Choosing the right size of Gutters and Downpipes

It is important that the sizes of gutters and pipes used in a rainwater system are selected with consideration of the size of the roof and the position of downpipes in order to ensure that typical rainfall does not cause the system to overflow.

Step by step guide:

Step 1- Calculate effective roof area
BS EN 12056-3:2000 requires that wind driven rain is taken into account when considering the effective catchment area. The standard assumes that rain falls at an angle of 2 units vertical to 1 unit horizontal which means that half the vertical height of any surface must be taken into account in calculations.

For a typical roof surface this means that the effective area is calculated from:
Effective area (m2) = L x (W + H/2)
L= Roof length (m)
W= Eaves to ridge width (m)
H= Eaves t ridge height (m)

Step 2- Calculate flow rate
In order to select the appropriate gutter and pipe size combination it is necessary to understand the flow rate required and the flow capacity of the system.
There is no standard rate of rainfall in the UK so for accurate calculations refer to an approved weather authority to get a site-specific rainfall level. A worst case storm of 0.021 litres per second per square metre is recommended by British Standards.

Flow rate is calculated from:
Flow rate (l/s) = Effective are x rainfall intensity

Step 3- Refer to gutter flow capacity charts
Reputable guttering manufacturers such as Lindab can provide flow capacity charts for their range of gutters. The presence of corners in the gutter reduces the capacity, where this is applicable a 15% reduction should be assumed for calculations.

Flow capacity (l/s) of Lindab Rainline gutters is shown below:

Step 4- Adjust for pipe position
The position of the pipe has a significant effect on the flow capacity of a rainwater system, this can be seen by comparing the flow capacities table for a gutter run with a centre outlet to the table for a gutter run with an end outlet.

Lindab provide charts (shown below) which allow the gutter and pipe size to be easily selected using Effective Roof Area- AE (m2) and a gutter ratio which accounts for the pipe position.

For typical gutter runs up to 10m length the gutter ratio can be calculated from:
Gutter Ratio = L1 / (L1 +L2)
Where: L1 and L2 are the lengths of gutter run at either side of the pipe
And: L1 ≥ L2

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