Déports autour des obstacles

To offset conduit around an obstruction, measure the rise from where the pipe runs now to where it has to run to clear the thing — underside to underside, with room for a strap — then measure the straight room you have before it and pick the angle whose run fits: the sloped part of an offset covers rise ÷ tan θ. The pull is on how to bend an offset; this page is the part before the bender.

Measure the rise off the real thing

The rise is centre to centre — or underside to underside, the same thing. Measure to the obstacle's edge, then work out where your pipe's underside has to be.

• A beam flange. Deck to the underside of the flange, plus clearance for the strap, minus where your pipe's underside sits now. Measure the flange in front of you, not the drawing.
• A pipe. To the outside of its insulation, and allow for the hanger right where you cross.
• Ductwork. To the widest point: seams, flanges, reinforcing angles. A flat-looking duct has a lip at every joint.
• A box or a panel. To its face plus the cover and any connector on the side.
• A wall jog. Off the finished faces, square across — and check both are plumb, or the rise changes from top to bottom.

Then clearance: for the coupling (fatter than the pipe), the strap, and your hand to drive the screw. A 1/4" (6.4 mm) short offset hits; a 1/4" (6.4 mm) long one does not.

When you cannot reach it

Three ways to get the rise from a ladder, without climbing the beam:

• A level as an extension. Hold a level flat against the underside of the flange and slide it out toward you. Its underside is the flange line, out where you can tape from it to your pipe.
• From a common reference. Deck to flange, deck to pipe, subtract. Two short, square measurements beat one long slanted one.
• A laser line. A level line across both, each measured to it, subtract.

Measure square to the surface; a slanted tape reads long.

Pick the angle by the room you have

The run an offset takes is rise ÷ tan θ. Measure the straight room from the last thing you cannot move (a coupling, a strap, a box, another bend) to the obstacle, and find the shallowest angle whose run fits.

The run is for the slope only; each bend's radius adds a little at both ends, so leave an inch or two of spare. Shallower is better when it fits: less shrink, an easier pull, fewer degrees counted against the limit in your code book. 10° is for small rises over long room; 45° and 60° are for when the obstacle is right on top of you.

The procedure

1. Find the line. Decide where the pipe has to end up: under the flange, past the duct, off the face of the box. Add clearance for the strap, the coupling and your fingers.
2. Measure the rise. Measure from the underside of the pipe where it runs now to the underside of where it has to run, or top to top. That is the centre-to-centre rise, with no pipe diameter counted.
3. Measure the room. Measure the straight distance available for the offset: from the last fixed point (a coupling, a strap, a box) to the face of the obstacle.
4. Pick the angle. Multiply the rise by the run figure for each angle and take the shallowest angle whose run fits the room with a little to spare.
5. Mark. Mark 1 where the offset starts, mark 2 at rise × the multiplier for that angle past it. Add the shrink to the overall length.
6. Bend and check. Bend as an ordinary offset, both bends in one plane, then hold it up to the obstacle before you strap anything.

Worked examples

The offset calculator does this arithmetic for any rise and angle and shows the run, so you can try angles against the room before marking anything.

Rolled offset, saddle, or a rack

When a rolled offset beats two offsets

If the pipe has to come 6" (152 mm) off the wall and move 8" (203 mm) over past the same thing, two separate offsets are four bends. One rolled offset does it in two: the true offset is √(6² + 8²) = 10" (254 mm), so at 30° the marks are 20" (508 mm) apart and it eats 2 11/16" (68 mm) of run, against 3 3/4" (95 mm) for the pair. Roll the pipe to the angle the calculator gives, then bend it as one offset. Two offsets are still the better call when the two moves happen at different places, or when a strap has to go between them.

When a saddle beats an offset

An offset leaves the pipe at the new level. If it has to come back to the original line on the far side — across a pipe, over a duct on a wall — it is a saddle: a three-point saddle over something round and narrow, a four-point saddle over anything flat or wide.

A rack of pipes through the same offset

Pipes that run side by side in the plane of the bend have to offset together and stay evenly spaced. Same rise, same angle, same marks apart on every pipe — but each pipe's marks start a little further along than the one beside it: centre-to-centre spacing × tan(θ ÷ 2). Start with the pipe on the inside of the first bend (the side the offset moves toward) and move each next pipe's marks along by that much. At 30° with the pipes 3" (76 mm) apart it is 3 × 0.268 = 13/16" (20 mm) per pipe, so the third pipe's marks sit 1 5/8" (41 mm) further along than the first's. The parallel offset calculator lays out the whole rack.

Questions

How do you measure an offset around an obstruction?

Measure the rise from the underside of the pipe where it runs now to the underside of where it needs to run, clear of the obstruction with room for a strap. Underside to underside is the same as centre to centre. Then measure the straight room you have before the obstruction, and pick the angle whose run fits that room.

How do I offset conduit around a beam?

Measure from the deck or wall down to the underside of the beam flange, add clearance for a strap, and subtract where the underside of the pipe sits now: that is the rise. If the flange is out of reach, hold a level against its underside and measure down from the level to the pipe. Pick the angle by the room before the flange, and bend an ordinary offset.

How much room does an offset take?

The sloped part covers rise ÷ tan θ of run: 1.732 × the rise at 30°, 1.000 × at 45°, 2.414 × at 22.5°. A 6" (152 mm) rise at 30° needs about 10 3/8" (264 mm) between where the offset starts and where it finishes, plus a little for the bend radius at each end.

When should I use a rolled offset instead of two offsets?

When the pipe has to move up and sideways past the same thing at the same place. One rolled offset is two bends instead of four, eats less run, and puts fewer degrees in the run. Two offsets are easier to get right and easier to strap, so use them when the two moves are at different places along the run.

Should I use a saddle or an offset?

An offset if the pipe stays at the new level afterwards. A saddle if it has to come back to the original line on the far side of the obstruction: a three-point saddle over something round and narrow, a four-point over something flat or wide.

Related

• Conduit offset calculator — rise and angle to the marks, the shrink and the run.
• Offset angle calculator — the angle that fits the room you have.
• Rolled offset calculator — up and over in two bends.
• Parallel offset calculator — the stagger for a rack.
• Three-point saddle calculator — over a pipe and back down.
• Four-point saddle calculator — over a duct or beam and back down.
• How to bend an offset — the pull, once the marks are on.