. 5-7

 

Sustainable Development Goals

  • contribution to Sustainable Development Goals
    In view of all the advantages mentioned above, green roofs offer an integrated sustainable solution for Health & Well-being (SDG 3), Sustainable Cities (SDG 11), Climate (SDG 13) and Living onshore (SDG 15). By merging the benefits with stakeholders involved, new Partnerships are created (SDG 17).
    source: United Nations Sustainable Development Goals

Green roofs to help to deal with water challenges as part of climate adaptation

  • Green roofs retain rainwater (water storage) and enable evaporation. As a result, less water enters sewers and requires treatment in a mixed sewer system. On average, about 850mm of rain falls every year.
  • Green roofs reduce peaks in drain water during and after downpours. The delayed drainage ensures that water is discharged more gradually and reduces street-level flooding. The number of downpours of 60-120mm/hour is increasing every year.
  • Due to the reduction of peaks in run-off water, there is also less chance of sewers overflowing. This keeps our surface water cleaner, which is healthier for plants, animals and people, and saves money. Without green roofs, 5.5% of sewer water enters the surface water due to overflow. With green roofs this is reduced to 2.2% of the annual amount of rain.

The extent to which green roofs store water and enable evaporation depends on their design features, such as the depth of the substrate layer, the type of planting used and whether there is an extra water drainage layer with or without a water management system.

Costs and water savings on four types of green roofs

sources:

 

This fact sheet has been compiled by the Dutch Green Deal on Green Roofs partners: the City of Rotterdam, Nelen & Schuurmans, Van der Tol and Vereniging Bouwwerkbegroeners in collaboration with STOWA / COP Measuring and Monitoring Green-Blue Roofs. The English version was created with the support of the European Federation of Green Roof Associations.

No rights can be derived from this fact sheet.
© Green Deal Groene Daken, 2018

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