Why standard duct fails for grease and fire service
A normal comfort-air duct is built from light-gauge galvanised sheet (typically 0.5–0.8 mm) and closed with a mechanical lock seam — a Pittsburgh corner lock or a snap-lock, folded and rolled but never sealed. That construction is fast, cheap and perfectly adequate for moving conditioned air. It fails on two counts the moment the duct has to carry grease-laden vapour or survive a fire.
First, a folded seam is not liquid-tight. In a commercial kitchen exhaust, hot cooking vapour condenses on the cool duct wall, and the condensate is grease. It runs to the lowest point of the run and finds the seam. A lock seam will weep grease into the ceiling void, and that deposit is fuel: a grease fire in the hood can flash the length of the duct in seconds. Second, light galvanised sheet warps, splits and lets its zinc coating fume at flue-fire temperatures. The seam opens, the duct loses integrity, and the fire spreads into the building cavity it passes through.
The answer in every major code is the same: build the duct from heavier gauge and weld every seam continuously, liquid-tight. Grease, fire-rated and smoke-extract duct are welded fabrications, not folded ones. That single requirement is what drives the material, the gauge and the machinery choice on this whole class of duct.
Grease duct — commercial kitchen exhaust construction
Grease duct carries the exhaust from a commercial cooking hood to atmosphere. Its governing document in North America is NFPA 96, Standard for Ventilation Control and Fire Protection of Commercial Cooking Operations, and its requirements are echoed in the International Mechanical Code. The core fabrication rules:
- Continuous liquid-tight weld. All seams and joints are continuously welded — typically a continuous external weld — so no grease can escape and no seam sits on the bottom of a horizontal run where condensate pools.
- Heavier gauge. NFPA 96 sets a minimum of 16-gauge (~1.5 mm) carbon steel or 18-gauge (~1.2 mm) stainless steel — substantially heavier than comfort-air duct. Stainless is chosen where corrosion, hygiene or appearance matter.
- Slope and drainage. Horizontal runs slope back toward the hood or to a listed grease reservoir so condensate drains and does not stand against a seam.
- Cleanability and access. Cleanout openings are placed at intervals and at every change of direction; the duct must be reachable for the regular cleaning the code mandates.
- Clearance and enclosure. The duct is held clear of combustibles, or wrapped in a listed fire-rated enclosure where it passes through the building.
Round welded grease duct is generally preferred over rectangular: it self-drains, has no internal corners for grease to lodge in, and is far easier to clean. It can be formed in heavier gauge on a spiral tubeformer and then have its longitudinal seam welded continuously. Rectangular grease duct is built from heavier-gauge sheet that is cut, formed on a folder or roll line, and then continuously welded along every seam and corner. NFPA 96 permits both, provided the result is liquid-tight and accessible.
Fire-rated and smoke-extract duct
Fire-rated and smoke-extract duct exist to control where fire and smoke can travel inside a building. They are distinct duties:
Fire-rated (fire-resisting) duct maintains its integrity, insulation and load-bearing performance for a rated period — commonly 60 or 120 minutes — so a fire in one compartment cannot pass through the duct into the next. In Europe this is tested to EN 1366-1 and classified under EN 13501-4 (the familiar EI / E ratings). In Australia and New Zealand, fire-resistance levels for ductwork are established by test to AS 1530.4, within the air-handling fire-and-smoke control framework of AS 1668.1.
Smoke-extract (smoke-spill) duct does the opposite of containing the fire — it must keep running while hot, pulling smoke out of a building to keep escape routes clear. Because it operates at elevated temperature, it is tested to the smoke-extraction parts of the series, EN 1366-8 (single-compartment) and EN 1366-9 (multi-compartment), and must not split, sag or leak when hot. AS 1668.1 likewise sets out smoke-control duct construction and the temperature it must withstand.
Construction follows the same logic as grease duct: heavier gauge and continuously welded seams, so the duct does not open up under thermal load. Both fire-rated and smoke-extract duct interface with fire and smoke dampers wherever they cross a fire compartment boundary — a fire damper closing on a fusible link or heat, a smoke damper closing on an alarm signal. The duct-to-damper connection detail (often a breakaway connection at the rated wall) is itself part of the tested assembly. For how those dampers integrate into the duct, see the fire and smoke damper integration reference.
The construction comparison
The table sets the three fire-duty duct types against ordinary comfort-air duct, so the fabrication difference is explicit.
| Property | Comfort-air duct | Grease (kitchen exhaust) | Fire-rated / smoke-extract |
|---|
| Seam | Folded lock seam (Pittsburgh / snap-lock) | Continuous liquid-tight weld | Continuous welded seam |
| Typical gauge | ~0.5–0.8 mm GI | ≥16-ga (~1.5 mm) carbon / 18-ga (~1.2 mm) stainless | Heavier gauge, welded |
| Material | Galvanised steel | Carbon or stainless steel | Carbon or stainless steel |
| Primary standard | SMACNA / EN 1505 / AS 4254 | NFPA 96 | EN 1366-1, -8, -9 / AS 1668.1, AS 1530.4 |
| Defining requirement | Air-tightness class | Liquid-tight, drainable, cleanable | Fire integrity / hot-running for a rated period |
| Damper interface | Volume / balancing dampers | No dampers in the grease stream | Fire / smoke dampers at compartment lines |
Gauge equivalents are nominal. NFPA 96, AS 1668.1, AS 1530.4 and the EN 1366 series are the authoritative texts — always confirm the edition adopted in your jurisdiction. SBKJ supplies the machinery; the duct is fabricated and certified by the duct contractor to the applicable code.
Which SBKJ machines and processes apply
Because this whole class of duct is welded rather than folded, the defining machine is a welder. SBKJ is an Australian HVAC duct machinery supplier with an office in Box Hill North, VIC, and the welding range covers every joint these ducts need. The figures below are taken verbatim from the SBKJ Product Catalog 2026.
- Seam welder (SBFN-35 / 55 / 75 / 100). A resistance roller welder that lays a continuous gas-tight lap seam — exactly the liquid-tight seam grease and fire-rated duct demand. Material thickness runs 0.4–1.0 / 0.4–1.2 / 0.4–1.5 / 0.4–2.0 mm across the four models, at 45 / 55 / 75 / 110 KVA, welding speed 0.5–3 m/min, machine weight 328–478 kg. The SBFN-100 reaches the 2.0 mm gauge band that suits grease-duct sheet.
- Stitch welder (SBSW-30-2Z). Welds the longitudinal seam of round duct from Φ100 to Φ1000 mm, length 50–1000 mm, 6 mm overlap, material 0.4–1.0 mm, welding speed 2 m/min, 40 kW, 850 kg. This is the machine for closing the seam on round welded exhaust duct.
- Medium-frequency welder. For the heaviest stainless work, the MF inverter reaches up to 3.0 mm stainless and carbon steel, 2.5 mm aluminium and 2.0 mm galvanised, with cleaner heat control than a mains-frequency machine — useful for thick stainless grease and smoke-extract duct.
- Forming and cutting upstream. Heavier-gauge blanks are still cut and formed before welding. Round welded duct can be rolled on a heavier-gauge spiral tubeformer (the SBKJ SBTF-1602 and SBTF-2020 carry galvanised up to 2.0 mm and stainless up to 1.2 mm), and rectangular duct is cut, notched and folded on standard auto duct line equipment, then passed to the welder.
One practical note on capability rather than location: SBKJ welders run on a 380 V, three-phase, 50 Hz supply (60 Hz on request), and resistance welders need a cooling-water circuit. SBKJ provides a 2D workshop layout drawing with every quotation and offers a factory acceptance test (FAT) before shipment, so the welding cell, water and power are sized to the floor before the equipment is committed.
Spec a welding cell for grease & fire-rated duct →
FAQ
Why can't standard lock-seamed duct be used for a grease exhaust?
A folded lock seam is mechanically closed, not sealed. Grease vapour condenses inside the duct, runs to the seam and leaks — and that grease is fuel for a flue fire. NFPA 96 requires a continuous liquid-tight weld with no seam on the bottom, in heavier gauge carbon or stainless steel. It is a welded fabrication, not a folded one.
What gauge and material is grease duct built from?
NFPA 96 sets a minimum of 16-gauge (~1.5 mm) carbon steel or 18-gauge (~1.2 mm) stainless steel — heavier than the 0.5–0.8 mm of comfort-air duct. The defining feature is the continuously welded liquid-tight seam, not the gauge alone.
What standards govern grease and fire-rated duct?
NFPA 96 for commercial kitchen grease exhaust; the EN 1366 series in Europe (EN 1366-1 fire-resisting ducts, EN 1366-8/-9 smoke extraction, classified to EN 13501-4); and in Australia/NZ, AS 1668.1 for fire and smoke control, with fire dampers to AS 1682 and duct fire ratings tested to AS 1530.4.
Which SBKJ machines fabricate grease and fire-rated duct?
The welders: the seam welder (SBFN series) for a continuous gas-tight lap seam up to 0.4–2.0 mm at 45–110 KVA, the stitch welder (SBSW-30-2Z) for round-duct seams Φ100–Φ1000 mm, and the medium-frequency welder up to 3.0 mm stainless. Heavier-gauge blanks are cut and formed on standard SBKJ forming equipment first.
Is grease duct round or rectangular?
Both. Round welded duct is preferred because it self-drains, has no corners for grease to collect and cleans easily; it is rolled in heavier gauge and its seam welded. Rectangular grease duct is cut, formed and continuously welded along every seam. NFPA 96 allows either if liquid-tight and accessible.
How does fire-rated duct differ from smoke-extract duct?
Fire-rated duct keeps fire from spreading and holds its integrity for a rated period (EN 1366-1 / AS 1530.4). Smoke-extract duct must run hot to pull smoke out of a building and is tested to EN 1366-8/-9 at elevated temperature. Both use welded seams and heavier gauge, and both interface with dampers at compartment lines.
