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Calculating Pipe Offsets and Swan Neck Sizes for Cast Iron Rainwater Systems

How-to-measure-downpipe-offsets-and-swannecks-for-cast-iron-downpipes

In the conservation and restoration of period properties, attention to detail is paramount—especially when specifying rainwater goods. One such detail, often underestimated, is the calculation of pipe offsets and swan neck dimensions to ensure that cast iron downpipes align accurately with sub-surface drainage infrastructure.

At Tuscan Foundry, we understand the importance of both heritage accuracy and functional precision. This guide will support professionals in specifying the correct components for their cast iron rainwater systems—be it for listed buildings, conservation areas, or traditionally styled new developments.

Understanding the Need for an Pipe Offset

Downpipes must connect precisely with the below-ground drainage or gully, yet in many period buildings, the gutter outlet is not vertically aligned with this point. This misalignment may be due to:

  • Eaves overhangs or soffits, which create a projection between the gutter and the building’s external wall
  • Plinth details, requiring a further change in alignment lower down the elevation
  • Architectural elements, such as parapet walls or corbelled brickwork


To bridge this gap and ensure a continuous, watertight path for rainwater, offsets—often referred to as swan necks—are introduced.

Bespoke-cast-iron-pipe-offset_round-downpipe

Determining Your Offset Projection

Determining Your Offset Projection

The key measurement required is the horizontal distance between the centreline of the gutter outlet and the centreline of the intended downpipe run. This is known as the projection.

For accuracy:

  1. Measure horizontally from the rear of the gutter outlet to the wall face where the pipe will be fixed.
  2. If a plinth offset is also required, measure the additional horizontal projection at ground level.

Unlike modern materials that allow on-site adjustment, traditional cast iron downpipes are cast with fixed offset dimensions. As such, precise planning is essential to avoid on-site complications.

Standard Cast Iron Offset Sizes

Measuring_cast-iron-downpipe-offset_swanneck-projection_size

Tuscan Foundry offers a selection of fixed offset sizes to accommodate common architectural requirements:

  • 75mm (3”)
  • 115mm (4.5”)
  • 150mm (6”)
  • 225mm (9”)
  • 305mm (12”)
  • 380mm (15”)
  • 457mm (18”)
  • 533mm (21”)
  • 610mm (24”)


Bespoke sizes can also be manufactured to suit complex or non-standard installations.

Offset Angles & Alignment Considerations

  • Eaves/Soffit Offsets typically use a 112.5° bend
  • Plinth Offsets are usually cast at 135°, allowing the downpipe to follow the lower wall profile

Correct pipe alignment is essential for both performance and visual integrity:

Square/Rectangular Pipes:

  • Front Offsets are used where the gutter outlet is directly above the gully.
  • Side Offsets are needed where the outlet is laterally offset—commonly below parapet outlets or hopper heads.

Due to their geometry, square and rectangular pipes cannot be rotated to achieve alignment, so precise ordering is crucial.

Round Pipes:

  • Offer greater flexibility, as the offset can be “swung” or twisted slightly to align with the gully—particularly helpful on uneven façades or where minor tolerances exist.

Best Practice: Installation Planning

  • Ensure the outlet position corresponds with your intended downpipe path—don’t leave it to chance.
  • Account for all projections, including cornices, mouldings, and plinths.
  • For projects involving bespoke hoppers, ensure their projection aligns with the chosen offset.
  • Consider ground conditions—especially for listed buildings where connections to historic drains must be handled with care.


Tuscan Foundry Products provides site surveys, CAD drawings, and bespoke casting services to ensure every component is manufactured for a precise fit. When required, we also fabricate custom offsets to suit complex or heritage-specific requirements.

Conclusion

Offset calculations may appear minor, but in heritage and conservation work, precision in rainwater system design can make the difference between a seamless installation and a costly rework. Whether you’re restoring a Georgian townhouse or specifying pipework for a converted chapel, Tuscan Foundry’s expertise in cast iron rainwater goods ensures your project maintains both its function and historic integrity.

Need help calculating your pipe offsets? Contact our team for advice, technical drawings, or to arrange a site visit.

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