Custom Duct Offsets, Made to Order in Any Size

Custom Rectangular Duct Offsets: The Complete Guide

A duct offset, also called a jog, a dogleg, a double-angle or an S-fitting, is the fitting that moves a rectangular duct sideways or up and down without changing its direction or its size. The duct enters one end, bends gently off its line, runs a short way at an angle, bends gently back, and leaves the other end parallel to where it came in but displaced by the offset distance. It is the fitting for going around a beam, a pipe, a column or another duct; for shifting a trunk from between one pair of joists to the next; for lining up a run that was framed a few inches off; and for dropping a duct from tight against the deck to below a beam. Every job has at least one obstruction that nobody drew, and the offset is how the ductwork gets past it in one sealed piece instead of two elbows and a stub. We build every offset to the exact numbers you type in, and this page explains how to choose those numbers.

Everything below is what we tell customers on the phone: what the fitting is, why one offset beats two elbows, how the offset angle sets the length, how to size the jog for the duct width, which gauge and material to specify, how the ends connect, and how to measure the obstruction you are going around. If you already know your numbers, the designer above prices them instantly and the popular-sizes table links to a page for each common size.

What a duct offset actually is

Geometrically the fitting is a rectangular duct whose centerline has been sheared sideways. The inlet and outlet are the same width and depth and face the same way; the outlet is simply moved over. Between them, two of the four walls are flat parallelograms that carry the jog, and the other two are straight panels bent at each end so the duct steps over without twisting. The whole thing is laid out flat from four numbers, cut, formed and joined with locked seams, so both openings are square, both are on the same axis direction, and the jog happens in two clean bends with no internal step.

The four numbers that define the fitting are the duct width, the duct depth, the offset distance (how far the centerline moves), and the overall length from the inlet face to the outlet face. The angle of the jog is not a separate input: it follows from the offset and the length, and the calculator at the top of this page turns the angle you want into the length you need. Both ends are finished with the connection you specify, slip-and-drive being the default.

One offset versus two elbows

Two elbows and a short straight will move a duct over. They will also give you three joints to seal, two sharp turns for the air to fight through, and the job of cutting and fitting a stub of exactly the right length in the field. A single offset has one joint at each end, turns the air twice through a shallow angle instead of twice through 90 degrees, and arrives as one part that already matches the jog you measured. The pressure loss is a fraction of the two-elbow version, which matters on a trunk that feeds a whole floor, and the installation is faster. On a job with a dozen small jogs, the offsets pay for themselves in the time saved fitting stubs alone.

Where offsets are used

• Around beams, pipes and columns. The trunk has to drop below a steel beam and come back up, or slide sideways past a soil stack. The offset distance is the beam depth or the pipe clearance; the length is whatever the joist bay allows.
• Between joist bays. A branch that has to move from one joist space to the next moves over by the joist spacing, typically 16" or 24" on center, in the depth direction of a flat duct.
• Lining up with equipment. The furnace plenum outlet is a few inches off the trunk line. A short offset makes the connection without a kink.
• Dropping a run. A duct tight to the deck has to come down to clear a hanger or a sprinkler main and go back up. A vertical offset does it in one piece.
• Commercial low-pressure duct. Engineered drawings call the offset by duct size, offset distance and length, which is what the designer builds.

How the angle sets the length

The offset angle is the angle the sloped middle of the fitting makes with the duct axis. Because the sloped section spans the offset distance, the length from face to face is

L = offset distance ÷ tan(angle)

which gives these lengths for the common angles:

The rule of thumb is to use 30 degrees or shallower wherever there is room, and 45 degrees where there is not. At 30 degrees the air barely notices the jog; at 45 degrees an offset still adds much less loss than two elbows, but it is audibly a fitting. Above 45 degrees the middle section is so short that the part is really two mitered elbows joined together, and you should either lengthen it or accept the loss. The published sizes are at 18" and 24" lengths, which put a 4" to 8" jog in the 15 to 30 degree range for the duct sizes most people order.

Sizing the jog for the duct width

The other thing to watch is the offset distance compared with the duct dimension in the plane of the jog. If a 12" wide duct offsets 4" sideways, the sloped section still overlaps the inlet by most of its width and the air flows nearly straight through. If the same duct offsets 12", the inlet and outlet no longer overlap at all and every bit of the air has to make the full jog. When the offset is more than about half the duct dimension in the direction of the jog, keep the angle at 30 degrees or shallower so the turn stays gentle; the calculator shows the ratio and flags it. For a very large jog on a narrow duct, two long-radius elbows with a straight between them can actually be the better fitting, and the elbow page sizes those.

Gauge

Sheet-metal gauge for low-pressure duct is set by the largest dimension of the fitting. The size pages on this site default to the gauge that a two-inch-water-column residential or light-commercial system would use: 26 gauge up to about 14" on the long side, 24 gauge to about 26", 22 gauge to about 40", and 20 gauge above that. An offset is a little less stiff than a straight duct of the same size because two of its walls are bent twice, so on larger sizes or longer offsets it is reasonable to go one gauge heavier than the straight duct next to it. Step up a gauge as well for kitchen exhaust, dust collection, outdoor or rooftop work, anywhere the fitting will be walked on, or any medium-pressure system. Every size page lets you change gauge in the designer and the price updates on the spot.

Galvanized, aluminum or stainless

Galvanized steel is the standard for heating and cooling duct and is what nearly every offset we build is made of. It is the least expensive, it takes duct sealant and tape well, and it matches the rest of the system. Aluminum weighs about a third as much, does not rust, and is specified for pool and spa air handlers, coastal installations, some laboratory exhaust, and anywhere a fitting has to be lifted into place by hand. Stainless steel is the material for commercial kitchen grease exhaust, chemical and process exhaust, washdown environments, and any fitting that will be exposed and must stay bright. Each material is priced live; the popular-sizes table above shows all three for two common sizes so you can see the difference.

How the ends connect

Both ends are finished with the connection you choose, and the two ends can be different. Slip-and-drive is the residential and light-commercial standard: the two long edges receive an S-cleat and the two short edges receive a drive cleat, so the offset joins a trunk section the same way any rectangular joint does. Raw leaves the edge plain for a field connection or for a collar to slip over. FoldOut and DoubleFoldOut turn the edge into a flange for larger commercial duct, and a DuctMate-style flange is the four-bolt transverse connection. Because the inlet and outlet of an offset are the same size, the same cleats and drives fit both ends. Install with the drive cleats on the short sides, seal the joint with mastic or UL 181 foil tape, and support the fitting at both ends; an offset carries a small side load from the jog and should not hang from one end.

How to measure the obstruction

1. Duct size: the inside width and depth of the duct the offset joins. The nominal size written on the duct is the inside dimension.
2. Offset distance: how far the centerline of the duct has to move, measured perpendicular to the run. Around a beam, that is the beam depth plus the clearance you want, plus half the difference if the duct also has to change which face it rides on.
3. Direction: whether the jog is in the width direction (sideways on a flat duct) or the depth direction (up or down). The designer takes both, so a compound jog is one piece.
4. Length: the space available along the run between the two connections, less the overlap at each end. Compare it with the calculator's length at 30 degrees; if the space is shorter, use 45 degrees, and if it is longer, use it all.

Why field-made offsets go wrong

The field version of an offset is two elbows from the truck and a stub cut to fit, or a piece of straight duct with the corners notched and the walls kinked over. The elbow version leaks at three joints and turns the air twice at 90 degrees. The kinked version has no true angle at all, the walls buckle where they were forced, and the outlet comes out twisted so the next section will not sit flat. A shop-made offset has the sloped walls laid out to the true parallelogram, corners that are locked rather than overlapped, and inlet and outlet that are parallel and square to each other by construction. On a supply trunk the difference is measurable in static pressure; on a kitchen or process exhaust, where the duct must be sealed grease-tight or airtight, the field version is not acceptable at all.

Ordering, price and turnaround

Set the four dimensions in the designer, choose material, gauge and connections, and the price shown is the price you pay; there is no quote step for standard configurations. Each offset is made to order in our Pittsburgh shop from full sheets, so an unusual size costs the same as a common one of the same amount of metal, and quantity does not change the unit price the way it does for stamped fittings. Add the offset to the cart with the rest of the job, or order any of the 338 published sizes from its own page. For offsets that also change size, TDC flanges, heavier gauges than the designer offers, or a takeoff of a drawing, call 412.725.2744 or email sales@pmxductwork.com with the dimensions and we will price it the same day we read it.

Read more

• Offset duct fittings: how to clear obstructions without losing airflow: the angle rule in more depth.
• Custom duct offsets made to order: what to specify and where the fitting shows up on real jobs.
• Duct elbow sizing guide: for the cases where two long-radius elbows beat an offset.
• Every published duct offset size with its price.