Additional information:
The photo below shows a woefully fitted overflow pipe on the left (no mitigation) and an equally woeful vertical riser on the right fitted with an equally woeful first flush diverter near the top. So...why are (1) the vertical riser and (2) the first flush diverter also woeful?
(1) The vertical riser allows mosquitoes to access the standing water because a flap valve cannot be fitted to a 45 degree elbow.
The flow path is archaic and ugly.
(2) The first flush capacity is too small (it probably holds about 6 litres) BUT in this situation, it actually wastes less water needlessly collected.
The first flush (the initial dirty roof wash) only fills with the standing settled water retained in the riser since it last rained while the first flush will still be in the downpipe(s) at the end of the wet system!
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