Design of a blue roof

The incorporation of a blue roof into a project can be at rooftop or podium level. A blue roof is designed to attenuate storm water within a void which sits directly above the waterproofing layer and beneath a surface finish such as a vegetated green roof or hard landscaping.

A blue roof should not be designed as a water storage facility but should allow all the water to evacuate from the roof over a 24-hour period from the end of the projected maximum rainfall event, with over half of the attenuated water having evacuated from the roof after a 12-hour period.

Key aspects for inclusion:

1. Single Point Source and Guarantee. Consider the waterproofing, blue roof and surface design finish as one element, to ensure compatibility and guarantee clarity.

2. Void Space. Water flow in the blue roof void space must be multi-directional and free flowing above the waterproofing membrane, that is, no baffles or structures designed to divert flow on the roof surface.

3. British Standards. Adhere to the British Standards and Systems Codes of Practice for waterproofing and roof detailing, with waterproofing upstand heights ending standing 150mm above the landscaped / surface finish, including suitable termination to the building structure.

4. Roof Penetrations. The blue roof design must minimise or eliminate penetrations in the area where water is to be attenuated, other than the rainwater outlets or emergency overflows that are required for drainage functionality. In some instances, it may be possible to individually isolate the roof penetration from the blue roof to prevent any possible water ingress contaminating the insulation. This is achieved by forming a secondary seal between the vapour control layer and the underlay or the underside of the waterproofing, set 250mm back from the penetrations.

5. Emergency Overflow. Designed as an emergency overflow (example below), unconnected to the blue roof outlet flow restrictor, to discharge the rainwater should the maximum height of the blue roof design void be exceeded.

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