Storm Water Attenuation



Green roofs absorb rainfall that would normally run straight off hard surfaces and help with storm water attenuation. Green roofs have an important role to play in reducing urban flooding.

Water landing on man-made hard surfaces cannot be absorbed and runs off through drainage systems to rivers. Around 75% of rainfall on towns and cities is lost directly as surface run-off compared to around 5% in a forested area. The outcome is that rivers flood.

Heavy run off can result in large areas of polluted water, when sewage treatment plants cannot cope, as well as erosion problems. Roofs represent nearly half of the impermeable surfaces in urban areas and so have a key role to play in sustainable urban drainage systems.

Lindum green roofs are specifically designed to maximise storm water attenuation benefits. Lindum’s wildflower and sedum vegetation mats require deeper substrates of 75mm-100mm which will give average annual water retention of 50% (based on average annual rainfall of 650-800mm). Lindum’s deeper substrate retains more water than a shallow substrate system – increasing the storm water attenuation value by approximately 20 litres per square metre at a substrate depth of 100mm

Lindum’s drainage layer Roofdrain 20 acts as a water reservoir holding an additional 5.3 litres of water per square metre to further reduce the runoff from the roof. This stored water can be released slowly, used as grey water or even used as irrigation for the vegetation mat. 

The graph below shows run-off from a conventional flat roof and an extensive green roof over a 22 hour period, highlighting that green roofs not only create a time delay in storm water runoff but also a green roof will capture most of the water that hits it. (Source: Dunnett & Kingsbury 2004)    

Storm Water Runoff from a conventional flat roof and an Extensive Green Roof over a 22 hour period


How Green Roofs reduce water runoff

  • Absorbed by the substrate
  • Taken up by the plants and either stored in plant tissues or transpired back into the atmosphere
  • Stored in drainage system on the roof acting as a buffer
  • Water absorbed by the green roof is released over time evening out the flow after heavy storms
  • Improved quality of water runoff by reducing pollutant release and water acidity