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”
(See appendix for more specifications)
The degree of eaves offset is usually 112.5°
and often 135° for a plinth offset.
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 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.
- Specs for different Cast Iron offsets:
Methods for creating different Galvanised Steel, Copper, Zinc and Stainless steel downpipe offsets:
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