September 26, 2021

Early engagement during blue/green roof construction essential following Mayor elections

Following Mayor of London Sadiq Khan’s pledge to increase the number of green roofs in his re-election manifesto; architects, engineers and contractors are being urged to ensure designs meet both performance needs and British standards.

Urban greening projects are expected to increase across the UK, following changes to local and national planning policies. One of the key challenges in constructing blue/green roofs, according to ACO Building Drainage, is meeting the needs of the Building Regulations, required standards – BS EN 12056-3:2000 and BS6229: 2018 – and achieving effective rainwater retention.

Currently, as blue/green roofs are flat, they are required to meet the British standards for gravity drainage on flat roofs. However, while traditional flat roofs are designed for water to quickly drain in the event of heavy rainfall, a blue/green roof retains water for 24 hours or more meaning the primary drainage functions are not the same. As such, there is increased risk of flooding if attenuation systems are incorrectly specified and not maintained.

Neill Robinson-Welsh at ACO Building Drainage, said: “If insufficient consideration is given to the drainage design, particularly for exceedance events, then the building integrity can be compromised by flooding. Blue Roof systems that utilise a single roof waterproofing have to trade off increased risk with a compromised flat roof drainage design.

“It is recommended that attenuation is considered at the earliest stages of the design, where architects, engineers and contractors collaborate with drainage specialists. This will allow project leads to find ways in which the roofing design is not compromised while meeting wider sustainable drainage system (SuDS) requirements.”

To overcome the challenge, ACO’s RoofBloxx blue roof attenuation system enables engineers to design to the correct standards, while meeting planning conditions without the need to compromise. The ACO RoofBloxx system is compatible with standard civils hydraulics, while it also allows inverted roof designs to be achieved without a reduction in U-values and can be incorporated into any roof build up whether warm or inverted, new build or retrofit. 

Mr Robinson-Welsh added: “The unique drainage requirements of blue/green roofs – coupled with the need to form part of a wider SuDS scheme – means that the initial hydraulic design of rainwater attenuation systems should include drainage specialists. Given the expected influx of blue / green roof requirements across metropolitan cities, collaborating at the earliest opportunity is critical for the smooth delivery of roof attenuation projects.”

To support professionals working on a blue/green roof, ACO has a free on-demand webinar which supports those working on a project. To register and watch the webinar, please visit