Page 542 - The City and Guilds Textbook: Plumbing Book 1 for the Level 3 Apprenticeship (9189), Level 2 Technical Certificate (8202) and Level 2 Diploma (6035)

The City & Guilds Textbook: Plumbing Book 1

1. 8m In drawing 1, the outlet has to be able to cope with the total rainwater
run-off from the whole roof area. The outlet in this situation could be positioned
3 m at either end of the roof, but the total flow rate would be the same. Running
outlets are designed to cope with rainwater from two directions, so the outlet
at either end can cope with only half the flow rate. Only half the capacity of the
outlet can effectively be used. Placing the outlet centrally would increase the

A total area of roof that the gutter can serve.
2. 4 m 4m
Outlet design
3 m
This is the least effective outlet design. The corners are
sharp edged, which restricts the flow of water down the
outlet by causing a clash of water streams at the shaded
area. This creates turbulent water flow. Some water will
travel across the outlet and against the flow on the opposite
A C B side of the outlet.
Alternative position C: Here the
single outlet is equal to two outlets
either end because of the outlet Here the corners are slightly rounded, which assists the flow
design
3. 2 m 2m 2m 2m of water down the outlet. However, the two water streams
are likely to clash, creating some turbulence.
3 m

Fully rounded corners give a much better flow of water
down the outlet. The two streams are kept more or less
A B separate, which assists gravity flow down the rainwater pipe.
p Figure 8.9 Outlet positions This is known as hydraulic efficiency.

The outlet position in drawing 2 is more effective than drawing 1 simply
because there are now two outlets and each outlet is coping with half the
expected rainwater run-off. Again, an alternative, but equally effective, layout
would be one outlet placed in the centre of the gutter run.

With outlets placed as in drawing 3, each half of the outlet has only a quarter of
the flow rate to cope with and so layout 3 is much more effective at discharging
the rainfall without the risk of flooding because both outlets are being used to
their full flow rate capacity.
Each manufacturer will have different rainwater flow rates for its own running
outlet designs. It should not be assumed that all manufacturers’ flow rates
will be equal. Therefore, manufacturers’ data should be considered before the
installation begins.
To find out how many outlets are required on a rainwater system design, simply
divide the expected flow rate of the roof area by the flow rate for the outlet
given in the manufacturer’s technical literature.

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