Public health Rainwater Blue-roof
The custom has been to remove storm water from a building as quickly as possible, but there is a different way of managing rainwater, says Carl Harrop
he strict attenuation of storm water from developments is now commonplace, with the flow control normally taking place close to the outfall from the site.
More recently, the idea of reducing the volume of
thinking T
water through using a ‘green roof’, which also provides a habitat for wildlife, has been used. But a third, and commonly overlooked, way is gaining credence with engineers – the blue roof. ‘Blue roof’ is a term used to describe a system which
allows rainwater to temporarily build up where it lands on the building roof, reducing or potentially eliminating the need for other downstream attenuation methods, such as tanks or oversized drains, and is particularly suited to new-build flat-roof developments. A blue roof normally has zero degrees fall, maximising the volume available for attenuating the rainfall in a relatively thin film across the maximum possible surface area. It can also be cheaper than more established methods. So what is wrong with the current approach? All too
often, barriers to a holistic surface water disposal plan are created through the number of different parties
The traditional approach to rainwater disposal
The architect designs the roofspace including the falls, flashings and waterproofing details
The public health engineer sizes the rainwater outlets and downpipes to remove the water from the roof as quickly and completely as possible
The public health engineer provides downpipe positions, sizes and flow rates to the underground drainage designer
The drainage engineer sizes the underslab drainage in line with the flow rates provided by the public health engineer
The drainage engineer considers restrictions imposed on discharge rates and engineers mechanisms and structures to attenuate flows before outfall
involved in the design for taking the rainwater from the roof, through the building and to its point of discharge. The timing of appointments reinforces the difficulty for any particular discipline to propose an approach that may cross several contractual boundaries. Using traditional methods creates a concentration
of flows that inevitably leads to large-bore rainwater pipework connected to the underground drainage, which is also sized to take the peak flows resulting from short, high-intensity storms. However, does removing the water based on a two-minute storm duration really provide the best value? A typical flat roof office building will have been
designed to withstand the weight of snowfall, which is usually at least 0.6 kN/sq m. Also, to help convey the rainwater to the various outlets as efficiently as possible, the roof will normally be laid to falls, often with screed weighing in at between 1.2 kN/sq m and 4.8 kN/sq m. This means that if we remove the screed, there is
already the structural capacity to allow more than 180mm of water to build up on the roof. So, what if we use this inherent strength to contain the peak flows from a storm, as opposed to sizing the entire system to convey it to a remote attenuation point? The key to designing a blue roof or similar system
is to understand not just the peak flow rates generated by a particular storm, but also the volumes of water delivered by the downpour. Every building will have different requirements,
and this approach would only be suitable for flat-roof buildings. But as the costed example in the box on the third page shows, if a building is well suited to the approach, there is the opportunity to actually reduce the cost of a building by adding storm attenuation. A blue roof system needs a means to control the
flow rate from the roof. Currently, however, there are very few components on the market designed to accurately restrict flows. But there are several ways of approaching this. One is to control the flow at the outlet. Although this can be difficult to calculate and control effectively due to the very small head of
44 CIBSE Journal September 2010
www.cibsejournal.com
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