Problem Addressed

This solution was sourced in response to UKGBC’s Innovation Challenge: “How can existing buildings be made more resilient to climate change, with as little disruption to their occupants as possible, by 2030?”

Climate change is causing a rapidly growing number of extreme stormwater events. Flood risks in urban areas are of huge concern and The Sempergreen Detention and Retention Roofs can help with managing the impact of these extreme events.

Solution Overview

Building green infrastructure into cities, such as water retention and/or detention on the roof, contributes to effective stormwater management. Sempergreen Detention and Retention Roofs harvest rainwater on the roof, allowing water to be efficiently stored without having to dig underground, which is especially useful in urban areas with little space and on existing buildings.

The Detention Roof is a lightweight, green roof stormwater management solution which works on existing buildings and new builds. It can be applied to roofs with a slope of up to 2º. The system ensures that rainwater run off is temporarily detained and slowly drained delaying run-off by up to 24 hours. A sedum vegetation blanket is the top layer which also helps to mitigate excessive rainfall by evapotranspiration.

The Retention Roof is a water storage solution for new-build projects with flat zero fall. It is possible to cover the roof with sedum, wildflower and other biodiversity vegetation blankets as this doesn’t have to be a lightweight system. The system can also be used in conjunction with solar panels.

  • Retention Roof: £110 plus VAT per m2 plus installation for sedum-herb blanket and 8cm extensive substrate.
  • Detention Roof: £140 plus VAT per m2 plus installation for sedum mix blanket, 40mm lightweight substrate roll, honeycomb and detention layer.

Returns include not only water retention, but also extra benefits such as increased local biodiversity, extended lifespan of the roof covering, and natural cooling of indoor and outdoor areas.

Case Study

A Detention Roof was implement for a project in Rotterdam, the Netherlands. During a rain shower R10, 35.7 mm of rain water falls in 45 minutes. The maximum permitted run-off speed on the sewer system is 2.78 litres per second per hectare. When a Detention Roof is applied the calculated peak run-off falls within this standard.

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