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.

Our 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 attenuation 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.

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 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 provide permanent UV protection if the membrane is to be left exposed.

Alternatively, mitigated water can be concealed under a raised pedestal and paving system or by the installation of modular load bearing ‘blue roof’ attenuation cells, which means that the flat roof can still be used for access and a variety of other purposes. Conversely, a living roof would also temper the amount of water flowing through it while increasing  biodiversity.

By far the most important waterproofing aspect to note is that ‘blue roofs’ should be a warm roof construction, and inverted roofs should be avoided. Inverted roofs rely on a water flow reducing layer (WFRL) installed above the inverted insulation to restrict the flow of water, and this is not a waterproofing membrane. Its unsealed laps will not be watertight when a three inch head of water is temporarily retained above. This will result in the calculated thermal performance of the roof not being achieved, higher than expected heating and cooling costs and a poorly performing building.  This should be of significant concern to all parties involved.

Go bespoke

Standard thermal calculation methods cannot be used when designing an inverted roof with ‘blue roof’ finishes. For example, a 0.15 W/m2K design U-value on a standard inverted roof would require approximately 220 mm of extruded polystyrene (XPS). Whereas on a ‘blue roof’ this thickness would increase to 1000 mm of XPS and a design U-value of 0.12 W/m2K could not be achievable.

In conclusion, because every building is different, it is a key maxim to remember that a bespoke engineered drainage strategy must be designed for every project. The specified waterproofing system must be robust, and installed to the highest quality standards. And, as stated earlier, the roof should be a warm construction and inverted roof designs avoided. 

If these three basic principles are followed, I believe we can quickly achieve a successful national strategy for sustainable and affordable flood mitigation at roof level.

Phillip Wilcox-Moore is managing director at Axter