Introducing blue roofs: sustainable engineering drainage

Alumasc has pioneered the development of blue roofs in the UK – a new concept in stormwater management at roof level that allows attenuation of rainwater rather than instantaneous runoff. Simon Poë, technical director for Alumasc Roofing Systems, explains the need for these advances in rainwater drainage.

The recent increased frequency and severity of flooding in the UK has served to highlight the need to complement conventional drainage systems with a range of sustainable measures that can provide additional drainage capacity.

The Flood and Water Management Act (2010) aims to manage flood risk within the built environment by placing responsibilities on the Environment Agency to develop and implement a national flood and coastal erosion risk management strategy, which will inevitably have important implications for building design. Furthermore, with the government’s Water Strategy for England (published by DEFRA in 2008) having acknowledged that ‘below-ground piped systems can never be built large enough to cope with the extreme rainfall events’, the new strategy will need to have a key focus on Sustainable Drainage Systems (SUDS).

Blue roofs are a relatively inexpensive option as, unlike most other SUDS tools, they occupy otherwise-redundant roof space without extending beyond the building’s footprint into scarce and expensive urban ground space. They are designed to alleviate flood risk by reducing and controlling the peak rate of discharge in compliance with design requirements; facilitating the detention of stormwater, up to a prescribed maximum hydraulic head, for subsequent discharge over an elongated period of time. Blue roofs can include open water surfaces but can also be used in buried applications.

How does a blue roof work?

Blue roofs restrict the maximum permissible discharge of stormwater from the roof. As a result, during more intense storm events, rainfall will exceed the rate of discharge leading stormwater to temporarily back-up on the roof until such time as the outlet has the capacity to discharge it.

The reduction in peak rates of runoff, and the associated elongation of the runoff period, is important to flood risk mitigation. By restricting the peak rates of runoff from the roof, rainfall does not instantly discharge from the roof into the subterranean drainage or waterways; reducing the probability of sewer overflow and localised flooding. This is particularly important where sewers carry both waste and storm flows, due to the added health risks of a surcharge. However, it is essential that any method employed for the sustainable detention of stormwater adopts a controlled approach and avoids neglecting important safety requirements.

Firstly, the restriction of flow must be carefully estimated; establishing the peak rates of runoff and the associated depth (or stage) of water that forms on the roof surface. Secondly, the appropriate safety measures must be in place – from the provision of safety overflows (in the event of rainfall exceeding design storm criteria) to the adoption of a robust planned and preventative maintenance programme. And, thirdly, the roof’s waterproofing layer, and the detailing of it, is integral to the success of a blue roof system; protecting the building against water ingress.

Waterproofing

Clearly, the temporary storage of water at roof level requires the highest confidence in the waterproofing solution. A single point warranty, covering the waterproofing and drainage systems, should be obtained for added peace of mind.

Blue roof response

A blue roof can contribute to the attenuation of the kind of storm event that can induce localised flooding. For example, in response to a storm event with a return period of 1 in 100 years, when rainfall depth is 109mm, and peak intensity is 5.01 l/s, the peak rate of runoff is 0.285 l/s (a peak reduction of 94 per cent). In this case, the maximum stage of water reaches 90mm, before draining slowly over a period of 27 hours.

Conclusion

It is often said that the key to good design is simplicity; and the blue roof concept has adopted this governing principle. While anybody can reduce the size of an outlet to restrict flow, the consideration and control of the wider issues is imperative to the engineered drainage approach that has been employed in the development of blue roof solutions.