Downpipe and rainwater discharge

Frequently asked questions

How is a downpipe sized, and why is rainfall RATE the input rather than annual rainfall?

By the peak flow it has to carry, which comes from the roof area draining to it multiplied by a design rainfall INTENSITY — millimetres per hour, not millimetres per year. The distinction matters because drainage fails in the worst few minutes of the worst storm, not in a wet year: a location with modest annual rainfall and intense summer downpours needs larger pipes than a wetter place with steady drizzle. That is why codes publish a design intensity for a location and a return period rather than an annual figure, and why a system copied from one region to another can be undersized without anybody having made an arithmetic error. The second input is how many downpipes the roof is split between, since halving the area each one serves is usually cheaper than enlarging the pipe.

Why does the gutter's outlet position matter as much as the pipe?

Because a gutter's capacity depends on where the water leaves it, and the pipe is often not the constraint. A gutter with an outlet at one END behaves as a long channel with all the flow travelling to that point, and its capacity is limited by the depth of water it can hold before overtopping at the far end. The same gutter with a CENTRE outlet serves two shorter runs and carries substantially more, and outlets at both ends more again. Sharp corners, stop ends, and a run that has to negotiate an angle all reduce it further. So a gutter overflowing is very often not a blocked pipe but a run that was always going to overflow at that intensity, and adding a second outlet fixes what cleaning cannot.

Where should the bottom of it discharge?

Somewhere that takes the water away from the building, and the three normal answers have different consequences. Into a trapped GULLY connected to a surface water drain is the conventional arrangement and the one that needs the trap kept wet. Into a SOAKAWAY disperses it into the ground and is subject to how fast that ground accepts water and how far it is from the building. Onto a SHOE discharging over a path is the cheapest and puts a concentrated stream against the base of the wall every time it rains, which saturates the masonry there and is a common cause of damp at low level. The version that causes most damage is none of these: a downpipe that discharges into a drain which is blocked or broken below ground, so the water goes into the ground immediately beside the foundation and nobody can see it.