When do I use a four-point saddle instead of a three-point?

A four-point saddle goes over something with a flat top — a beam, a duct, a tray. It is two offsets with a flat run across the top, so the pipe follows the shape of a squared-off obstruction. A three-point saddle peaks to a single centre bend and suits a round crossing pipe. Match the saddle to the top you are clearing.

How many bends is a four-point saddle?

Four, all the same angle. The first two make an offset up to the top of the obstruction; the second two make an offset back down the far side. Between them the pipe runs flat across the top, which is where the obstruction width comes in.

Why is the shrink bigger than on a plain offset?

Because there are two offsets, not one. Each offset pulls the run in by its own shrink, and a four-point saddle has one going up and one coming down, so the shrink is doubled. Add the full figure to your measurement or the far end lands short.

Does the flat width change the mark spacing?

It sets the gap between the two middle marks — the flat run across the top is exactly the obstruction width. The two offset spacings on either side come only from the depth and the angle, and the width does not change them.

Four-point saddle calculator

A four-point saddle carries a run over a flat-topped obstruction with two matched offsets and a flat run between them. Enter the depth you need to clear, the width across the top, and the bend angle; the calculator gives the four mark positions measured along the pipe and the total shrink to add so the far end still lands right.

Worked example

Say a 6 inch wide duct sits 3 inches proud of your run and you cross it at 30°. The 30° multiplier is 2, so each offset spaces its two marks 3 × 2 = 6 inches. The four marks fall at 0, 6, 12 and 18 inches from the first: up over 6, flat across the 6 inch top, down over 6. Two offsets of 3 inches at 30° shrink the run by 2 × 3 × 0.268 ≈ 1 5/8 inches, which you add to your measurement before the first mark.

The formula

Each offset spaces its marks by the depth times the multiplier; the flat between the inner marks is the obstruction width:

offset spacing = depth ÷ sin θ · flat = width · shrink = 2 × depth × (1/sin θ − 1/tan θ)

The two outer marks and the two inner marks are just two ordinary offsets back to back, so each obeys the same rise ÷ sin θ. The flat top is measured straight across — the pipe does not bend there — so the middle gap equals the width exactly. Shrink is the offset shrink counted twice, once for each jog.

Exact vs. the field rule

The multiplier and shrink are the ordinary offset figures, applied to the depth and then doubled for the shrink. Rounded shop multipliers are fine on a shallow saddle; on a deep one over a wide obstruction the doubled rounding adds up, so take the spacing off the exact figure when the depth is large.

Angle	Multiplier	Field rule	Shrink / in of depth (per offset)
22.5°	2.613	2.6	0.199 in
30°	2.000	2	0.268 in
45°	1.414	1.4	0.414 in
60°	1.155	1.15	0.577 in

Questions

When do I use a four-point saddle instead of a three-point?: A four-point saddle goes over something with a flat top — a beam, a duct, a tray. It is two offsets with a flat run across the top, so the pipe follows the shape of a squared-off obstruction. A three-point saddle peaks to a single centre bend and suits a round crossing pipe. Match the saddle to the top you are clearing.
How many bends is a four-point saddle?: Four, all the same angle. The first two make an offset up to the top of the obstruction; the second two make an offset back down the far side. Between them the pipe runs flat across the top, which is where the obstruction width comes in.
Why is the shrink bigger than on a plain offset?: Because there are two offsets, not one. Each offset pulls the run in by its own shrink, and a four-point saddle has one going up and one coming down, so the shrink is doubled. Add the full figure to your measurement or the far end lands short.
Does the flat width change the mark spacing?: It sets the gap between the two middle marks — the flat run across the top is exactly the obstruction width. The two offset spacings on either side come only from the depth and the angle, and the width does not change them.

Related tools

Three-point saddle calculator — for a round obstruction.
Conduit offset calculator — the single jog a saddle is built from.
Conduit shrink calculator — the length an offset pulls out of the run.