The roof drainage outlets and rainwater downpipes are calculated based on the design principles for gravity drainage systems established in ss525 2006 code of practice for drainage of roofs.
Rain water roof pipe.
As an example if the run of fall from a notional high point to a point of drainage is 20 meters then a minimal fall of 1 80 will see the roof level rise by 250mm.
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A rainwater downpipe is a pipe that is used to direct rainwater away from a building typically from roof guttering to a drainage system.
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It is sometimes also referred to as a downspout drain spout roof drain pipe or leader.
The calculations below are meant for drainage from flat roofs where water are expected to spread over the entire flat roof during rain.
On large roof areas having fewer outlets may cause a further design issue.
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