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Blue-green roof

When it rains, where does the water go? How can we prevent cities from getting flooded?

Diagram of blue-green roofs in the urban landscape.

When rain falls onto a city like London, the water runs off a range of hard surfaces like roofs and pavements and then runs down the drain. When rainfall is extremely heavy, the rainwater can overwhelm London’s Victorian sewer system and cause major problems to the river Thames and the wider environment. We can help this problem by slowing down the journey of the water by holding it on our roof rather than it washing away instantly from hard, impermeable surfaces.

Part of the Institute of Physics building is covered by a ‘blue-green roof’ – green because plants can grow on the flat roof and blue because it collects and stores rainwater before being discharged into the drains.

Diagram showing the different layers of a blue-green roof.


  • Excess rainwater that’s been stored can be released into the drains more slowly, so reducing the risk of flooding.
  • Plants growing on the roof insulate the top of the building which reduces the heat leaving the building in winter and so reduces the heating bill.
  • Insects and birds are attracted to the roof, increasing biodiversity in the city


  • Discover more about the physics behind the IOP’s King’s Cross home in a feature from Physics Review (PDF, 1MB). (Originally published November 2019 and reproduced with kind permission of Hodder Education)