Blue roof design considerations after BS 6229:2018 update

Nigel Blacklock, technical director at Bauder, and Chris Roddick, green and blue roof product manager for Bauder, discuss the changes in the BS 6229:2018 update that impact rooftop water management solutions.

Chris, tell us about Bauder’s rooftop SuDS system, BauderBLUE

Chris: BauderBLUE is a blue roof system which attenuates heavy rainfall and controls its discharge over a 24-hour period to tackle standing water and flash flooding in built up areas.

How will the revisions in BS 6229 update impact the design considerations for a BauderBLUE roof?

Chris: The 2018 update clarifies and reinforces the meaning of a zero falls roof used in most blue roof systems. A zero falls roof is defined as having no more than a 1:80 fall, but more importantly the roof must not have any back falls away from outlets. This will mean all back falls must be corrected with localised latex screed or additional outlets.

Nigel, what structural considerations are required when thinking about zero falls on a blue roof?

Nigel: A detailed structural analysis of the deck should account for construction tolerances, settlement and deflection under load with a concrete slab level survey carried out to identify any back-falls prior to the waterproofing being installed. Hence the advice “design a fall of 1 in 80 to achieve not less than zero”, starting at zero inevitably will lead to back-falls.

How does the thermal performance update apply to a blue roof?

Chris: The U-value achieved in an inverted roof greatly depends upon the amount of water that passes through the joints of the insulation and sits on the waterproofing, as this will cause rainwater cooling. This has a negative effect upon the roof insulation value.

Nigel: There is now a note that suggests that not less than 10 per cent is added to the thickness of the insulation to take account of rainwater cooling. However, water on an inverted blue roof might sit on the waterproofing for longer and with a greater depth, BS6229:2018 states no correction factor should be used which will lead to much thicker insulation than is currently used.

BauderBLUE is supplied as part of a full Bauder roofing system and therefore has a single point warranty. For more information call Bauder on 0845 271 8800.