Galvanized steel sheet for HVAC ductwork is most often a choice between mill edge and cut edge coil, and the edge type drives how the duct is seamed, the wastage rate and the cost per square metre. This guide explains where each is the right call for rectangular duct, spiral duct and fittings.
HVAC ductwork is roll-formed and seamed from galvanized steel sheet, and the call between mill edge and cut edge coil is one of the first decisions a duct fabricator makes. Mill edge coil is supplied as rolled, with the natural edge of the steel strip; cut edge is slit or sheared to the exact width the customer needs. The edge type affects the seaming method, the fit of Pittsburgh lock seams and drive cleats, and the wastage from edge trimming, so the wrong edge type shows up as either weak seams or excessive scrap.
What Mill Edge and Cut Edge Mean in Practice
Mill edge coil has the rough, slightly out-of-round edges produced by the hot strip mill and the cold rolling reduction; the edge is not perfectly straight and has a small variation in width across the length of the coil. Cut edge coil is slit on a precision slitter to the exact width ordered, with a clean square edge and tight width tolerance. For Pittsburgh lock seams and drive cleats, which are the dominant seam methods for rectangular duct, the square clean edge of cut coil is required to make a tight, leak-free seam. Mill edge coil is acceptable for spiral duct, where the edges are continuously overlapped and the lock seam is formed by the spiral machine regardless of the edge condition.
When Cut Edge Is the Right Specification
Cut edge galvanized steel sheet is the right specification for rectangular duct fabrication using Pittsburgh lock seams, drive cleats, or TDF/TDC flanges. The tight width tolerance allows the fabricator to use coil directly without edge trimming, which reduces scrap and speeds production. For galvanized sheet in the typical duct gauge range of 0.5–1.0 mm, cut edge coil is the standard stock for most HVAC fabrication shops. The cut edge also has a slightly smoother zinc surface at the edge, which improves the appearance of the seam when the duct is visible.
When Mill Edge Is the Right Choice
Mill edge galvanized steel sheet is the right specification for spiral duct production, where the coil is fed into a spiral forming machine that wraps the strip into a tube and continuously forms the lock seam. The edge condition does not affect the spiral seam, so the small width variation of mill edge is acceptable and saves the slitting cost. Mill edge is also used for general sheet metal fittings and for fabricators who have their own precision slitting capability and want to slit multiple widths from one master coil.
Coating Weight, Thickness and SMACNA Standards
For HVAC ductwork, the galvanized coating is typically G60 (Z180) for commercial and residential ducts in dry indoor environments, and G90 (Z275) for outdoor duct, kitchen exhaust, or any duct in a humid or corrosive space. The substrate thickness is governed by SMACNA (Sheet Metal and Air Conditioning Contractors' National Association) HVAC Duct Construction Standards, which call out 26 gauge (0.55 mm) for residential ducts, 24 gauge (0.70 mm) for low-pressure commercial, and 22 gauge (0.85 mm) or heavier for medium- and high-pressure commercial duct.
What to Specify on the Duct Sheet RFQ
State the substrate standard (ASTM A653 or EN 10346), the coating weight (G60 or G90 / Z180 or Z275), the substrate gauge, the edge type (mill edge or cut edge), the coil width (or set of widths if the order covers multiple duct sizes), the coil inner diameter (typically 508 mm or 610 mm), and the maximum coil weight for the fabricator's decoiler. For rectangular duct, default to cut edge; for spiral duct, mill edge is acceptable and cheaper. Confirm that the mill supplies the test report per EN 10204 3.1 with the actual coating weight and substrate thickness.
Related Resources from Baobin Steel
- Steel sheet catalog – galvanized and cold rolled sheet
- Steel coil catalog – mill edge and cut edge options
- Request a quote – send your duct schedule for galvanized sheet pricing