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STRUCTURAL ELEMENTS A good case for the blues


Phillip Wilcox-Moore of Axter offers advice on how ‘blue roofs’ can elevate a scheme’s sustainability credentials by retaining rain water


O


ur climate is rapidly changing. Climatologists are predicting wetter winters, significantly


increased rainfall over short periods and a certainty of more extreme weather events. This means we need to look at more sustainable and affordable solutions for flood mitigation.


The traditional approach to storm water management was to remove water from the building as quickly as possible causing downstream flooding. Currently, below ground storage tanks with specialist attenu- ation control mechanisms minimise the impact of storm water on the downstream drainage system. These solutions require substantial earth works and are often expensive, especially if collected storm water must be pumped out of storage. However, there is a simpler and more cost-effective solution to storm water mitigation and SuDS compliance that is gaining credibility with architects, designers and engineers alike. These measures have been driven by legislative requirements imposed upon new developments within Sustainable Urban Drainage Systems (SuDS) regulation.


Extreme care must be taken when ensuring the waterproofing


membrane is not affected by standing water


Cost-effective attenuation


SuDS demands the control of water from where it first lands, at roof level, and attenuating through gravity such as when filtering through earth and rock. Water on the flat roof is engineered to drain away slowly over several hours after the storm event, without overloading the downstream drainage system, thus making it a ‘blue roof’.


Gravity provides a simple and


47


cost-effective way to attenuate storm water flows, by temporarily holding rainwater back on the flat roof. A temporarily retained rainwater depth of just three inches (less weight than a heavy snowfall) on a flat roof would severely reduce ground level flooding and the impact of drainage system overload of even a big storm. The only additional cost to that of a standard flat roof are for slightly higher waterproofing details and a simple flow restriction insert to the roof outlet. That said, extreme care must be taken when ensuring the performance and composition of the waterproofing membrane is not affected by standing water. Some products are prone to plasticiser migration and premature ageing when submerged for a prolonged time. A waterproofing failure on a ‘blue roof’ will have catastrophic consequences over and above those experienced during a standard roof leak.


A two layer fully-bonded waterproofing system is more robust than a single layer system. Constant water immersion can lift the granules on a self-protected granular finished membrane, therefore it would be advisable to apply chippings to


GETTING THE BLUES


Blue roofs are so called because they are designed to allow water to drain away over several hours to avoid overloading downstream drainage


ADF JUNE 2017


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