Insights · Reference
Galvanized steel is the default material for HVAC ductwork — zinc-coated, low-cost, and easy to form. This reference covers what galvanized duct steel is, the coating grades (G60, G90, G115), the gauges used, why G90/Z275 is the SMACNA default, and the machines that turn it into round and rectangular duct.
Galvanized steel is cold-rolled steel hot-dip coated with zinc on both faces. The zinc is sacrificial — it corrodes in place of the steel — so a galvanized sheet resists rust for decades in conditioned air. That corrosion resistance, combined with low cost, good strength and clean forming behaviour, is why galvanized steel is the material for the large majority of supply, return and exhaust ductwork worldwide.
It forms cleanly on a spiral tubeformer (round duct), an auto duct line (rectangular TDF duct) or a lockformer (Pittsburgh-lock rectangular duct), and takes both Pittsburgh and TDF seams well.
The number after the G is the zinc coating weight, in ounces per square foot across both surfaces. The metric Z-grade is the same coating in grams per square metre. More zinc means more corrosion protection.
| Grade (imperial) | Grade (metric) | Zinc coating | Typical duct use |
|---|---|---|---|
| G60 | Z180 | 0.60 oz/ft² · 180 g/m² | Dry, conditioned interior duct; lightest protection |
| G90 | Z275 | 0.90 oz/ft² · 275 g/m² | SMACNA default for HVAC duct — standard supply/return/exhaust |
| G115 | Z350 | 1.15 oz/ft² · 350 g/m² | Humid, coastal or mildly corrosive air; longer service life |
Coating weights follow the ASTM A653 (imperial G-grade) and EN 10346 (metric Z-grade) designations. Going from G60 to G90 adds roughly 5% to raw-material cost; G90/Z275 is the usual specification because the extra zinc pays for itself in service life.
Galvanized duct gauge is set by duct size and pressure class, not preference. Most low-pressure duct sits in the 26 to 20 gauge band (0.55–1.0 mm), increasing as duct grows or pressure rises — confirm against SMACNA HVAC Duct Construction Standards or AS/NZS 4254 for your job. For the full gauge-to-millimetre conversion and which machine forms each gauge, see the sheet metal gauge & thickness chart.
SBKJ machines form galvanized steel across this whole range and beyond: the SBTF-1500/1500C spiral tubeformers roll 0.4–1.2 mm galvanized, and the SBTF-1602/2020 roll 0.4–2.0 mm — heavy enough for industrial round duct. Rectangular auto duct lines run 0.5–1.5 mm galvanized.
Round duct — a spiral tubeformer winds a continuous 137 mm galvanized strip into a lock-seamed tube, from Φ80 to Φ2500 mm. The zinc coating survives the forming rollers because the lock seam folds rather than scrapes the surface.
Rectangular duct — an auto duct line takes galvanized coil and notches, folds and seams it into TDF-flanged duct in one pass; a lockformer rolls the Pittsburgh lock that closes the corner. Galvanized takes both seams cleanly at full line speed, which is part of why it is the default — stainless runs about 20% slower.
Cut edges expose bare steel, but the surrounding zinc protects them galvanically, so standard duct needs no edge treatment. For the material trade-offs against stainless and aluminium, see galvanized vs stainless steel duct.
G90/Z275 covers ordinary conditioned air. Step up to G115/Z350 or stainless when the air is wet (kitchen exhaust, pool halls), salty (coastal sites) or chemically aggressive (labs, some industrial process exhaust). Specifying stainless where G90 would do can add 200% to the material bill, so match the grade to the environment rather than over-specifying.
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Steel duct hot-dip coated with zinc on both faces. The zinc resists corrosion, making galvanized steel the default for most supply, return and exhaust duct. It is formed on spiral tubeformers (round) or auto lines / lockformers (rectangular).
Zinc coating weight. G60 (Z180) is 0.60 oz/ft² / 180 g/m²; G90 (Z275) is 0.90 oz/ft² / 275 g/m². G90/Z275 is the SMACNA default — heavier zinc, longer life, for about 5% more material cost.
Mostly 26–20 gauge (0.55–1.0 mm), rising with size and pressure per SMACNA / AS NZS 4254. SBKJ machines form 0.4–2.0 mm galvanized (SBTF-1602/2020 reach the full 2.0 mm).
Cost, strength, clean forming and good corrosion resistance. Stainless or G115/Z350 is reserved for wet, coastal or corrosive air where the extra cost is justified.
Building galvanized duct and choosing a machine? Tell us your gauge, coating grade and duct sizes and an SBKJ engineer replies within 12 hours — not a salesperson.
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