Reading the Abbreviations: GI, SPCC, SECC, CRC and HRC
Coil enquiries often mix substrate and coating terminology in one line, which hides real differences. GI means hot-dip galvanized, the product of passing steel through a molten zinc bath. CRC means cold rolled coil: pickled and cold reduced to final thickness, with good surface finish and tight thickness tolerance. HRC means hot rolled coil, produced at final thickness by hot rolling, with a rougher surface and a wider tolerance band, and it is normally the feedstock that is later cold reduced. SPCC is a cold-reduced commercial quality grade defined in JIS G3141, while SECC is an electrolytically zinc coated cold-reduced steel defined in JIS G3313, where the E in the designation identifies the electroplated coating rather than the hot-dip coating.
The distinction matters in roofing. Hot-dip coated coil carries a thicker, more abrasion-tolerant zinc layer for a given mass, while electrolytically coated coil offers a thinner, smoother and very uniform coating with excellent paint appearance. On a trapezoidal roof profile exposed to weather, the hot-dip product is the usual choice; where the requirement is a smooth painted surface for an appliance or a panel, the electrolytic product can be the better match.
Substrate Comparison and Coating Options
| Designation | Substrate | Coating | Typical roofing role |
|---|---|---|---|
| SPCC | Cold reduced, commercial quality | None (uncoated) | Feedstock for painting or galvanizing |
| SGCC | Cold reduced, commercial quality | Hot-dip zinc | General profiled sheet and walling |
| SECC | Cold reduced, commercial quality | Electrolytic zinc | Smooth painted panels, appliance parts |
| DX51D+Z | Cold reduced, forming grade | Hot-dip zinc | Profiled roofing and cladding |
| AZ150 | Cold reduced, forming grade | 55% aluminium-zinc | Long-life roofing, coastal exposure |
JIS G3302 defines the hot-dip galvanized family SGCC, SGCH and SGCD, and its coating symbols are read directly: the number following Z or the equivalent symbol multiplied by ten gives the coating mass in grams per square metre, so Z27 corresponds to 270 g/m2 over both faces. Under EN 10346 the equivalent commercial and forming grades are DX51D+Z to DX54D+Z, with A80 elongation rising from at least 22% for DX51D+Z to at least 36% for DX54D+Z, and tensile strength falling from 270-500 MPa to 260-350 MPa as formability increases.
Selecting Coil for Roofing and Cladding
Roof sheet selection starts with the required corrosion life, which drives coating mass, then the profile depth, which drives the forming grade, and finally the paint system. For a standard industrial roof in a low-corrosion inland atmosphere, a Z180 to Z275 zinc coating with a 20 micrometre polyester topcoat is a normal specification. For coastal or industrial atmospheres, an AZ150 aluminium-zinc coating with a high durability polyester or PVDF topcoat is the common upgrade, because the aluminium-rich oxide layer tolerates combined salt and sulphur exposure better than zinc alone.
Thickness selection is a structural question. The second moment of area of a trapezoidal profile rises steeply with base metal thickness, so a 0.5 mm sheet in a deep profile can outperform a 0.6 mm sheet in a shallow profile for wind uplift. Profile suppliers publish load tables against span and thickness, and those tables assume a stated steel grade, normally 250 MPa or 350 MPa minimum yield, which must match the delivered coil.
Forming, Fixing and Paint Compatibility
Hot-dip coated coil forms readily, but the zinc coating is softer than the steel and is easily marked by slitting burrs, dirty rolls and metal-to-metal contact during transport. Electrolytically coated coil is thinner-coated and therefore needs even more careful handling. Both products accept the same primer and topcoat families, but adhesion testing after bending must be done on the actual coil, because conversion coating weight controls whether the paint follows the coating through a tight bend. Fixings must be compatible: a galvanized sheet fixed with uncoated carbon steel screws will lose the fixing long before the coating is consumed, so coated or stainless fasteners are standard practice on exterior work.
Quality Evidence and Handling in Storage
The delivery document set should identify substrate grade and heat number, coating type and mass, surface finish, mechanical properties and dimensional tolerances. Coils should be stored indoors on a dry surface, clear of the floor, in a stack height that does not mark the wrap, and used in rotation. Condensation between coils is the main cause of white rust in transit, so desiccant and moisture barrier wrapping are used on long sea voyages, and coils should be allowed to reach ambient temperature before unwrapping.
Frequently Asked Questions
Q: Is SPCC a stainless steel?
A: No. SPCC is a cold-reduced carbon steel of commercial quality under JIS G3141. It has no chromium addition, so it cannot form the passive surface layer that defines stainless steel and must be protected by painting or metallic coating.
Q: What is the difference between SGCC and SECC?
A: SGCC is hot-dip zinc coated under JIS G3302, and SECC is electrolytically zinc coated under JIS G3313. The hot-dip coating is heavier and more abrasion resistant; the electrolytic coating is thinner, smoother and more uniform, which suits painted appearance panels.
Q: Can HRC be used directly for roofing?
A: Not for exposed roofing. Hot rolled coil has a rougher surface and a wider thickness tolerance, and it is normally the feedstock for cold reduction. Roofing coil is a cold-reduced substrate with a metallic coating, usually with an additional painted finish.
Q: How do I read a coating designation such as Z27 or G90?
A: JIS symbols are read as the number times ten grams per square metre over both faces, so Z27 equals 270 g/m2. ASTM A653/A653M uses G designations in ounces per square foot over both faces, so G90 equals 0.90 oz/ft2, approximately 275 g/m2.
Q: Which coating should be specified for a coastal site?
A: An aluminium-zinc coating of at least 150 g/m2 combined with a high durability polyester or PVDF topcoat is the usual specification, supported by cut-edge protection and stainless or coated fasteners, because cut edges and fixings are the first points to corrode.
Q: Does a heavier zinc coating reduce formability?
A: A heavier coating is slightly stiffer at the surface but the decisive factor for forming is the steel grade and its elongation. The practical risk with a heavy coating is coating flaking at tight radii, which is controlled by the forming grade and by the roll former's radius limit.