Menu Close


Log in to personalise your experience and connect with IOP.

Blue-green roof

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

Blue-green roofs help towns and cities to manage excess rainwater, keep buildings cool and support urban wildlife.

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 the sewer system and cause major problems to the wider environment. We can help this problem by slowing down the journey of the water by holding it on the roof rather than it washing away instantly from hard, impermeable surfaces.

At the Institute of Physics building in London, part of the roof is covered by what's known as 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.

Blue green roof layers from the top include: plants, growing medium, fibre fleece, drainage medium, waterproofing and concrete.


  • 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.