Design Solution · Water Management
Design Solution · Dream about it
Green roof attenuates run-off; harvested rain reused.
An extensive green roof slows and stores rainwater run-off while harvested rainwater is reused for irrigation and other non-potable demand. The combination reduces peak discharge to the sewer and lowers mains water consumption.
Extensive green roofs as a stormwater attenuation strategy are well-established in UK sustainable drainage guidance and have been required or encouraged on major developments since the early 2000s; pairing that with rainwater harvesting for irrigation closes a logical loop by making productive use of what the roof collects. The attenuation performance of an extensive green roof is real but modest compared to a blue roof or dedicated storage tank — a saturated substrate has limited remaining capacity, and attenuation efficiency drops sharply in long storm events or when the roof is already wet from antecedent rainfall, which is precisely the condition during the kind of multi-day rainfall events that cause urban flooding. As a harvesting source, an extensive green roof produces water with higher sediment and biological load than a hard roof surface, requiring more filtration before it is suitable even for drip irrigation, and the collected volume is typically small relative to building demand unless the roof catchment is very large. No deployment evidence appears on this record, which understates how common green roofs are in practice, but specific co-use as an attenuation and harvesting system in a formally verified installation is less documented. The combination makes most sense on constrained urban sites where planning requires SuDS compliance, biodiversity credit and water efficiency simultaneously; size the storage to demand rather than catchment maximisation to keep the balance of the system sensible.
#green roof #rainwater harvesting #SuDS