Sheet metal duct fabrication and installation field guide

What duct construction covers, and why it decides whether the air arrives

Sheet metal duct construction is the gauge, the seam, the joint, the reinforcement, the sealing, and the support that turn a sized opening on a drawing into a duct that holds its pressure. The design picks the dimensions so the blower can move the air. Construction decides whether the air gets where it was sent or leaks into a ceiling cavity on the way.

Keep the two jobs straight. Sizing is a separate discipline, covered in the Manual D duct design guide: it sets how big the duct is from the load and the static budget. This guide is the other half, the building of it. The same duct that calculates perfectly on paper will starve the far room if it is built under-gauge, sealed loose, or hung so it sags and opens its joints.

A leaky, under-built duct wastes the air and the energy that moves it twice. Once because the conditioned air dumps where nobody asked for it, and again because the blower runs harder to make up the loss and slides down its curve. SMACNA, the Sheet Metal and Air Conditioning Contractors' National Association, publishes the HVAC Duct Construction Standards that tie every one of these choices to one input: how much static pressure the duct has to hold.

What is a SMACNA pressure class?

A SMACNA pressure class is the operating static pressure the duct is built to hold, and it is the single input that drives the gauge, the reinforcement, the seam, the joint, and the seal. SMACNA defines seven classes by inches of water gauge: 1/2 in, 1 in, 2 in, 3 in, 4 in, 6 in, and 10 in w.g. Each is specified as positive, negative, or both.

Positive and negative matter because they fail differently. A duct under positive pressure tries to balloon outward and blow its seams. A duct under negative pressure, common on the return and on the suction side of the fan, tries to collapse inward, so the gauge and reinforcement have to resist buckling, not just bursting. One system routinely contains both: the supply runs positive, the return runs negative, and each section is built to its own class.

Where the designer does not call out a class, SMACNA treats 1 in w.g. as the basis of compliance. Do not let that default ride on a high-static system. A variable-air-volume system, a long run with a lot of fittings, or a fan with a strong shutoff head can push a duct well past 1 in, and a duct built to the wrong class is the cheapest mistake to make and the most expensive to chase later. Confirm the class against the project specification and the design static before the shop cuts metal.

What gauge should ductwork be?

Duct gauge comes from a SMACNA table, not from habit, and it is set by three things together: the pressure class, the duct's longest side, and the reinforcement you add. A lower gauge number is thicker metal, so 20 gauge is heavier than 26 gauge. The bigger the duct and the higher the pressure, the heavier the gauge, because a wide unsupported panel under pressure wants to flex.

The trade-off SMACNA builds into the table is gauge against reinforcement. You can carry a lighter gauge on a large duct if you add stiffeners at the right spacing, or you can run a heavier gauge with fewer stiffeners. Both can satisfy the standard. The table gives the allowed combinations for each pressure class, and the right answer is whichever the shop and the spec settle on, not whichever the cutter reached for.

The representative numbers below are for galvanized steel in the common low-pressure range and they move with the pressure class and the reinforcement, so treat them as orientation and pull the actual gauge from the SMACNA table for the class on the job. The failure to watch for is a shop standard that uses one gauge for everything. That over-builds the small duct and, far worse, under-builds the large high-pressure duct that needed heavier metal or more stiffeners than the house default ever gave it.

Reinforcement: stiffeners, cross-breaking, and beading

Reinforcement is what keeps a large flat panel from oil-canning, the popping in-and-out flex that makes a duct bulge and drum when the fan cycles. The wider the unsupported sheet and the higher the pressure, the more it wants to move, and the noise and the bulge both come from the same place. SMACNA sets the reinforcement by pressure class and duct size alongside the gauge.

Three methods do the work, and they are not interchangeable. Cross-breaking presses two diagonal creases corner to corner so they intersect, which stiffens the whole panel and sheds any water that lands on a top surface. Beading rolls a series of parallel lines into the sheet that stiffen it without intersecting, which is faster on a machine and common on lighter gauges. Stiffeners are separate members, angle, hat channel, or formed metal, attached across the duct at a spacing the table sets, and they carry the large or high-pressure duct that a crease alone cannot hold.

There is a ceiling on the cheap methods. Heavier gauges do not take a cross-break or a bead well, and a panel can get so large that a crease will not support the metal's own weight, let alone the pressure. Past that point the table calls for stiffeners on a spacing, and skipping them is exactly what produces the wall that booms every time the unit starts.

Longitudinal seams: Pittsburgh, snap-lock, and standing seam

A longitudinal seam runs along the length of the duct, parallel to the airflow, and it is what closes a rectangular duct into a tube. The seam you pick has to suit the gauge and the pressure, because the seam is a candidate leak path and a candidate weak line all at once.

The Pittsburgh lock is the workhorse for heavier gauge, larger, and higher-static duct. One edge is formed into a pocket with a standing lip, the other into a flange, and a Pittsburgh machine folds the lip over to lock the two together mechanically. It holds well and seals well when it is run tight. The snap-lock, sometimes called a button-punch snap-lock, is for the smaller, lighter, low-pressure duct and fittings: the male edge carries a button that catches inside the female edge when the two are knocked together, so it goes up fast without a separate locking pass. The standing seam is a raised seam used to add stiffness on large duct, doing double duty as a seam and a stiffener.

Snap-lock and Pittsburgh are the two seams most shops run and most specs name. The mistake is carrying a snap-lock up into a pressure class it was never meant for. On medium and high-pressure duct the Pittsburgh, run tight and sealed, is the seam that holds; a snap-lock pushed past its range works open and leaks along its whole length.

Transverse joints: slip-and-drive and TDC/TDF flanges

A transverse joint connects two duct sections end to end, perpendicular to the airflow, and it is where most field leakage lives because there are so many of them. SMACNA classes transverse connections by strength, and the class has to match the pressure the joint will see.

Slip-and-drive, also called S-and-drive, is the most common low and medium-pressure connection. A flat S-slip caps two edges and drive cleats are hammered over the other two to pull the sections together. It is fast, it is cheap, and for low and medium-pressure work, built and sealed right, it meets the standard. The drives have to be fully engaged and the corners sealed, because that is where a slip joint leaks.

For higher pressure and tighter leakage, the flanged systems take over. TDC and TDF roll a flange directly onto the end of the duct, then a gasket is laid in, the flanges are mated, corner bolts go in, and cleats are crimped along the flange at a close spacing. The four-bolt flange is the strongest rectangular transverse connection SMACNA recognizes and it is the one to reach for on minimum-leakage and high-static work. The gasket is not optional on these. Metal to metal without a continuous gasket leaks no matter how tight the bolts are.

Round and flat-oval duct

Round duct holds pressure better than rectangular for the same gauge because the shape carries the load in hoop tension instead of flexing a flat panel, so it resists oil-canning without the reinforcement a rectangular duct needs. It also moves air with less friction for the same area, which the duct design guide gets into; here the point is that the round shape is stronger and tighter as a built thing, not just as a flow path.

Two seam styles dominate. Spiral duct is made by locking a continuous strip into a helical seam, which is both the seam and the stiffener and is why spiral runs straight and rigid in long pieces. Longitudinal-seam round, rolled and seamed along its length, is the other. Round fittings are typically gored elbows and stamped or welded tees and takeoffs. Flat-oval is round duct flattened to fit a shallow space, keeping most of round's strength and seal while losing some of the height.

Round and oval reach a lower leakage class than rectangular for the same effort, because there are fewer linear feet of seam and the slip-and-coupling joints seal cleanly. When the ceiling space and the architecture allow it, round is the easier duct to build tight.

Rectangular or round: which to build?

Build rectangular when the space is tight and the duct has to fit a shallow plenum or a chase, because a rectangular duct can be made wide and short to fit where a round of the same area never would. Build round when you can, because it is stronger, quieter, seals tighter, and needs less reinforcement for the same pressure.

The honest trade-off is fit against performance. Rectangular wins on fitting into the building and on tapping branches anywhere along a trunk. Round wins on strength, on leakage, on noise, and on the lower friction loss the design guide covers. Most commercial jobs run a mix: rectangular trunks where the structure is tight, round and oval branches and risers where there is room to swing them.

Flat-oval is the compromise built for exactly this conflict. It keeps most of round's seal and strength while flattening into a space rectangular would otherwise own. When the spec leaves it open, the run that has room should be round.

How do you seal ductwork?

You seal ductwork by closing every leak path the air can find, to the seal class the pressure demands, using mastic, mastic-and-tape, or a gasket rather than relying on the metal joint alone. SMACNA defines three seal classes by what must be sealed. Seal Class A seals all transverse joints, all longitudinal seams, and all duct wall penetrations. Seal Class B seals the transverse joints and the longitudinal seams. Seal Class C seals the transverse joints only.

The seal class scales with the pressure class, because higher pressure pushes air through any opening harder. SMACNA pairs the lower pressure classes with the lighter seal requirement and the higher pressure classes with Seal Class A, so the 4 in, 6 in, and 10 in duct gets everything sealed. Confirm the required seal class against the project specification and the current SMACNA edition, since the energy code often drives it tighter than the pressure class alone would.

Material matters as much as coverage. Duct mastic brushed or troweled over the joint, with fabric tape embedded where the gap is wide, is the durable seal. Pressure-sensitive tape on its own is not accepted as a primary sealant unless it is listed to UL 181A or 181B and used per that listing, because plain tape lets go as it ages and the joint reopens. Gasketed flanges seal at the gasket, so the gasket has to be continuous and unbroken at the corners, which is exactly where a hurried crew leaves a gap.

Duct leakage and the energy code

Leakage is rated by leakage class, written CL, the leakage in cfm per 100 square feet of duct surface at 1 in w.g. The allowable leakage at any tested pressure follows the SMACNA relationship F equals CL times P to the 0.65 power, where F is the cfm per 100 square feet and P is the test static in inches of water. A lower CL is a tighter duct.

The energy code is what turns sealing from good practice into a requirement. ANSI/ASHRAE/IES Standard 90.1 calls for ducts and plenums to be built to Seal Class A and holds rectangular metal duct to a leakage class around CL 4, with round and oval expected tighter still because they leak less for the same effort. The IECC carries parallel sealing and testing language. The exact numbers and the duct that has to be tested move between code editions, so confirm against the adopted edition and the project spec.

Testing is triggered by pressure. Under 90.1, duct designed to operate above 3 in w.g. is leakage-tested, commonly all of the outdoor-air duct and a representative fraction of the rest. A built duct that is never tested leaks quietly for the life of the building, which is the spec-versus-reality gap the static pressure guide runs into from the other direction when the installed static comes back high and the airflow comes back low.

How often do you support ductwork?

You support ductwork on the spacing SMACNA sets by duct size and type, and you add a hanger near every fitting and every heavy item. For typical rectangular duct the maximum spacing commonly lands in the 8 to 10 ft range, with closer spacing on larger and heavier duct; round duct can run farther between supports. Pull the exact spacing from the SMACNA hanger table for the size on the job rather than eyeballing it.

The hanger type follows the duct. A galvanized strap wrapped under the duct carries small rectangular and small round. A trapeze, two rods dropping to an angle or a channel under the duct, carries the larger rectangular. A split band or saddle carries round so the strap does not crush it. Whatever the type, the hanger has to reach sound structure and the attachment to that structure has to be rated for the load, because a hanger that pulls out of a weak anchor is the same as no hanger.

A sagging duct opens its joints, so support is a sealing issue as much as a safety one. The other common failure is the run hung tight everywhere except next to a fire damper or a heavy coil, where the concentrated weight needs its own support and rarely gets it. In seismic regions the supports also have to resist lateral movement: the SMACNA Seismic Restraint Manual sets transverse and longitudinal bracing on a maximum spacing and exempts smaller duct by cross-section and diameter, with the required Seismic Design Category coming from ASCE 7. Confirm whether bracing applies and on what spacing for the SDC and the duct on the job.

Flexible duct: where it helps and where it kills the system

Flex is fine for the last short connection to a register or a diffuser and nowhere else. Kept short, pulled tight, and fully supported, a flex tail is a quiet, fast connection. Run long, left slack, kinked over a joist, or crushed above a light fixture, it becomes the single biggest avoidable loss in the system.

The reason is friction. A flex duct has far more resistance per foot than smooth metal even when it is perfect, and a sagged or compressed flex multiplies that several times over, which the duct design guide quantifies as a flex penalty when it sets the friction rate. Build it like the design assumed: hold the length to the minimum, support it so it does not sag between hangers, and never compress it to make it fit. A compressed flex throttles the branch it feeds, and the room downstream starves.

The inspector and the commissioning agent both look at flex first, because it is where the field most often departs from the drawing. A clean flex tail is short and stretched. A long, droopy, kinked tail is a callback waiting to happen.

Fittings: elbows, takeoffs, transitions, and turning vanes

Fittings are where the air turns, splits, and changes size, and a badly built fitting wastes more pressure than a long straight run. The duct design guide covers the loss numbers; the construction job is to build the low-loss version the design assumed instead of the cheap one the shop reached for.

An elbow turns the air, and a wide radius turns it with less loss than a tight one. Where a tight square elbow is unavoidable in a rectangular trunk, turning vanes installed inside it guide the air around the corner and cut the loss and the noise a bare square elbow makes. The vanes have to be the right profile, anchored top and bottom, and actually present, because the vanes left out of a square elbow is a common and invisible loss until someone reads the static.

Takeoffs tap a branch off a trunk, and a conical or angled takeoff pulls air into the branch far better than a square hole cut in the side of the trunk. Transitions change the duct size, and a gradual transition, commonly held to a shallow slope so the air follows it, beats an abrupt step that trips the flow. Build the radius, the vane, the takeoff, and the transition the way the design priced them, or the friction the design budgeted is gone before the air reaches the room.

Fire and smoke dampers in the duct

Where duct passes through a fire-rated or smoke-rated assembly, the opening needs a damper that closes to keep fire or smoke from riding the duct through the barrier. A fire damper holds back flame and is listed to UL 555. A smoke damper holds back smoke and is listed to UL 555S. A combination fire/smoke damper does both and is listed to both. The rated wall or floor and the applicable mechanical code, not the duct, decide where one is required.

Installation is governed by the damper's listing, the manufacturer's instructions, and the code, and the details are not negotiable. The damper sits in a sleeve, the sleeve extends a limited distance past the barrier, and the duct ties to the sleeve with a breakaway connection so the duct can drop away in a fire without dragging the damper out of the wall. The retaining angles, the clearances in the opening, and the actuator on a motorized damper all follow the listing.

Every damper needs access to reset it and to inspect it, and the access door has to be labeled in letters at least 1/2 in high reading FIRE DAMPER or SMOKE DAMPER so the next person can find it. A damper with no access is a damper nobody can test, and an untestable life-safety device fails the inspection it exists to pass.

Access doors and cleaning access

Build access doors wherever something downstream of the metal has to be reached: a fire or smoke damper to reset, a coil to clean, an in-duct device to service, or a run that has to be cleaned. An access door is a framed, gasketed, latched opening that seals to the same class as the duct around it, not a cut panel screwed back on.

Put the door where the work actually happens and size it for a hand and a tool, not just for code-minimum. The two failures are a door too small to reach the device through, and a door that leaks because the gasket was an afterthought. On a sealed system an unsealed access door is a hole in the seal class you worked to hit everywhere else.

Where a duct serves a kitchen or a system that will be cleaned on a schedule, the cleaning access has its own spacing and sizing rules in the applicable code and the NFPA standards for that system. Lay those openings out during fabrication, because cutting them in later means breaking back into a sealed, insulated, hung duct.

Liner and external insulation

Duct is lined or wrapped for two reasons that get confused: acoustic control and thermal control. Internal liner, glued and pinned to the inside of the duct, absorbs fan and air noise and adds thermal value. External wrap, blanket insulation banded around the outside, adds thermal value and controls condensation without touching the airstream. The insulation by topic and the project spec set which one and what thickness.

Liner has a failure mode wrap does not: erosion. Liner sits in the airstream, so it has to be the right density for the velocity, pinned on the spacing the standard sets, and coated or cut so the leading edges do not lift. Liner that erodes sheds fibers into the air and into the coil, and once it starts lifting it peels. On high-velocity duct the liner spec is tighter for exactly this reason, and a generic liner pinned loose is a coil-fouling problem two years out.

External wrap fails differently, usually at the seams and the hangers, where a gap or a crush in the insulation lets the metal sweat and drip. On cold supply duct in a humid space the vapor barrier on the wrap has to be continuous, because a break in it condenses inside the insulation where nobody sees it until the ceiling stains.

Kitchen grease duct and welded duct

A commercial kitchen exhaust duct is a different animal from comfort duct and it is not built from snap-lock and mastic. Grease-laden exhaust runs in a welded, liquid-tight duct with continuous external welds, because the duct carries flammable grease and has to contain a fire long enough for it to be controlled. The gauge, the welding, the slope to drain, the clearances to combustibles, and the access for cleaning all follow the mechanical code and NFPA 96, not the comfort-duct standard.

The make-up air that replaces what the hood exhausts is ordinary supply duct and gets built to its pressure class like any other run, which is where this connects back to the rest of the guide. The exhaust side is the specialty. Treat it as welded, listed, code-driven work, and confirm the grease-duct construction and reinforcement against NFPA 96, the mechanical code, and the SMACNA grease-duct guidance for the application.

Do not let a comfort-duct crew freelance a grease duct. The welds have to be continuous and tested, the cleanouts have to land on the required spacing, and the listing on a factory-built grease duct controls how it is assembled. This is the section where the wrong build is a fire, not a callback.

Installing it right: level, sealed, supported, and not kinked

Good fabrication dies in a bad install. The duct can leave the shop built to class and still leak, sag, and roar if it goes up wrong, so the field work carries the same weight as the shop work. The workmanship that matters is plain and it is checkable.

Run the trunk straight and level so the joints stay square and the slip-and-drives stay engaged; a duct that is racked to fit pulls its joints open. Seal each joint as it goes up, not at the end, because a joint sealed after the next section is hung is a joint half-sealed. Support on the SMACNA spacing and add a hanger at every fitting and every concentrated load. Keep flex short, stretched, and supported, and never crush it to clear an obstruction. Protect the open ends during construction, because a trunk left open collects dust and debris that ends up at the coil.

The blunt version: the duct only performs as well as the worst joint, the longest unsupported span, and the most crushed flex tail in the run. A crew that builds tight and hangs loose has built nothing.

Commissioning and the leakage test

The leakage test is how you prove the duct holds its class instead of hoping it does. A section is capped, pressurized with a calibrated fan to the test static, and the make-up airflow needed to hold that pressure is the leakage, compared against the allowable from the leakage class. Pass means the build met the spec. Fail means there are open joints to find and seal before the duct gets buried.

Test before the duct is hidden. A leakage test run after the ceiling closes and the insulation goes on is a test you cannot act on, because finding and sealing the leak now means demolition. The agent's job is to make a quiet failure loud while it is still cheap to fix, which means pressurizing the duct while every joint is still reachable.

Leakage and installed static are two readings of the same system from opposite ends. The leakage test pressurizes a built section to confirm the seal; the static pressure check, covered in the external static guide, reads what the running system actually fights and ties it back to airflow with a tool like blowercfm. A duct that passes leakage and still reads high static has a sizing or a fitting problem, not a sealing one, and knowing which is which is the difference between sealing joints and re-cutting fittings.

High-pressure, data center, and critical air-handling duct

On high-pressure central systems, data center cooling, and critical air handling, the duct construction stops being routine and becomes the thing the whole airflow depends on. These systems run higher pressure classes, demand Seal Class A and a tight leakage class, and carry airflow that the facility cannot afford to lose into a ceiling. The build margin that is optional on a small comfort system is mandatory here.

Two things change with the stakes. The leakage allowance gets tighter and is verified by test rather than assumed, because a few percent of leakage on a large high-pressure system is a large absolute volume of conditioned air and a measurable energy cost that runs every hour. And the joints lean toward the flanged, gasketed connections that hold pressure and seal repeatably, because a slip joint that is fine at 1 in is the wrong call at 6 in.

Confirm the pressure class, the seal class, and the leakage class against the project specification, which on these jobs is usually stricter than the SMACNA default and is the document that controls. Build to the spec, test to the spec, and document both.

What to document

Build the duct to its pressure class and the work still disappears above the ceiling the day it goes up, so when the airflow later comes up short the only thing that tells the story is what you recorded. The record is what tells the next crew, the commissioning agent, and the inspector that the duct was built to its class, and it is what isolates the problem when something downstream runs wrong.

For each run or system, capture the pressure class it was built to, the gauge and the reinforcement, the seam and the transverse joint used, the seal class and the sealing method, the leakage class targeted and the leakage test result, the hanger type and spacing, and any dampers with their access. If the build departed from the drawing, write down what changed and why, because the person reading it later needs to know whether the duct in the ceiling matches the duct on the plans.

Common mistakes

• Running one shop-default gauge for everything, which under-gauges the large high-pressure duct.
• Leaving out the stiffeners or the cross-break on a wide panel, so the duct oil-cans and drums.
• Carrying a snap-lock or a slip-and-drive up into a pressure class that needed a Pittsburgh or a flanged joint.
• Treating the 1 in w.g. default as the class on a system that actually runs higher static.
• Sealing to the wrong seal class, or using plain tape that is not listed as a primary sealant.
• Skipping the gasket, or breaking it at the corners, on a flanged transverse joint.
• Hanging supports past the SMACNA spacing, or missing a hanger at a fitting or a heavy damper.
• Running flex long, slack, kinked, or crushed instead of short, stretched, and supported.
• Leaving turning vanes out of a square elbow and the loss invisible until someone reads the static.
• Closing the ceiling before the leakage test, so a failure can only be fixed by demolition.

Field checklist

Standards and references

SMACNA's HVAC Duct Construction Standards, Metal and Flexible, is the controlling document for the build: it sets the pressure classes, the gauge and reinforcement tables, the seam and joint construction, the seal classes, and the hanger and support spacing. The HVAC Air Duct Leakage Test Manual gives the leakage classes and the test method, and the SMACNA Seismic Restraint Manual covers the bracing where the Seismic Design Category requires it. Cite the section by topic and confirm it against the edition the job is built to, because the tables and the numbers move between editions.

The energy code drives the sealing and the testing. ANSI/ASHRAE/IES Standard 90.1 calls for Seal Class A and a leakage class for ducts and triggers leakage testing above a static threshold, and the IECC carries parallel requirements. The adopted code edition and any local amendments control, so verify them rather than quoting a remembered number.

Fire and smoke dampers are listed to UL 555 and UL 555S and installed per the listing, the manufacturer's instructions, and the mechanical code, which also sets where dampers and access are required. Kitchen grease exhaust duct follows NFPA 96 and the mechanical code, not the comfort-duct standard. Above all of these, the project specification governs, and on critical and high-pressure systems it is usually stricter than the SMACNA default.

Units, terms, and conversions

Duct construction crosses a few unit systems and a lot of shorthand, so the same value reads differently on a drawing, a spec, and a SMACNA table.

Pressure class is in inches of water gauge, written in w.g. or in. wc, and one inch of water gauge is about 249 pascals. Gauge is the sheet thickness, where a lower number is thicker metal, and the metric equivalent is millimeters. Leakage class CL is cfm per 100 square feet of duct surface at 1 in w.g., and allowable leakage at another pressure follows F equals CL times P to the 0.65 power. Seal class A, B, or C describes what gets sealed, while leakage class is the measured tightness; the two are related but not the same thing.

Pressure class

The operating static the duct is built to hold, in inches of water gauge, positive or negative

Gauge

Sheet metal thickness, where a lower gauge number is thicker, heavier metal

Seal class (A/B/C)

What must be sealed: A is everything, B is joints and seams, C is transverse joints only

Leakage class (CL)

Allowable leakage in cfm per 100 sq ft at 1 in w.g., used in F = CL x P^0.65

Longitudinal seam

The seam along the duct length, parallel to airflow, such as a Pittsburgh lock or snap-lock

Transverse joint

The connection between duct sections, perpendicular to airflow, such as slip-and-drive or a TDC flange

Oil-canning

The popping flex of an unsupported panel under pressure, stopped by reinforcement

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