There are numerous reasons to opt for a blue roof. Flat roofs that temporarily store rainwater and then gradually release it form a Sustainable Urban Drainage System (SuDS), significantly reducing the risk of flooding in areas with few permeable surfaces. Blue roofs can also harvest rainwater for, for instance, indoor use. Moreover, they help manage storm water, are able to cool buildings, and even serve recreational purposes such as swimming pools. Blue roofs are very individual in terms of design, the starting point always being the site report that dictates exactly how much water will be stored at roof level in each particular case. That said, there are some general design considerations as well.
Calculating weight load and the number of outlets
SOPREMA offers a full design service for blue roofs, from weight load calculations (the structural integrity of the roof must be able to handle the often excessive weight of standing water) to calculating the number of outlets required to ensure drainage for the runoff volume.
The ‘NFRC Technical Guidance Note for the construction and design of Blue Roofs’ prescribes that “blue roofs should attenuate water for no more than a 24-hour period from the end of the maximum designed rainfall event. The discharge rate should be calculated to allow the roof to be half empty of attenuated water in a 12-hour period.” SOPREMA can provide control devices to regulate the drainage outlets according to the required water discharge rate.
As stated by BS 6229-2018, water may be discharged “to external gutters and downpipes or to internal downpipes connected to outlets within the roof area: the number, size and disposition of those outlets should be determined by reference to BS EN 12056-3. They should not be located adjacent to columns but rather at points of maximum deflection in order to avoid ponding.” With a loose-laid aggregate on top of the roof, extra provision should be made to prevent blockage of the outlets.
The NHBC says that the flow restrictor outlets should be “accessible for inspection and maintenance.” Indeed, outlets not only deserve careful attention during the design phase, but during maintenance checks as well. Like any roof, a blue roof should be inspected every six months, and that includes lifting the cap of the inspection chamber to make sure nothing is obstructing the discharge tubes and outlets.
The Fall Of A Blue Roof System
Another important aspect to consider in a blue roof situation is the fall of the roof. For standard roofs, the fall guidance design is 1:40 to reach a minimum of 1:80. This allows for deviations or imperfections within the substrate to start with. In blue roof applications, where water is meant to stay on the roof and eventually drain off naturally or by design, BS 6229-2018 takes into account the eventuality of zero falls for blue roofs. Zero fall corresponds to a fall of 0 – 1:80 and ensures that there are no back falls where water can pond significantly. If any back falls are detected, extra outlet provisions should be made and a detailed substrate and fall analysis should be undertaken by a structural engineer.
Thermal Performance
Lastly, there are thermal performance effects that come about with blue roof applications, too. Excessive ponding water can influence the thermal performance of the insulation in inverted roof situations. In addition, the water that passes through the joints of the insulation will cause rainwater cooling and will negatively affect the roof insulation value. Hence, the insulation layer must have sufficient thickness to compensate for this cooling effect. For inverted blue roofs with overlapping or tongue-and-grooved insulation, for instance, the British Standard prescribes that the “default factor for increased heat loss caused by rainwater flowing on the waterproof layer should be those detailed in ETAG 31-1 clause 6.6.1.3.”
BBA Approved Waterproofing Systems
SOPREMA’s DUOFLEX and SOPRALENE are both BBA certified waterproofing systems for blue roofs. With a blue roof, the waterproofing layer obviously must be very robust and completely sealed to prevent water transmission to the interior. We therefore recommend an integrity test, utilising electricity (developing an electric circuit) and then wetting the roof to locate any breaches in the membrane tested.
Do you want to get expert advice on the design or materials required for your inverted roof project? Please don’t hesitate to get in touch with our experts.