Sustainable drainage systems (SuDs) are essential for effective rainwater management and blue/green roofs have an important role to play. Increasingly, their specification is being driven by regional guidance and planning requirements, with major cities, such as London, which already has 700 green roofs, wanting to increase their percentage of green cover further. When it comes to designing effective blue/green roof rainwater attenuation systems, it is essential to understand competing drainage requirements. This is what we call the design dilemma.
The design dilemma
This is a major challenge faced by architects, civil engineers, designers, and most notably MEP engineers. Blue/green roofs are flat, and as a result are required to be designed to the British Standard for gravity drainage on flat roofs, BS EN 12056-3:2000. However, the reality is that the primary drainage function of a flat roof and a blue/green roof are not the same. One requires the roof to drain very quickly in an extreme event (a conventional flat roof), and the other slowly. As a result, the design of a blue roof attenuation system is at odds with the British Standard. Unlike conventional flat roofs, blue roof attenuation systems are designed to retain water for up to 24 hours or longer. This creates a dilemma for the designer of any blue/green roof attenuation system. Put simply, do they design to BS EN 12056-3:2000 or to meet planning conditions and performance requirements? With standard blue roof systems it is very difficult to do both!
The current compromise
The roofing industry and its suppliers have developed solutions that offer a single roof waterproofing layer and a modified drainage system that aims to accommodate both drainage extremes. However this approach by its very nature involves a degree of compromise, and therefore also requires an acceptance of increased risk – a failed blue roof will not create a leak but a ‘flood’. For this reason, there is a reluctance to employ these systems over habitable and sensitive buildings, which limits the extent to which they can contribute towards the mitigation of climate change.
ACO believes that the solution to the design dilemma is to separate the competing requirements into distinct issues, with a specifically engineered blue roof attenuation system that simplifies design responsibility without compromising performance of the roof or the drainage. By creating a separate, but linked, attenuation system that sits above the standard roof waterproofing and flat roof drainage system, the ACO RoofBloxx system enables engineers to design to BS EN 12056-3:2000, and to meet planning conditions without compromising either requirement.
The ACO system also delivers the following additional benefits. The roof waterproof membrane is not compromised so there are no warranty issues. It can operate on half drain down time in 24 hours so it’s compatible with standard civil hydraulic calculations. Inverted roof designs – the most common type of blue, blue/green roof design – can be achieved without a reduction in U-values or potential buoyancy issues. They are also suitable for use above habitable areas due to the increased resilience of separate waterproofing layers. ACO’s in-house design team can assist with attenuation sizing and the design of blue roof drainage systems, including modelling the behaviour of the roof during various storm events.
The unique drainage requirements of blue/green roofs and the need for any blue/green roof rainwater attenuation system to form part of the wider SuDs scheme means that the initial hydraulic design of the system tends to be undertaken by a civils drainage specialist. However, the detailed design often falls to other design specialists, who are not usually equipped with the tools to undertake this process. While a blue/green roof is an integral part of any roof build up it is important to note that its primary function is that of a rooftop SuDS solution.
Surface water management systems for blue roof applications are fundamentally different to conventional drainage systems. It is therefore essential that any blue roof attenuation system is designed by a drainage specialist who understands the key differences with regards to functionality and design.
Collaboration is the key to success
The design requirements of a blue roof include everything from non-standard drainage flow rates to differing structural loads, fire break and wind lift considerations. Consequently, it is important that drainage companies, architects, engineers and roofing contractors work collaboratively and pool their specialist knowledge to ensure that every aspect of a blue/green roof design is fit for purpose. Only by adopting this approach, can the industry address the design dilemma and ensure that the blue/green roofs we create today meet the needs of tomorrow’s sustainable urban environments.