Most suppliers will provide you with the maximum design depth of water for the blue roof - often referred to as 'H-max'.

This is needed both to ensure that waterproofing systems and upstands are appropriately detailed and to provide to the structural engineer to calculate the loadings.

What happens however in the event of a blockage or if the system capacity is exceeded?  The water will continue to rise until such point as there is adequate depth of water above the overflows in order for them to operate at the required flow rate.

This depth of water, sometimes referred to as the peak water depth or accidental loading is in fact the depth which should be given to the structural engineer and NOT the H-max depth, which could potentially underestimate the loading from collected rainwater.

The successful design of blue roofs requires early engagement with all stakeholders. Blue roof designs cross multiple disciplines and allocating and tracking responsibility can often be overlooked, opening the potential for error, omissions or gaps in design liability, responsibility and insurances.

If your project has a blue roof, the design team should be clear who holds responsibility for each of the following:

If the design calculations provided by your blue roof designer indicate that the plan area of the attenuaton zone (blue roof) is the same as the catchment area - alarm bells should ring.

Contact me on 07464225547 and I will explain why...

Blue roofs provide the more efficient results when used on absolute zero gradient roofs.  Gradient flat roofs can be used but require more detailed analysis and are more complex to detail,  design and construct.  They also result in concentrated structural loadings.

That said, they can be perfectly effective when well detailed.

Building warranty providers should be consulted at the earliest opportunity as most have specific requirements in regard to rainwater drainage and blue roofs and gradients.

Particularly if roofs are laid to falls, care should be taken in early design estimates, not to provide an excessive number of outlets as this may result in orifice sizes in flow control outlets being too small.

I will be adding more important watchpoints, so come back and take another look soon