Rotterdam's initiative to promote the creation of green roofs within the city has seen just under 10% of the roofs suitable for this converted into green roofs. The project is part of the Rotterdam Climate Initiative, run by the Rotterdam city council, Port Authority and the Environmental Protection Agency with the aim of reducing the city's CO2 emissions by 50% and helping the city adapt to climate change.

Although large areas of green roofs have many benefits for cities, such as reducing air pollution and helping to combat the heat island effect, Rotterdam's priority was for water retention, since the city has a shortage of areas where water can be stored following heavy rainfall. Water management has always been a major concern in the Netherlands, since approximately 60% of the country lies below sea level. The analysis of the potential of green roofs in Rotterdam that preceded the introduction of the subsidies focused heavily on their capacity for water storage in order to reduce peak water discharge following a rain storm and help prevent flooding.

4,623,000 square meters of roof area in Rotterdam was identified as being potentially suitable for green roofs in the initial analysis (only available in Dutch; PDF), based mainly on whether they were flat or not. A surprising amount of the roofs were flat: 70% of the residential buildings owned by housing corporations and 90% of the non-residential buildings within the study area. The maps on pages 38 to 40 of this booklet (PDF) show the buildings that could potentially have green roofs installed, together with the amount of water storage that this represents. If this strategy is to be effective in combating flooding, then a significant proportion of these green roofs actually need to be created.

To promote the installation of green roofs, an 'information point' has been set up on the (green) roof of a prominent building in the center of Rotterdam and the initiative is being promoted at festivals and community events throughout the city. Green roofs have been installed on a series of municipal buildings, including the local hospital, city library and government offices. Subsidies are provided to building owners by the local council and local water board to the tune of 30 Euros per square meter, where the average cost of installing a green roof is approximately 45 Euros per meter. Demand has been high and in March this year the minimum size of roof that could qualify for a subsidy was reduced from 40 square meters to 10 square meters.

While the Rotterdam Climate Initiative stresses the many benefits to building owners of having a green roof, such as improved insulation and an extension to the lifespan of the roof, the largest benefit to the city as a whole will be in a reduction of the flood risk. This initiative needs to be pursued at scale if it is to have a measurable impact on water retention, something that the City of Rotterdam and the water board will be monitoring over the coming two years, together with the Technical University in Delft and Hogeschool (university) Rotterdam.

Alison Killing is an architect and urbanist based in Rotterdam, the Netherlands.

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