Last edited 03 May 2021

Blue roof

A blue roof is a roof designed for the retention of rainwater above the waterproofing element of the roof. This is as opposed to more conventional roofs which allow for rainwater to drain from the roof. Blue roofs are typically flat, without any fall, with control devices regulate drainage outlets that enable water to be retained or drained.

The SuDS Manual published by CIRIA in 2015 suggests that a blue roof is: ‘A roof construction that stores water, can include open water surfaces, storage within or beneath a porous media or modular surface or below a raised decking surface or cover.’

Some of the reasons for incorporating a blue roof into a building, include:

As a form of Sustainable Urban Drainage System (SuDS) to try and alleviate urban flooding caused by stormwater run-off.
In areas of urban density, the space created by a blue roof can avoid having to provide attenuation for rainwater at ground level.
Rainwater harvesting for use, independent from, or supplemental to the mains water supply, such as wc flushing or green roof irrigation
Cooling solar panels, or reducing load on mechanical refrigeration or cooling equipment.
For recreational purposes, such as rooftop swimming pools or water features.

Unlike some other forms of SuDS, blue roofs make use of spaces that might otherwise be redundant without extending beyond the footprint of the building or into ground space which, depending on the density of the location, may be expensive.

To ensure safety, there must be careful estimation of the flow restriction, which calculates the peak rates of run-off and the water depth that will be formed on the roof surface. This informs the design of the safety overflows, the preventative maintenance programme, and the design and construction of the waterproofing layer.