Your AJ Specification article on roofing (08.05) was fine in terms of describing the most efficient methods of removing rainfall from roofs but us poor devils trying to deal with that run-off when it reaches the ground have a different agenda; ie how to stop it reaching the drainage system too rapidly?
One of the methods we would like roof designers to contemplate is to retain water on roofs so that it does not have to be attenuated at ground level, bearing in mind that on many sites there is no room for storage or no suitable sewer/watercourse to receive the run-off at high rates. Thus tight city centre/urban sites are a prime target for the 'keep it on the roof' approach, especially in cities with inadequate sewerage and high flood risk.
As a rough rule of thumb we reckon that if 50mm of rain could be retained on a flat roof, with special controls to attenuate flow into the downpipes, attenuation at ground level could be done away with, saving excavation, disposal of material (possibly contaminated) and bringing many other cost/ programme benefits.
Green roofs are one way of achieving this effect but they are not necessarily appropriate to many building types or client preferences. Therefore someone needs to come up with a flat roofing system that can be totally watertight despite up to 50mm of ponding, with a controlled outlet system, easy to maintain and cost effective to install. It's that simple!
An added benefit in terms of sustainability would be to 'siphon' off the stored water at high level for use in WC flushing etc. If well thought out this would avoid pumping it up from ground level as in most current recycling schemes.
Martin Jones, infrastructure group team leader, SKM, Cirencester, Glos