Seams vs joints — two different problems
Before comparing seam types it helps to separate two jobs that are often confused. A longitudinal seam runs along the duct and closes a single section into a tube (round) or a four-sided box (rectangular). A transverse joint runs around the duct and connects one finished section to the next, end to end. They are formed by different machines and judged by different criteria. This page is about the longitudinal seam; the transverse joint — TDF flange, angle-iron flange, drive and S-cleat — is covered in TDF vs angle flange. Get the seam right first, because no flange can rescue a duct whose longitudinal seam leaks or pulls open.
Six longitudinal seam families dominate commercial and industrial duct. The first three (Pittsburgh, snap-lock, button-punch) are mechanical locks for rectangular duct; the standing seam is a mechanical lock used on both rectangular panels and round-to-rectangular transitions; the spiral lockseam is the continuous seam that is spiral round duct; and the welded seam is the choice when nothing mechanical will hold.
1. Pittsburgh lock — the rectangular standard
The Pittsburgh lock is the workhorse longitudinal seam for rectangular HVAC duct and the SMACNA default for low- and medium-pressure work. It is a five-bend interlock: one edge of the flat blank is roll-formed into a female pocket with an inner hook, the opposite edge into a single male flange. The blank is folded into a box, the flange is dropped into the pocket along the corner, and a peening hammer or closing roll flattens the lock for the full length of the section. The result is a multi-ply mechanical interlock that is self-supporting before any sealant — air-tight to SMACNA Seal Class C as-formed, and Class A or B with a water-based sealant run into the pocket.
Machine. The Lockformer (SBLC Series) roll-forms the Pittsburgh profile from flat sheet in a single pass through progressive forming rolls. The SBLC handles 0.4–1.2 mm in galvanised, stainless and aluminium, draws 1.5–3 kW depending on model, and weighs 150–350 kg on a 380 V / 50 Hz / 3-phase supply. On a fully automatic line the lockformer is integrated in-line: the SBAL-V auto duct line delivers a blank with both edges already formed and the box folded ready for closure, and the SBAL-III runs the same station at lower line speed. For the full rectangular-seam deep-dive — Pittsburgh against snap-lock and double-seam on leak class, gauge and assembly time — see Pittsburgh lock vs button-punch snaplock.
2. Snap-lock — the speed seam
Snap-lock trades the Pittsburgh's mechanical strength for assembly speed. The female edge is a simple C-shaped channel and the male edge a matching tab; the two are pushed together and the seam snaps shut by hand, with no hammering or rolling. It is the fastest rectangular seam to assemble on site and the natural choice for high-volume light-commercial and residential trunk duct. The trade-off is gauge and pressure: snap-lock is best on lighter sheet and lower pressure, and on its own it is not as positively locked as a Pittsburgh.
Machine. Snap-lock is a different roller set on the same Lockformer (SBLC Series). The SBLC is rated 0.7–1.5 mm for snap-lock forming (stainless 0.7–1.0 mm), and a fabricator can switch a single lockformer between Pittsburgh and snap-lock tooling, so one machine covers both seams.
3. Button-punch snap-lock — snap-lock that stays shut
Button-punch snap-lock is a snap-lock that is mechanically locked against pulling apart. After the seam is snapped together, a punch press dimples (button-punches) the joint every 75–100 mm (3–4 in). Each dimple deforms both plies and pins them so the seam cannot work itself open under handling or pressure cycling. It keeps almost all of plain snap-lock's assembly speed while adding positive retention, which is why it is the common factory finish on snap-lock duct. It remains a low-pressure seam (about 2 in. w.g. / 500 Pa), and the dimples must be evenly spaced — a crew skipping every second punch leaves a seam that passes first inspection and opens later.
Machine. The Lockformer (SBLC Series) forms the snap-lock profile; a separate punch press (often air-powered and portable) applies the button-punch dimples. Many shops run the punch as a bench station immediately after the folder.
4. Standing seam — the stiffened lock
A standing seam is a mechanical lock that, instead of being flattened against the duct wall, is folded so the joined plies stand up proud of the surface. The raised seam acts as a built-in stiffener, adding rigidity to a flat panel without a separate reinforcing bar, and it is used where panel stiffness or a particular profile is wanted — large flat sides, plenum panels, and some round-to-rectangular transitions. It is more positively locked than a plain snap-lock and does not need fasteners, but the standing profile costs internal clearance and is slower to form than a flat lock.
Machine. The standing seam is roll-formed on a lockformer or folder roller set, then the joined edges are stood and locked. On the SBKJ floor it is produced on the same forming family as the other mechanical locks, with the roller set matched to the chosen profile and gauge.
5. Spiral lockseam — the seam that is the duct
Round spiral duct does not have a separate seam-closing step — the seam is how the duct is made. A continuous strip of coil (137 mm standard on the SBKJ SBTF series) feeds into a spiral tubeformer and is wound helically; as the tube grows, the two adjacent strip edges are folded over each other and rolled flat into a continuous helical lockseam. The seam therefore forms at the same rate as the tube and runs the full helical length, which is why spiral duct is fast, strong and dimensionally consistent. The raised lockseam also stiffens the tube, so spiral duct holds its shape at a lighter gauge than welded round duct.
Machine. The Spiral Tubeformer SBTF-1500 forms Φ80–Φ1500 mm round duct from a 137 mm strip in galvanised steel 0.4–1.2 mm and stainless 0.4–0.8 mm, with saw-blade cut-off, a 5.5 kW forming head plus a 4 kW saw, at 1700 kg. Larger and heavier-gauge machines in the same series extend the range — see the spiral duct machine spec comparison for the full SBTF line — and the spiral duct forming guide walks through the helical lockseam step by step. Spiral machinery is grouped under spiral duct machinery.
6. Welded longitudinal seam — when nothing mechanical will hold
A welded longitudinal seam replaces the mechanical lock with continuous metal. It is specified when the duct must be fully gas-tight, carry high pressure, or handle aggressive fume, grease or temperature — stainless laboratory and hospital exhaust, kitchen-grease duct, fume-extraction and high-pressure industrial duct. Because the joint is fused, it is limited by the sheet rather than by a lock profile, so it reaches the highest pressure class and meets SMACNA Seal Class A with no sealant. The cost is speed and skill: welding is slower than roll-forming, needs trained operators and fume extraction, and adds heat that can distort thin sheet.
Machine. A continuous lap seam is made on the Seam Welder (SBFN-35 / 55 / 75 / 100), which welds 0.4–1.0 / 0.4–1.2 / 0.4–1.5 / 0.4–2.0 mm at 45 / 55 / 75 / 110 KVA and 0.5–3 m/min, 328–478 kg. Where a fully gas-tight seam is not required on round duct, the Stitch Welder (SBSW-30-2Z) lays an intermittent seam on Φ100–Φ1000 mm duct at 0.4–1.0 mm and about 2 m/min. The full welder line-up is compared in the duct welder comparison and grouped under duct welding machines.
Comparison table
Machine figures below are taken verbatim from the SBKJ Product Catalog 2026. Pressure classes are the practical ceilings the industry applies to each seam; achievable gauge and speed depend on material temper and model.
| Seam type | Forming / closing machine | Material & gauge | Pressure class (typical) | Speed | Where used |
|---|
| Pittsburgh lock | Lockformer SBLC Series | GI / SS / Al 0.4–1.2 mm | Up to 4 in. w.g. (1000 Pa) | Single-pass roll-form | Commercial rectangular trunks & branches |
| Snap-lock | Lockformer SBLC Series (snap-lock rolls) | 0.7–1.5 mm (SS 0.7–1.0) | Low pressure | Fastest hand assembly | Light-commercial & residential duct |
| Button-punch snap-lock | Lockformer + punch press | 0.7–1.5 mm | ≈ 2 in. w.g. (500 Pa) | Snap + punch every 75–100 mm | Factory-finished low-pressure duct |
| Standing seam | Lockformer / folder roller set | Light–medium gauge | Low–medium pressure | Roll-form + stand | Flat panels, plenums, transitions |
| Spiral lockseam | Spiral Tubeformer SBTF-1500 | GI 0.4–1.2 / SS 0.4–0.8 mm, 137 mm strip | Medium pressure (round) | Continuous, Φ80–Φ1500 mm | Round supply, exhaust, dust & mine vent |
| Welded (continuous) | Seam Welder SBFN-35/55/75/100 | 0.4–2.0 mm | High pressure / Seal Class A | 0.5–3 m/min, 45–110 KVA | Stainless, fume, grease, high-pressure industrial |
| Welded (stitch) | Stitch Welder SBSW-30-2Z | 0.4–1.0 mm, Φ100–Φ1000 mm | Medium (not fully gas-tight) | ≈ 2 m/min | Round duct where a sealed seam is not required |
Source: SBKJ Product Catalog 2026, manufacturer nameplate specifications. Mechanical-lock seams conform to SMACNA / EN 1505 rectangular and EN 1506 round construction. Pressure classes are practical industry ceilings, not machine limits. Most machines run 380 V / 50 Hz / 3-phase (60 Hz on request).
How to choose the seam — a short decision path
1. Round or rectangular? Round duct off a coil is spiral lockseam by definition — the tubeformer makes the seam as it makes the tube. Rectangular duct is a mechanical lock or a weld.
2. What pressure and leak class? Low-pressure residential and light commercial → snap-lock or button-punch snap-lock. Medium-pressure commercial → Pittsburgh lock. High pressure, or Seal Class A with no leakage budget → welded seam.
3. What material and gauge? Light galvanised → any mechanical lock. Heavier galvanised, or stainless and aluminium that must stay gas-tight → welded. Stainless that only needs structural joining on round duct → stitch weld.
4. What is the constraint — speed or strength? If site assembly speed rules, snap-lock wins. If the seam must never open and must seal, weld it. For the broad middle of commercial HVAC, the Pittsburgh lock is the balance, which is exactly what auto duct lines produce natively.
A fuller process picture — coil to cut blank to folded box to closed seam to flanged end — is set out in how HVAC duct is made, and seal-class targets are defined in SMACNA seal classes explained.
Tell us your seam, gauge and pressure for a machine recommendation →
FAQ
What is the difference between a longitudinal seam and a transverse joint on HVAC duct?
A longitudinal seam runs along the length of a section and closes the sheet into a tube or box — Pittsburgh, snap-lock, standing seam, spiral lockseam and welded seams are all longitudinal. A transverse joint runs around the duct and connects one section to the next end-to-end — that is the role of TDF flanges, angle-iron flanges and drive/S-cleats. This page covers longitudinal seams; transverse joints are covered in TDF vs angle flange.
What is a Pittsburgh lock seam?
A Pittsburgh lock is the standard interlocking longitudinal seam for rectangular duct. One edge is roll-formed into a pocket and the opposite edge into a single flange; the flange inserts into the pocket and is hammered or rolled closed into a multi-ply mechanical lock that is self-supporting before any sealant. SBKJ forms it on the Lockformer (SBLC Series) for 0.4–1.2 mm sheet.
How is the seam on spiral duct formed?
Spiral duct uses a continuous lockseam. A 137 mm coil strip (SBKJ SBTF series) feeds into a tubeformer, is wound helically, and the two strip edges are folded together and rolled flat as the tube grows — so the seam is made at the same time as the tube and never needs separate closing. The SBTF-1500 forms Φ80–Φ1500 mm round duct this way.
When is a welded longitudinal seam used instead of a mechanical lock?
When the duct must be fully gas-tight or carry high pressure, aggressive fume or high temperature — stainless laboratory, hospital, kitchen-grease and industrial-exhaust duct. A continuous resistance seam weld (SBKJ SBFN, 45–110 KVA, 0.5–3 m/min) gives a leak-tight seam to SMACNA Seal Class A that a mechanical lock cannot match without heavy sealing.
What machine closes each type of duct seam?
Pittsburgh and snap-lock are roll-formed on a lockformer (SBKJ Lockformer SBLC Series); button-punch snap-lock adds a punch press. The spiral lockseam forms continuously inside a spiral tubeformer (SBKJ SBTF series). Standing seam is roll-formed on a folder or lockformer and stood at the corner. Welded seams are made on a seam welder or stitch welder (SBKJ SBFN and SBSW series).
Which duct seam holds the highest pressure?
A continuous welded seam — it is limited by the sheet, not the joint, so it suits high-pressure industrial duct above 4 in. w.g. Among mechanical seams the Pittsburgh lock is the high-pressure default (up to about 4 in. w.g. / 1000 Pa) and button-punch snap-lock is restricted to low pressure (about 2 in. w.g. / 500 Pa).
Does a mechanical duct seam need sealant?
For SMACNA Seal Class C a well-formed Pittsburgh lock or snap-lock is air-tight enough unsealed. For Class A or B you must run a water-based duct sealant into the seam — into the Pittsburgh pocket before closing, or along the finished seam. A welded seam reaches Class A with no sealant because the joint is continuous metal.
