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How green is your roof?
How clear is your water?


16th October 2018

Rainwater harvesting and green roofs are well developed technologies.
Both systems have numerous benefits for the environment as well as making a strong long term financial contribution to running costs.

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Although the two technologies are not really competitive they are rarely seen as compatible. Nevertheless we often get involved in solving issues where building design suddenly includes a green roof, although a rainwater harvesting system has already been planned or installed.

Research shows that run-off from green roofs is usually of a high quality. 

Green roofs are vegetated ground filters in which the loosened root area increases the breakdown and retention of pollutants. The run-off is generally fit for storage and usage.

Different build up of the green roof can have different impacts (see picture above).

The significant drawback is that the captured water is discoloured.

The discoloration of the water is caused by humic ‘acids’, which stain the water and give an unpleasant optical effect. This reduces the appeal of the water for re-use. In order to minimize the discoloration the substrates should contain as little organic substance as possible.When using the water for toilets, it is recommended to inform the occupants about the green roof as a cause for the discoloration to allay any misconceptions. The water should certainly not be used for washing machines. However, it is not an issue for irrigation and garden use.

The other drawback using green roofs with rainwater harvesting systems is that roof run-off is significantly reduced.

If you are planning a green roof and want to capture the optimum amount of water it would be wise to consider the various merits of the different systems and the use of greywater re-cycling as well.

Greywater recycling adds additional dimensions to all the opportunities

Greywater recycling has taken off recently and is becoming very popular with major developments particularly in the South and London region of the UK.