Polypipe Civils & Green Urbanisation’s [Polypipe CGU] involvement in the project officially began at the end of 2024, when the CMT team reached out following an initial enquiry three years prior. The Polypipe CGU team, working in collaboration with the Environmental Protection Group [EPG], were able to provide a full solution for the blue-green roof element of the project, comprising of its Permavoid product in a unique configuration, installed beneath permeable paving to provide full interception and attenuation of rainfall across the entire podium deck.
In September 2018, planning permission was granted for KNT Developments’ Trego Road scheme – a project which sees the site redeveloped into a new residential building. Drawing on the concept of shared space, the scheme also creates 1,915m² of commercial floorspace and landscaped open space complete with blue-green roof.
George Grant, EPG’s Principal Engineer in Water, said: “By employing varying Permavoid depths (Permavoid 150mm and 85mm combined to form areas of 235mm and 385mm, encapsulated in high performance capillary wicking geotextile), the system was engineered to optimise available storage volume while accommodating complex structural constraints, irregular geometries, and level changes within the podium itself.
“Multiple internal rainwater pipes discharge into the system via diffuser inlets, ensuring uniform distribution and minimising point loading. Outflows are regulated through orifices located within accessible chambers, maintaining discharge rates in line with greenfield rates. This approach not only achieves compliance with planning and drainage requirements but also safeguards against increased local flood risk.
“The adaptability of the Permavoid system enabled attenuation to be seamlessly integrated into the podium design without compromising usable space, demonstrating the effectiveness and flexibility of blue roof technology in delivering sustainable, high-performance water management on constrained urban sites.”
Nathan Burns, Specification Sales Manager at Polypipe CGU, said: “Permavoid is a fantastic product, and, because of its great versatility, we were able to achieve the required storage volume in what was a geometrically challenging space, all while keeping cover levels to a minimum. The capillary wicking geotextile allowed the collected rainwater to be retained and dispersed laterally across the surface of the tank. It also helps to prevent ponding and pooling in any lower lying areas caused by small undulations and ruts in the insulation and slab, assisting in keeping the water in constant motion.
“As part of this project, I undertook a toolbox talk on site with CMT to run through best practice for Permavoid installation – to which the team responded very well and managed to successfully navigate the different challenges that arose during the installation process, which was great to see.
“Overall, this project represents a great showcase between Polypipe CGU, EPG and CMT Construction – one that’s been a pleasure to have been a part of.”