Green Roof Drainage Design

Ariel photograph of green roof commercial building

A common question we are asked on our CPD: Is siphonic drainage compatible with a green roof system?

The simple answer is yes, siphonic drainage is compatible with green roofs and in fact, is advantageous over a traditional gravity system.

A green roof on a building, partially or completely covered with vegetation, requires special consideration when it comes to designing a sustainable drainage system. Two key aspects in managing a siphonic system on a green roof are as follows.

Managing Excess Debris​

In managing a siphonic system on a green roof, it is crucial to ensure there is a permeable membrane between the outlets and the sedum. The membrane will prevent excess debris entering the siphonic system and is particularly important during times of low rainfall. Typically the outlets are placed within a grate which sits alongside the permeable membrane and acts to ensure any debris entering the system only enters during times of rainfall when the system is naturally self cleansing.

Regular Roof Maintenance

Regular roof maintenance should be implemented to ensure to continued performance of the system. BS EN 12056-3:2000 recommends the system be maintained four times in the first year, following which a maintenance regime can be put in place.

Advantage of Utilising Siphonic Drainage With Green Roof

Managing excess debris and regular roof maintenance are both key requirements for either a gravity or a siphonic system. However, if excess debris gets into a gravity system there may not be sufficient volume of rainwater to flush it through the system. In contrast, a siphonic system, once 10-15% of capacity is achieved, becomes self-cleansing as the debris is flushed from the system. 

Capcon Engineering have installed siphonic systems on multiple green roof projects over the last number of years including:

For further detail on design and implementation of a siphonic drainage system and green roofs, or to attend our Siphonic Drainage CPD, please contact us:

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Stephen Morris

Design Lead, BIM Level 2 Certified

Stephen Morris

Photo by Chuttersnap.