Blueroof restrictor chamber – key design features

Along with the attenuation void formers, the stormwater restrictor chamber is one of the key components of a Blue Roof SuDS system design. The chambers are fitted above each rainwater outlet (or to the side when close to parapet walls), and are designed to restrict rainfall outflow during key design storm events. When it comes to choosing a Blue Roof system, not all designs are equal however, and ABG’s patented ABG blueroof design includes some specific features to provide additional safeguards against blockages and debris entering the water network.

During typical rainfall events the water simply flows through the chamber’s orifices and drains into the rainwater outlets as it would on a normal roof drainage design (for over 84% of the year). During the high intensity or long duration rainfall of the key design storm events however, the chambers begin to restrict outflow.

Currently in the UK the key design storm event is the 1-in-100 year design storm event, plus 40% allowance for the effects of climatic change. This is either based on the Flood Studies Report (FSR) or Flood Estimation Handbook (FEH) rainfall data for the location. During the key storm events the water flow is reduced to the maximum discharge rate, as agreed with the project engineer at design stage, via sized orifices in the vertical wall of the restrictor chamber.

ABG Blueroof blue roof chamber illustration for a green roof

The ABG blueroof® restrictor chamber also features an integral internal overflow weir, which is set at the maximum depth of the Blue Roof system and only comes into effect in the case of an exceedance event or any blockages on the surrounding roof area. The ABG design is different to most competitor systems that feature a flat restrictor plate placed horizontally over the RWO to limit discharge. The limitations of this type of flat plate design is that they can potentially become more easily blocked with debris, or simply be damaged / removed and not replaced correctly. They also cannot be used for parapet outlet positions, unlike the ABG blueroof system.

As part of the ABG system design, the chamber’s control orifices are protected by an additional filter strip in order to prevent blockages and maintain the required maximum flow rates. These filters are then checked as part of the six-monthly maintenance checks.

The filter strips form part of a triple layer of rainwater filtration provided throughout the system build up, with the attenuation void formers being fully wrapped in a filter / separation geotextile, in addition to a perforated geocomposite drainage board layer with bonded geotextile installed on top of the void formers. All these geotextile layers serve to provide added protection and security to prevent debris from entering the system whilst filtering pollutants from the rainfall.

The ABG restrictor chambers are available in different formats and for different roof build-ups i.e. inverted, warm or un-insulated roof areas. Different designs are also available to suit the roof’s final surface finish or use, for example whether the roof is for maintenance access only, amenity pedestrian access or fire engine / traffic loading on an accessed podium deck.