Blue roof case study: Cambridge University, Department of Civil Engineering
This new build project in the centre of Cambridge combines a Sustainable urban Drainage Solution (SuDS) with vegetation and renewable energy in a warm roof construction.The client for the Department of Civil Engineering at Cambridge University, R H Partnership, identified sustainability as being a key driver in the design of the roof, but also sought a single source supplier that could provide a guarantee for both workmanship and products. Bauder’s systems range encompasses products to meet all the sustainability objectives, and with our network of approved contractors carrying out the installation, we could offer the comprehensive guarantee that was required.Bauder Building BoardProject:Cambridge University Department of EngineeringLocation:CambridgeRoof Area:1610m²Client:University of CambridgeSpecifier:RH Partnership ArchitectsMain Contractor:SDC LimitedApproved Contractor:Voland LimitedThe specificationTo meet the sustainability criteria set out by the client, the Bauder technical team designed a roof system combining an extensive blue roof, a green roof and a solar PV array. The first and most fundamental element of the roof system though is the waterproofing. Bauder Total Green Roof System (BTGRS) was selected as it provides robust and durable waterproofing with a life span in excess of 35 years, as validated by BBA Certificate 10/4744. The Plant E cap sheet has also been tested by the FLL to ensure long term root-resistance, making it ideal for use in conjunction with the BauderSOLAR G LIGHT system. The BauderSOLAR G LIGHT system is a unique way to combine rooftop vegetation with renewable solar energy. The PV panel mounting boards are held in position using the growing medium as ballast, meaning the waterproofing is not penetrated and therefore compromised. The mounting boards incorporate a water retention feature which allows a small amount of excess rainwater to be stored for short periods of time for the various plant species to feed from. The BauderSOLAR G LIGHT system’s unique mounting system lifts the panels above the vegetation allowing the plant life to grow freely and therefore qualify a greater area of the roof as biodiverse.In addition to the BauderSOLAR G LIGHT system, a blue roof was added to the build-up. Blue roofs are sustainable drainage solutions designed to manage stormwater on a flat roof to reduce water runoff rates and ease flash flooding and standing water in built up areas. The BauderBLUE Roof System attenuates water from a flat roof over a 24-hour period via a restrictive flow outlet. Calculations are carried out to determine the rate at which the water should egress the rooftop. This is determined by the local planning authority and is often set as low as 5-8 litres per second per hectare. With these calculations, Bauder can design each flow restrictorto meet the necessary runoff rate. For the Department of Civil Engineering, the main roof area of 1455.5m² required two flow restrictors, each with flow control holes to ensure the specified discharge rate of 0.77 litres per second was achieved. The maximum depth of water the roof is designed for is the H-max. When the water level reaches H-max, it will drain via the emergency overflow pipe. These overflow pipes are set at the same height as the H-max level. On this project the H-max was calculated to be 99mm. Although an unlikely scenario, this provision for a ‘once-in-a-hundred-years’ storm event will ensure that the structural integrity of the building is always preserved.Project challengesThe roof deck was constructed using a pretensioned concrete plank roof structure. Due to the large span of these planks, the dead load weight to the roof would be restricted. This being a blue roof with green landscaping and PV (a reasonably heavy build up), presented the issue of balancing the need of the roof design and the weight restriction of the supporting structure. To add to this challenging scenario, the pretensioned deck did not provide a completely flat finish. When installing a blue roof, a flat deck with no positive or negative falls is essential, as detailed in the 2018 update of BS 6229 – Flat roofs with continuously supported flexible waterproof coverings. To overcome this onsite challenge,the final deflection of the fully loaded roof was calculated and the concrete deck was screeded to give a flat finish.The resultBauder believes this project to be one of the first of its type in the UK to bring together so many rooftop systems in one space. Project management and clear communication was paramount in order to ensure a smooth installation. The Bauder project team consisted of green roof and solar PV product managers, area technical manager, technical design team, site technicians and approved installers for the waterproofing.With all the different elements being installed, green roof and solar PV product managers worked with our technical team to design the best possible PV array configuration for optimum energy generation whilst also qualifying for the maximum biodiverse area.Bauder site technicians worked closely with Bauder approved contractors, Voland Asphalte Ltd, to ensure works were kept to schedule and that a high quality of workmanship was maintained throughout.To ensure the landscaping is maintained, irrigation was installed by Access Irrigation Ltd. The system, Permadrip Pro, is a drip line irrigation system with anti-syphon design to resist clogging and pressure regulation. Due to the specialist nature of the irrigation system, Access Irrigation Ltd provided a bespoke design service.The result of this network of expertise was a multi-faceted support service for the client, specifier, main contractor and roofing and PV installers. System installed1 BauderPLANT E 42Green, 5mm torch-on root-resistant capping sheet.2 BauderTEC KSA DUOA self-adhesive, elastomeric, bitumen underlayer. This underlayer was used to ensure thorough sealing of lap joints in low temperatures when the roof was installed.3 BauderPIR FA-TETissue-faced flatboard insulation faced on both sides with aluminium foil to provide efficient thermal output.4 Bauder Super AL-ETorch-on air & vapour control layer. Once installed on the concrete pretensioned plank deck, it can act as a temporary waterproofing layer, allowing internal works to be carried out immediately which was a huge advantage on this new build project.