How a rolling offset is worked out
A rolling offset moves the pipe up (the rise) and sideways (the roll) at the same time. The trick is to combine the two into one number first, the true offset, and then treat it exactly like a simple offset.
true offset = √(rise² + roll²)
travel = true offset × travel constant
run = true offset ÷ tan(fitting angle)
Example: 16" rise and 12" roll with 45° fittings. √(16² + 12²) = 20" true offset. 20 × 1.414 = 28.28", so the travel is 2' 4 5/16" and the run is 20".
The mistake to avoid
Apply the fitting constant once, to the true offset. Multiplying the rise and the roll by 1.414 separately and then combining them puts the constant in twice and gives a travel that's too long. It's one of the most common errors on rolling offsets.
Travel constants by fitting angle
| Fitting angle | Travel = offset × | Run = offset × |
|---|---|---|
| 11¼° | 5.126 | 5.027 |
| 22½° | 2.613 | 2.414 |
| 30° | 2.000 | 1.732 |
| 45° | 1.414 | 1.000 |
| 60° | 1.155 | 0.577 |
| 72° | 1.051 | 0.325 |
Travel is not the cut length
Travel is centre to centre of the two fittings. To cut the pipe, take off each fitting's take-out and allow for the joint, such as the root gap on a butt weld. That part depends on the pipe size and the fittings on each end.
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