Reduce water run-off from roofs
Green roofs are a type of natural flood management. In urban areas roofs make up large areas of hard standing and runoff can be huge. The plants on a green roof use up a small amount of water, however the substrate (soil) beneath them will soak up a significant amount of water. The saturated soil will slowly release water off the roof, but the green roof slows the process down reducing the risk of a flash flood. A green roof can retain around 75% of the runoff in summer (source: livingroofs.org). Green roofs alone would not prevent flooding. However as part of a wider rainwater management scheme they can be a useful contribution.