Drainage and roof deck construction 

The discharge rate for the site is set by the local planning authority (LPA). Many LPAs are setting limits that equal that of greenfield run-off rates, 5 – 10 litres per second per hectare. Determining the total allowable discharge for the site is critical and will have been determined at planning stage. Once this is defined, the percentage of permeable and impermeable surfaces (roof, roadways, paving etc) is calculated so that the correct roof discharge rate can be worked towards.

As SuDS can take various forms, the blue roof may be included to supplement other methods, or designed as the sole solution. The blue roof may be designed to accept a higher or lower percentage of the controlled discharge depending on other attenuation or storage options available in other areas of the site.

The implementation of a blue roof will have considerable loading implication on the roof and its waterproofing. The roof deck construction will need to be designed not only to accept the dead and imposed loads*, but also the weight loading associated with the water to be attenuated on the roof.

As an example, at a depth of 100mm, a blue roof would exert an additional load of 1.0kN/m². Calculations must also consider the finish above the blue roof, be it a green roof, landscaped area or paved roof. Calculations must use saturated weights of all components and imposed loads, such as access by people and vehicles. It is strongly advised that the project structural engineer be involved throughout the design of the SuDS solution, and particularly when a rooftop solution is to be included.

A blue roof can be designed on zero falls providing the waterproofing system holds relevant certification and the roof is designed in accordance with British Standards. The Bauder systems used when constructing a BauderBLUE Roof have been BBA certified to be used on zero falls, with the appropriate precautions under BS 6229:2018 with falls of less than 1:80.

When the deck construction is complete, a deck deflection survey should be carried out to ensure the accuracy of the construction and that no depressions or back falls occur. Certification should be gained before the installation of the blue roof proceeds. Any falls created should be taken in to account as they will impact on the effective void space for holding rainwater.

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