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How to measure the offset size (from gutter outlet to downpipe) you’ll need to order

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Any downpipe needs to run down the wall in line with the subsurface drain/gully below it –  in order to join it or ‘let out’ into it.

But the gutter outlet may be at the end of the run of gutter and, therefore, anywhere between ‘a little’ and ‘some distance’ from this drain/gully.

Plus there is likely to be an over-hang/soffit distance of some kind to traverse,

Anywhere from ‘a little’ to ‘some distance’ from the roofline/eaves that the gutter is fixed to and the wall of the building the downpipe is attached to.

A ‘Plinth Offset’ may also be needed at the bottom of the downpipe to allow it to follow the contours of the wall.

The distance between these points – the centre of the gutter outlet and the centre of the downpipe neck – gives you your ‘projection’ measurement.
(and the size of offset you need to order)

While other materials offer two-part offsets (so that a cut-to-requirement length of pipe can added to create any offset required), Cast Iron – with its integral cast neck – traditionally comes in ‘set’ projection lengths (or ‘fixed offsets’) whether in square or round pipes, for example:

  • 76mm/ 3”
  • 114mm/ 4.5”
  • 152mm/ 6”
  • 228mm/ 9”
  • 304mm/ 12”
  • 381mm/15”
  • 457mm/18”
  • 533mm/21”
  • 609mm/24”
  • 685mm/27”
  • 762mm/30”

Offset Degrees

The degree of eaves offset is usually 112.5°

and often 135° for a plinth offset.

Pipe Alignment

Square Pipe
Where square or rectangular pipes are being installed and an offset is required, it will be either a left or right ‘Side Offset’,  or a ‘Front Offset’ projecting straight out from the wall.

The outlet must either be directly above the gully, for a ‘Front Offset’ (i.e. travels the the soffit depth)


The outlet must be level with the pipe when attached to the wall, the ‘projection’ is only to the left or right i.e. a ‘Side Offset’ is required, perhaps in the case of a hopper mounted below an outlet in a parapet wall.

Round Pipe

Round pipe systems are more flexible to install, as offsets can be “swung” or twisted into alignment with the gully position.

The Square pipes cannot be swung/twisted.

So, it is important to measure and calculate your projection carefully and then affix your outlet accordingly. Here is our short video guide to help you get it right.

The Video

Do you have more questions?

Get in contact with us on 0800 644 44 26 or email sales@rainclear.co.uk.

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