Rainwater management on green roofs

2 min read

Preventing Floods

Rainwater management and green roofs….2012 was one of the wettest years ever recorded in Britain. Many homes were destroyed in severe floods caused by heavy rainfall and inadequate drainage systems.

In urban and built up areas:

  • Up to 75% of rainwater becomes run-off.
  • Most rainwater run-off is pumped directly into streams, rivers, and lakes.
  • The run-off carries pollutants into open bodies of water having a negative effect on river and pond life.

However, in rural areas, there is less run off as almost all rainwater is absorbed by soils and local plant life. Some of this water will eventually make its way into rivers and streams. On its way it will be filtered by the earth removing most harmful pollutants.

Green Roofs can be effective in both urban and rural areas as a solution to managing rainwater drainage.

Up to 75% of rainfall is absorbed by an extensive living roof, and up to 90% is absorbed by an intensive green roof system.

Excess water drips from the roof gradually so drains are not overwhelmed by a sudden deluge.

A green roof will filter a large amount of pollutants from the water before releasing it – particularly heavy metals

A living roof works in the same way as the natural water cycle.

Rainwater can be stored in the roofing system for up to two months. From here, it is used by the green roof plants and released gradually into the atmosphere by evapotranspiration.

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Green roofs reduce the likelihood of overloading existing drainage systems, thus reducing the risk of flooding.

In the long term up to 45% of yearly rainfall can be completely absorbed by an extensive green roof system. With an intensive system this can be as much as 75%.

Despite having the potential to hold so much water on a long term basis, installing a green roof can also help the waterproofing layer on your roof to last longer.

Sustainable Urban Drainage (SUD) Systems

Green roofs are supported by many local councils and organisations in the UK. The Forestry Commission, for example, because they work so well in conjunction with SUD systems.

Sustainable Urban Drainage (SUD) schemes are a cost effective and environmentally friendly way of managing rainwater in built up regions.

Green roofs can be key to SUD schemes. They are one of the top ways to meet the demand for improved storm-water management.  A further advantage is they are generally installed on previously unused space.

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