Zinc Coating Classes for Galvanized Steel: Z275, G90 and Z27 Explained

The coating class on a galvanized coil — Z275, G90, Z27, Z08 — is the mass of zinc in grams per square metre counted across both surfaces of the sheet, not per side. Get that one point wrong and every calculation after it is out by a factor of two.

The second point matters more commercially. When an offer sheet says “Z80”, there are three completely different things it can mean, and they carry three different levels of protection for you:

• “EN 10346 Z80” is incoherent. EN 10346:2015 stops at Z100. There is no Z80 in that standard, so there is no triple-spot or single-spot floor to test against.
• “JIS G3302 Z08” is legitimate. It is a real class with an enforceable 80 g/m² triple-spot minimum and a 68 g/m² single-spot minimum, both total both surfaces.
• “zinc 80g” with no standard named is neither. It is a number on a proforma with nothing behind it, and it does not even tell you whether the mill means both sides or one side.

Which of the three you have received is worth ten minutes of your time before you open the LC. This guide gives you the class charts for EN, ASTM and JIS, the conversion arithmetic, and the checks that actually work on arrival.

On this page

• Is Z275 per side or total? (and the 42-micron problem)
• EN 10346 zinc coating class chart: Z100 to Z600
• ASTM A653 coating weight chart: G30 to G210
• JIS G3302: Z27, Z18 and Z12 explained
• Chinese offers, GB/T 2518 and ISO 3575
• Z275 vs G90 vs Z27: cross-standard equivalence chart
• Beyond pure zinc: AZ galvalume, ZM and ZF
• Which coating for a roof in Karachi vs inland Pakistan
• Gauge, BMT and TCT: the thickness traps
• Coil is sold by weight and used by area
• How GI buyers get caught
• How to actually check a coil
• Writing a specification line a mill can process
• Prime versus secondary
• Anti-dumping duty: GI and galvalume are not the same case
• Frequently asked questions

Is Z275 per side or total? (and the 42-micron problem)

EN 10346, ASTM A653/A653M and JIS G3302 all define coating mass the same way — total across both surfaces.

• EN 10346:2015 footnotes that a zinc mass of 100 g/m² on both surfaces corresponds to a zinc coating thickness of about 7.0 µm per surface.
• ASTM A653/A653M Table 1 is headed “Total Both Sides”. G90 requires a triple-spot test average of at least 0.90 oz/ft² total both sides.
• JIS G3302 states that coating mass for equally coated sheet and coil is expressed as the coating mass on both surfaces. Differentially coated product is designated separately.

So a Z275 sheet carries roughly 137.5 g/m² on the exposed side, not 275 — and that assumes an even split, which none of the standards require.

Mass to microns — the arithmetic

ASTM A653/A653M section 8.1.2.2 fixes the conversion: 7.14 g/m² of coating mass = 1 µm of coating thickness. That corresponds to a zinc density of 7.14 g/cm³; EN 10346 mill tables print 7.1 g/cm³, which is why EN’s rounded typical thicknesses sit a fraction above the computed values. Two rules follow:

• Total thickness both sides (µm) = Z ÷ 7.14
• Thickness per side (µm) = Z ÷ 14.28 (even split assumed)

For imperial-marked coil, GalvInfo’s conversion is 1.00 oz/ft² = 0.00168 in = 305 g/m² = 0.0427 mm. Multiply oz/ft² by 305 to get g/m², which is why G90 (0.90 oz/ft²) pairs with Z275. Across the whole EN table, Z ÷ 14.28 reproduces EN 10346’s own published typical per-side thicknesses to within 0.8 µm.

The 42-micron figure

You will see G90/Z275 described as “about 42 microns” total, or 21 microns per side. That figure appears on page 1 of GalvInfoNote 1.1, which states that a G90 coating contributes about 1.6 mils, 0.0016 inches or about 42 microns, and about 21 microns on each surface.

It does not survive either conversion route, including GalvInfo’s own:

• Imperial, using GalvInfo equation (1): 0.90 × 0.00168 in = 0.00151 in = 1.5 mils = 38.4 µm total.
• Imperial, using ASTM 8.1.2.1 (1 oz/ft² = 1.7 mils): 0.90 × 1.7 = 1.53 mils = 38.9 µm total.
• Metric, using ASTM 8.1.2.2: 275 ÷ 7.14 = 38.5 µm total.

The answer is about 38.5 µm total, roughly 19.3 µm per side, which is consistent with EN 10346’s typical value of 20 µm per side for Z275. Use 38.5, not 42. If you are checking a supplier’s claim against a coating gauge reading, the 3.5 µm difference is the whole margin you are arguing about.

EN 10346 zinc coating class chart: Z100 to Z600

All masses are total across both surfaces. Note that the lowest class is Z100 — there is no Z80, Z60 or Z40 in this standard.

Eight classes, lowest Z100. Z80, Z60 and Z40 appear as commercial designations rather than standard classes. They may well be real product, but if your order says EN 10346 there is no class in the standard for a test result to be measured against.

ASTM A653 coating weight chart: G30 to G210

ASTM sets three limits, not one, and the third is the one most buyers never read:

• a minimum triple spot test (TST) value, total both sides — the average of three readings taken edge, centre, edge;
• a minimum single spot test (SST) value, also total both sides, which is lower than the TST;
• a minimum one side value, published as its own column in Table 1. Footnote A defines it: the minimum triple-spot average coating mass on any one side shall not be less than 40 % of the single-spot requirement. This is a mandatory limit, not a guideline.

The full ASTM A653/A653M Table 1 for zinc coatings, reproduced with the inch-pound and SI designations on the rows the standard pairs them on:

The inch-pound and SI values in A653/A653M are independently specified, not exact conversions of one another. G90 is 0.90 oz/ft², which soft-converts to 274.5 g/m², while the paired SI class Z275 is specified at 275 g/m². Order to A653 and you get the oz/ft² numbers; order to A653M and you get the g/m² numbers. Say which one you mean.

Read the one-side column carefully. For Z275 the published minimum is 94 g/m² — that is 40 % of the 235 g/m² single-spot total-both-sides figure, and it is 13.2 µm against the 19.3 µm an even split would give. A coil whose exposed face carries 13.2 µm is fully compliant and will pass inspection.

In practice this is the legal floor rather than the norm. GalvInfo notes that a 50/50 split is by far the most typical coating distribution produced on modern coating lines, and that actual coatings tend to run a little above the minimum to guarantee compliance. But it is the reason a serious specification names the standard, the class and the test basis rather than just “Z275”.

ASTM footnote B is also worth quoting to anyone arguing that a lighter class is close enough: the atmospheric corrosion resistance of zinc-coated sheet is a direct function of coating mass, so selecting a lighter coating designation results in almost linearly reduced corrosion performance.

Where the letter sits changes what the number means

For hot-dip galvanized sheet ordered to A653/A653M Table 1, the letter comes before the numerals: “G” means inch-pound, “Z” means SI, and both are total-both-sides minimums.

When the letter comes after the numerals, the value is single spot, per side, and it carries a maximum as well as a minimum. For example 40G40G under A879M electrogalvanize is 40 g/m² minimum and 70 g/m² maximum on each surface.

One caution: the same nnGnnG format is also used for hot-dip sheet ordered to A653M Table S2.1, where 60G60G means 60 g/m² minimum and 110 g/m² maximum per side. So letter position tells you the figure is per side and bounded top and bottom — it does not tell you whether the product is hot-dip or electroplated. Check the specification number, not just the designation.

JIS G3302: Z27, Z18 and Z12 explained

The JIS symbol is an abbreviation, not the value. Z27 is 275 g/m², not 270. Single-spot minimums run at 85 % of the triple-spot figure throughout the table. Common Japanese grade names are SGCC (commercial, cold-rolled base) and SGHC (commercial, hot-rolled base).

This is the equally-coated table. JIS G3302 also covers differentially coated product, which is designated separately — if you are buying differential coating, the both-surfaces figures above do not describe what lands on your exposed face.

Chinese offers, GB/T 2518 and ISO 3575

A large share of the GI and PPGI coil arriving in Pakistan is Chinese. Chinese mills work to GB/T 2518-2019, Continuously hot-dip zinc and zinc-alloy coated steel sheet and strip, issued 10 December 2019 and effective 1 July 2020, covering 0.20 mm to 6.0 mm. Its grade names follow the European pattern — DC51D+Z and similar — and SGCC is widely used commercially on Chinese offers as well.

There is also ISO 3575, the international specification for continuous hot-dip zinc-coated carbon steel sheet of commercial quality, which a number of Asian mills certify to. ASTM A653 footnote C notes that ISO 3575 contains Z100 and Z200 designations that A653M does not.

The practical problem is not the Chinese standard. It is that Chinese proformas routinely quote “zinc 40g”, “zinc 60g” or “zinc 80g” with no standard named at all, and the offer sheet rarely says whether that number is total both surfaces or per side. Those two readings differ by a factor of two on the metal you actually receive.

Before the LC opens, get it in writing: the standard and edition, the coating designation in that standard’s own notation, and the words “total both surfaces”. A number without a standard is not a specification.

Z275 vs G90 vs Z27: cross-standard equivalence chart

ASTM A653M and EN 10346 share four classes exactly — Z275, Z350, Z450 and Z600, with identical triple-spot and single-spot minimums in both standards. But the light ends do not overlap at all: EN’s Z100, Z140, Z200 and Z225 have no ASTM equivalent, and ASTM’s Z90, Z120 and Z180 have no EN equivalent. That gap is where substitution quietly costs you zinc.

Two substitutions to watch. Accepting ASTM Z180 against an EN Z200 order gives up 20 g/m², ten per cent of the zinc you ordered. And there is no ASTM class at 100 g/m², so an “ASTM Z100” on an offer sheet is as incoherent as an EN Z80.

Grade equivalence runs alongside coating equivalence. The common commercial-quality forming grades map roughly as DX51D (EN) ↔ SGCC (JIS) ↔ CS Type B (ASTM). Structural grades are separate: S250GD or S280GD in EN, SS Grade 33 or higher in ASTM.

Beyond pure zinc: AZ galvalume, ZM and ZF

EN 10346 is not a zinc-only standard. It covers several metallic coatings, and the ones that matter commercially in Pakistan are:

• +Z — pure zinc, the classes above.
• +AZ — aluminium-zinc alloy, sold as galvalume or Aluzinc. Conventionally around 55 % aluminium and 43 % zinc with a small silicon addition.
• +ZM — zinc-magnesium, where a magnesium and aluminium addition raises corrosion resistance at a given coating mass.
• +ZF — galvannealed, a zinc-iron alloy coating, matt grey and intended to be painted.
• +ZA and +AS — zinc-aluminium and aluminium-silicon, mostly specialist.

The critical arithmetic point: do not apply the 7.14 g/m² per micron divisor to an AZ coating. The aluminium-zinc alloy is far less dense than zinc — EN 10346 uses about 3.8 g/cm³ — so the same mass buys roughly twice the thickness.

Read that table twice. AZ150 delivers about the same coating thickness per side as Z275 at 150 g/m² instead of 275 g/m², because the alloy is roughly half the density of zinc. Anyone comparing “AZ150 versus Z275” on coating mass alone is comparing the wrong quantity.

AZ coating mass is still quoted total both surfaces, same as Z. Grade names follow the same pattern with the coating suffix changed: DX51D+AZ, S250GD+AZ, S350GD+AZ. More on the product range in our notes on PPGI, GI and galvalume roofing sheets and colour-coated steel coil.

Which coating for a roof in Karachi vs inland Pakistan

ISO 9223 classifies atmospheres by corrosivity and gives a first-year zinc corrosion rate for each category. The categories relevant to Pakistan are C3 upward; C1 and C2 describe deserts and subarctic zones and are not useful here.

ISO states these are first-year rates from one-year exposures and cannot simply be extrapolated for long-term prediction. Zinc forms a protective patina and the rate falls over time, so dividing a coating thickness by a first-year rate understates service life — sometimes badly. Do not do that arithmetic and treat the answer as a roof life.

What has actually been measured in Karachi

Two published datasets exist, both measuring mild steel rather than zinc, so treat the output as an environmental classification rather than a zinc result.

Zafar et al. (2020, Materials and Corrosion) exposed coupons at ten urban, industrial and marine sites across Pakistan from July 2018 to June 2019. A 2023 study in npj Materials Degradation ran two years in Karachi specifically and classified three urban sites as C3, two urban and two industrial sites as C4, and three marine sites as C5.

The two datasets disagree on the urban sites but agree that Karachi’s marine sites reach C5. The 2023 study then applied ISO 9224 to project 20 years and came out one category lower in both cases: C3 for urban and industrial sites, C4 for marine sites. That is the number to specify against for a building, unless the structure sits in the splash zone.

No published ISO 9223 category exists for inland Punjab. What can be said without a measurement is that distance from the sea removes the chloride driver, which is the dominant one at C4 and C5 — but inland does not automatically mean mild. The environments that consume zinc inland are specific and local:

• Poultry and livestock sheds — ammonia plus near-permanent damp is aggressive on zinc regardless of how far you are from the coast. Specify heavier coating or a painted product, and expect shorter life than a plain shed nearby.
• Fertiliser, cement and sugar plants, and dense industrial belts — pollution-driven corrosion, ISO’s C4 descriptions.
• Ordinary dry inland exposure away from industry — genuinely low corrosivity, where lighter classes do last.

Choosing a class

Corrosion life is close to linear in coating mass — ASTM A653 footnote B states it directly. Against a Z275 baseline, Z180 carries 65.5 % of the zinc, Z140 50.9 %, Z120 43.6 %, Z100 36.4 % and a commercial “Z80” 29.1 %, and life scales in roughly the same ratios. A Z100 coil is not slightly lighter than Z275; it is about a third of the life in the same air.

Applying that to the categories above:

• Coastal and near-coastal Karachi (C4–C5 long-term). Bare GI is working hard here at any class. An AZ (galvalume) substrate or a pre-painted product over AZ is the better answer; if it must be plain GI, Z275 is the floor, not the aspiration.
• Inland Karachi and industrial belts (C4). Z180 to Z275 in GI, or painted product where appearance matters.
• Dry inland, low-industry sheds and boundary work (C3 or below). Z120 to Z180 is commercially normal and lighter classes genuinely do the job. This is a real cost-versus-life trade-off, not a scandal — provided the class is named against a standard and tested.

Two details decide roof life as much as the class does. Fasteners must be galvanized or stainless with a sealing washer, and must not put dissimilar metals in contact with the sheet. And cut edges, drilled holes and laps are where a roof actually starts to fail.

That last point is worth stating plainly, because it is usually said backwards. On a corrugated roofing sheet the sun-facing surface dries fastest and is not the first thing to go. The failures start at cut edges, at fastener penetrations, at end laps, and on the underside where night condensation sits and does not dry. Zinc’s sacrificial protection at a cut edge is exactly why coating class still matters there — but detailing matters at least as much.

Coating class and formability interact

Heavier is not automatically better. Thick coatings crack and flake at tight bends on thin gauge, which is precisely what corrugating and profile-rolling do. ASTM notes that minimized spangle is normally produced in coating designations G90 [Z275] and lighter. Z600 is not a sensible corrugating specification at 0.25 mm. Forming also thins the coating locally at the bend. Specify the grade for the forming severity — DX51D or SGCC for general forming, deeper-drawing grades above that — not just the coating class.

Gauge, BMT and TCT: the thickness traps

Coating class and base-metal thickness are independent specifications. A heavy-gauge Z100 sheet rusts before a light-gauge Z275 sheet at the same site.

Two thickness definitions get confused, and the difference is money:

• BMT — base metal thickness. The steel only. This is what carries load and what you are actually buying.
• TCT — total coated thickness. Steel plus zinc plus paint. ASTM A653 section 5.1 says it explicitly: the thickness of the sheet includes both the base metal and the coating.

On a painted sheet quoted as “0.25 mm” the real steel can be materially less once zinc and paint are stripped out. ASTM provides for this — section 5.2.16.1 allows the product to be ordered to a specified base metal thickness under Supplementary Requirement S1. Use it. Write BMT into the order, name the tolerance standard (EN 10143 for EN material, ASTM A924/A924M for ASTM material), and do not accept “0.25 mm TCT” as an answer to a BMT question.

Gauge callouts have the same problem built in. The Manufacturers’ Standard Gauge publishes a separate column for galvanized sheet that adds a fixed coating allowance of about 0.0037 in (0.094 mm) regardless of what coating class you actually ordered:

A real Z275 coating adds only about 0.0385 mm total. The gauge table’s allowance is roughly two and a half times that. So “24 gauge galvanized” does not tell you the base metal thickness, and two suppliers quoting the same gauge can be shipping different steel. Specify millimetres of BMT and a tolerance standard.

One more consequence worth internalising: coating mass does not show up on a micrometer. Z275 is 0.0385 mm across both faces combined — inside the thickness tolerance band of most orders. Thickness checks and coating checks are two separate tests and neither substitutes for the other.

Coil is sold by weight and used by area

You buy coil by the tonne and you install it by the square metre, so a heavier coating class buys you fewer square metres per tonne. Most first-time buyers never work this out, and it changes how two offers compare.

At steel density 7.85 g/cm³, the mass per square metre of a coated sheet is:

kg/m² = 7.85 × BMT(mm) + Z ÷ 1000
Coverage (m² per tonne) = 1000 ÷ kg/m²

Worked at 0.50 mm BMT:

Moving from Z100 to Z275 at 0.50 mm adds 0.175 kg of zinc per square metre — about 4.3 % more invoiced weight, and 4.2 % fewer square metres out of every tonne. Set that against the roughly 2.75 times longer life the same change buys under ASTM’s linear relationship, and the comparison is no longer close.

Use the formula when you compare two offers at different classes or different BMTs. Comparing per-tonne figures on coils of different coating class and thickness is comparing two different quantities of steel.

How GI buyers get caught

How to actually check a coil

“Ask for the coating distribution” is not a procedure. This is:

1. Magnetic or eddy-current coating gauge. This is the single most useful practical fact in the subject: a coating gauge reads microns on one side at a time. That is why a gauge reading never matches the g/m² figure on the certificate. To compare, multiply the reading by 7.14 to get g/m² on that side, then double it for the both-surfaces class. On Z275 expect roughly 19 µm per side. Take five readings across the width and repeat at both ends of the coil.
2. Strip test for a dispute-grade number. ASTM A90/A90M is the weigh-strip-weigh coating mass method that A653 references, and it is what an arbitrator will accept. Have it done at an accredited materials-testing laboratory, not in your own godown.
3. Micrometer for thickness, separately. Five points across the width, both ends. Remember the coating is only about 0.04 mm on Z275, so a micrometer tells you nothing useful about coating mass.
4. Pre-shipment inspection in the LC. This is the only control that works before the money leaves. Name a third-party inspection agency, name coating-mass testing to A90/A90M or the equivalent in your chosen standard, and state who pays.
5. Retain samples. Keep a marked offcut from each coil, tagged with the coil number, until the job is closed and signed off.

Check the paperwork with the same discipline: the MTC should carry heat and coil numbers that match the physical tags on the coils, and the mill named should be one you can find and verify independently.

Writing a specification line a mill can process

A specification that only names a coating class is not a specification. No mill can process “Z275” — the first thing on any real order is the steel grade, because that determines whether the coil can be roll-formed at all.

A complete EN-basis line looks like this:

DX51D+Z275-M-A-C to EN 10346:2015, 0.50 mm BMT × 1219 mm, coil, triple-spot and single-spot per standard, mill test certificate per coil quoting heat and coil numbers, third-party pre-shipment inspection.

Element by element:

• DX51D — steel grade. DX51D is the general forming grade; DX52D and above are for deeper drawing; S250GD, S280GD and S350GD are structural. In JIS the equivalents are SGCC and SGHC; in ASTM, CS Type B for commercial forming or SS Grade 33 and up for structural.
• +Z275 — coating type and class. +Z is pure zinc; +AZ, +ZM and +ZF are the alternatives above.
• M — surface finish. M is minimized spangle (micrograin), N is normal spangle. Spangle is appearance-critical for exposed roofing and for PPGI substrate.
• A — surface quality. A is standard, B is improved (skinpassed), C is best.
• C — surface treatment. C is chemical passivation, O is oiled, CO is both, P is phosphated, S is organic passivation. Passivation is your defence against white rust in humid storage.
• 0.50 mm BMT — base metal thickness, stated as BMT, with the tolerance standard named.

Then add the coil geometry the standards themselves ask you to specify: width, coil ID and maximum OD, maximum coil weight your decoiler can take, edge condition, and packing. ASTM A653 section 5.2 lists these as ordering information for exactly this reason.

If a supplier answers an EN order with an ASTM or JIS designation, convert it with the chart above and confirm the class actually exists in the standard you specified. Do not accept a near-equivalent silently. And ask your supplier to confirm they can provide the mill test certificate before you place the order, not after.

Prime versus secondary

A good deal of the flat steel in the market is secondary, and the class charts above quietly assume it is not. Secondary material is coil rejected or downgraded for coating voids, dross pick-up, uneven spangle, edge wave, off-gauge running, or age and white rust in storage.

Secondary carries no meaningful coating guarantee at any nominal class. There is usually no MTC, and where there is one it may not correspond to the coil in front of you. The discount is real and buyers take it knowingly — that is a legitimate commercial choice for temporary sheds, ducting and non-critical work. It is the wrong choice for anything structural, anything coastal, or anything you will be asked to warrant.

The trade shorthand, since it is used loosely on offer sheets: GP is galvanized plain (flat coil or sheet), GC is galvanized corrugated, GI is the general term for hot-dip galvanized product, and PPGI is pre-painted galvanized coil. When comparing GP coil from International Steels, GP from Aisha Steel and imported prime GP from China, compare coating class and prime status first, and price second. Zinc mass is what you are actually buying.

Anti-dumping duty: GI and galvalume are not the same case

This trips up buyers because the two products sit under different PCT headings at different duty exposures, and the recent news coverage was all about the second one.

Per the National Tariff Commission’s notice in A.D.C. No. 37/2015/NTC/GC/Circum/2024, signed 28 June 2025:

• Plain galvanized coil and sheet from China falls under PCT headings 7210.4110, 7210.4190, 7210.4990, 7212.3010, 7212.3090, 7225.9200 and 7226.9900. The Commission imposed definitive anti-dumping duties on these “in ad valorem terms ranging from 6.09% to 40.47%” with effect from 8 February 2017 for five years, extended for a further five years from 8 February 2022 following a sunset review.
• Galvalume (alu-zinc) coil and sheet falls under PCT headings 7210.6110, 7210.6190, 7210.6910 and 7210.6990. Following Pakistan’s first anti-circumvention investigation, the Commission extended the existing 40.47 % rate — the rate applicable to “all other exporters/foreign producers” — to galvalume imports from China, with effect from the date of that notification until 8 February 2027.

Three things follow for a buyer. The rate on plain GI is a range starting at 6.09 %, not a flat 40.47 % — your exposure depends on the exporter. The 40.47 % headline applies to galvalume from China at the residual rate. And material from origins other than China is not subject to this measure at all; the notice says so explicitly.

Check which PCT heading your invoice actually declares before you price the order, and remember the NTC’s own wording that the anti-dumping duty is “in addition to other taxes and duties leviable on its imports under any other law” — customs duty, additional customs duty, regulatory duty, sales tax and advance income tax at import all sit on top and all belong in a landed-cost calculation.

As at July 2026, the measure has a little over six months left to run to its 8 February 2027 expiry, and its validity is co-terminus with the underlying galvanized duty. Whether it is extended again depends on a sunset review; check the NTC’s published notices before committing to a Q4 or Q1 shipment on the assumption it lapses. See also our notes on anti-dumping duty on galvanized steel.

Frequently asked questions

Is Z275 zinc on each side or in total, and how many microns is it?

In total. EN 10346, ASTM A653 and JIS G3302 all express coating mass across both surfaces. Z275 is about 38.5 µm total, or roughly 19.3 µm per side on an evenly coated sheet, using ASTM’s conversion of 7.14 g/m² per micron. Each face carries about 137.5 g/m². The commonly repeated “42 microns total, 21 per side” does not follow from any of the standards’ conversions — the correct figure is 38.5 µm.

Is Z80 a real coating class, and what do Chinese mills mean by 40g or 80g?

It depends entirely on the standard named. EN 10346:2015 has no Z80 — its lowest class is Z100. JIS G3302 Z08 is a genuine class at 80 g/m² triple-spot and 68 g/m² single-spot, both total both surfaces. An offer that just says “zinc 80g” names no standard, has no enforceable floor, and does not even specify whether the figure is total both surfaces or per side. Get the standard, the edition and the words “total both surfaces” in writing before the LC opens.

What is the difference between G90 and Z275?

Very little in substance. G90 is the ASTM inch-pound designation for 0.90 oz/ft² total both sides; Z275 is the SI class ASTM pairs it with, and it matches EN 10346 Z275 and JIS Z27. Note the values are independently specified rather than exact conversions: 0.90 oz/ft² soft-converts to 274.5 g/m² against Z275’s specified 275. Z275, Z350, Z450 and Z600 all appear in both EN 10346 and ASTM A653M with identical minimums.

What is the difference between GI and galvalume (AZ) sheet?

GI is coated with pure zinc; galvalume, designated +AZ in EN 10346, is coated with an aluminium-zinc alloy of roughly 55 % aluminium and 43 % zinc with a small silicon addition. The practical difference is density: the AZ alloy is about 3.8 g/cm³ against zinc’s 7.14, so AZ150 at 150 g/m² gives about 20 µm per side — the same coating thickness as Z275 at 275 g/m². Never convert an AZ coating mass to microns using the 7.14 divisor.

Which zinc coating should I specify for a roof in Karachi?

Published Karachi measurements put marine sites at C5 and urban and industrial sites at C3–C4 in the first year, with the 20-year ISO 9224 projection one category lower in each case. For coastal and near-coastal sites, bare GI is working hard at any class — an AZ galvalume substrate or a pre-painted product over AZ is the better answer, and Z275 is the floor if it must be plain GI. Inland Karachi and industrial areas suit Z180 to Z275. Dry inland sheds away from industry are genuinely fine on Z120 to Z180. Fastener choice and cut-edge and lap detailing matter as much as the class.

How do I check the zinc coating on a coil, and can a compliant coil still rust early?

Use a magnetic or eddy-current coating gauge, which reads microns on one side at a time — expect about 19 µm per side on Z275, taking five readings across the width at both ends. For a dispute-grade number use the ASTM A90/A90M strip test at an accredited laboratory, and put third-party pre-shipment inspection with coating-mass testing into the LC. As for early rusting: yes, legitimately. ASTM A653M Table 1 publishes a one-side minimum of 94 g/m² for Z275, so a compliant coil’s exposed face can carry 13.2 µm rather than 19.3 µm. A 50/50 split is the common outcome on modern lines, but if the exposed face matters, specify the coating distribution.

Sources

• GalvInfoNote 1.1 Rev 4.0 — understanding coating weight designations for zinc-based coatings on steel sheet
• GalvInfoNote 1.6 — corrosion performance and service life
• ASTM A653/A653M — Table 1 coating mass requirements, footnotes A–D, section 5.2 ordering information and section 8.1.2 thickness conversion
• Mill reproduction of UNI EN 10346:2015 coating class table, surface finish and treatment codes
• thyssenkrupp Steel — hot-dip galvanized (Z), EN 10346
• JIS G 3302:2019 — hot-dip zinc-coated steel sheet and strip (standard preview, JSA)
• Kobe Steel — galvanized steel sheet product data, JIS G3302 coating mass symbols
• GB/T 2518-2019 — continuously hot-dip zinc and zinc-alloy coated steel sheet and strip (scope and status)
• ArcelorMittal Aluzinc — AZ coating designations, coating weights and thickness per side
• AGA — ISO 9223 corrosivity categories C1 to CX and zinc corrosion rates
• AGA — continuous sheet galvanizing process
• National Tariff Commission — Notice of Final Determination, A.D.C. No. 37/2015/NTC/GC/Circum/2024, 28 June 2025
• SEAISI — Pakistan extends anti-dumping duty to Chinese galvalume imports (secondary reporting)
• Zafar et al., Materials and Corrosion (2020) — corrosivity map of Pakistan
• npj Materials Degradation (2023) — mild steel corrosion in Karachi, ISO 9223 categories and ISO 9224 20-year projection

Specifying the right zinc class is one of the cheapest decisions in a project and one of the most expensive to get wrong. Contact Raw Easy Corp for a current quotation.