G90 Galvanizing Explained: What the Number Means and What It Does Not
G90 is a coating weight in hundredths of an ounce per square foot across both faces — not a grade, not a warranty, and not the same thing as hot-dip galvanizing a fabricated part.
G90 turns up on framing, decking, purlins, clips, flashings, fence components and plenty of roofing panel. It is one of the most quoted specifications in the construction metals trade and one of the least well understood. It is a number describing how much zinc is on a piece of sheet steel. That is all it is — and knowing exactly what it measures tells you a lot about what to expect.
The Definition
G90 comes from ASTM A653, the standard for steel sheet, zinc-coated by the hot-dip process. The G prefix identifies a zinc coating. The number is the minimum coating mass in hundredths of an ounce per square foot of sheet, counting both surfaces combined, measured as a triple-spot average.
So G90 is 0.90 oz/ft² of zinc across both faces — roughly 0.45 oz/ft² on each side. Converted to thickness, that is approximately 19 µm, or about three quarters of one thousandth of an inch, per side. The metric designation for the same coating under ASTM A653M is Z275, meaning 275 g/m² total across both surfaces.
The single most common error is reading G90 as 0.90 oz/ft² per side. It is not. It is the total across both faces, and every G-designation in the standard works the same way. The same convention applies to the AZ numbers used for Galvalume, which is why the two systems get confused with each other.
The Full Range of Designations
G90 is one step on a ladder, and picking the right rung is the actual specification decision:
- ✓G40 (Z120) — light coating; interior, dry, or fully painted service only
- ✓G60 (Z180) — light commercial; common as a substrate for painted interior products and light framing
- ✓G90 (Z275) — the general-purpose commercial exterior standard, and the most widely stocked
- ✓G115 / G140 (Z350 / Z450) — heavier coatings for prolonged wetness or aggressive atmospheres
- ✓G165 through G235 — severe service; less commonly stocked and often mill-order only
Because zinc protection is consumed over time at a rate set by the environment, coating thickness and time to first rust scale close to linearly. Doubling the coating weight roughly doubles the service life in the same exposure. That linear relationship is the basis for the service-life estimating charts the galvanizing industry publishes, and it is a genuinely useful mental model: specifying a heavier coating buys proportionally more time, not a step change in behaviour.
How Zinc Protects Steel
Zinc does two jobs at once, and both matter.
It is a barrier: the zinc layer physically separates the steel from water and oxygen. And it is sacrificial: zinc is electrochemically more active than steel, so when both are wetted by the same film of water, a galvanic cell forms in which the zinc is the anode and the steel is the cathode. The zinc corrodes and the steel does not. This is why a scratch, a drilled hole or a sheared edge on galvanized steel does not immediately rust — the surrounding zinc keeps the exposed steel cathodically polarized.
The reach of that protection, sometimes called the throwing distance, is short in a thin atmospheric moisture film — on the order of a couple of millimetres — and longer where the electrolyte is more conductive or the part is immersed. For ordinary sheet thicknesses that is plenty to keep a cut edge passive, which is the single biggest practical advantage galvanized has over aluminum-zinc coatings.
The Zinc Patina and Why Storage Matters
Exposed zinc weathers through a sequence: zinc oxide forms first, converts to zinc hydroxide in the presence of moisture, and then reacts with atmospheric carbon dioxide to form zinc carbonate. The carbonate is stable, adherent and comparatively insoluble, and it is what slows further zinc consumption. A galvanized surface that has been allowed to weather properly in open air develops that dull grey protective film.
Interrupt the sequence and you get white rust — wet storage stain. If bundled sheet or nested parts get wet and the moisture is trapped where air and carbon dioxide cannot reach, the hydroxide stage keeps repeating with no path to the stable carbonate. The result is a bulky white deposit that consumes zinc quickly and permanently reduces the coating. Sheet that arrives at a site and sits banded and wet on the ground can lose meaningful coating life before it is ever installed. Store galvanized product dry, off the ground, on a slope so water drains, with air movement around the bundle, and break bands on material that got wet in transit.
Sheet Galvanizing Is Not Batch Hot-Dip Galvanizing
This distinction matters enormously for fencing, railings and structural work, and the terminology hides it.
ASTM A653 covers continuous hot-dip coating of steel sheet in a mill, before the sheet is formed into anything. Coating thicknesses are measured in tens of micrometres. ASTM A123 covers batch hot-dip galvanizing of fabricated steel products — a finished post, bracket, gate frame or railing dipped whole in a kettle after welding and drilling. Batch coatings are typically several times thicker than sheet coatings, and because the part is dipped after fabrication, cut ends, welds and drilled holes are coated too.
That is the crux. A pre-galvanized tube cut to length and welded has bare steel at every cut and every weld, protected only by whatever sacrificial reach the adjacent coating has. A batch-galvanized assembly has zinc everywhere, including inside the tube. For a fence post going into the ground, or a railing that will live in road salt splash, the difference is not academic.
Where G90 Belongs in a Roofing Assembly
For exposed roof panels in Southern Ontario, painted Galvalume is generally the better substrate — an aluminum-zinc coating outlasts an equivalent zinc coating substantially in ordinary atmospheric exposure, and bare G90 on an exposed roof plane is under-specified for a long-life roof. G90 earns its place elsewhere:
- ✓Flashings and reglets built into masonry or bedded in mortar, where the alkalinity that attacks Galvalume does not attack zinc.
- ✓Structural components of the assembly — purlins, hat channel, sub-girts, clip bases — where drilled and cut connections benefit from sacrificial reach.
- ✓Liner panels and structural members in livestock buildings with ammonia-rich air.
- ✓Severely formed accessories where the more ductile zinc coating survives tight bends better than an alloy coating.
- ✓As the substrate under a paint system in light exposure, where the paint carries most of the barrier duty.
What G90 Does Not Tell You
It says nothing about steel thickness, steel strength grade, surface finish, spangle, whether the product is painted, or whether it is fit for any particular exposure. A complete sheet steel specification names the standard and coating designation, the base metal thickness or gauge, the strength grade, and the finish. G90 alone is one line of four.
Frequently Asked Questions
Is G90 the same as Z275?
Effectively yes — they are the imperial and metric designations for the same coating weight under ASTM A653 and A653M respectively. G90 is 0.90 oz/ft² total both sides, Z275 is 275 g/m² total both sides.
Is G90 enough for an exposed exterior roof panel?
For a painted panel in ordinary exposure it can be adequate, but painted Galvalume is the more common and generally longer-lasting substrate for exposed roofing. Bare, unpainted G90 on an exposed roof plane is a light specification for Ontario conditions; either a heavier zinc coating or an aluminum-zinc coating is the better choice.
Why does my galvanized fence post rust at ground level first?
The soil-to-air interface is the most aggressive zone on a buried post: it stays damp, it has oxygen access, it cycles wet and dry, and it collects road salt. On top of that, a pre-galvanized tube cut to length has an uncoated cut end, often the one in the ground. Batch hot-dip galvanized posts, or posts set with proper drainage rather than in a concrete cup that holds water, address both causes.
Can I repair damaged galvanizing on site?
Partially. Zinc-rich repair paints and zinc solder restore some sacrificial protection at cut ends, welds and scratches, and they are the accepted field repair. They do not reproduce the thickness or the metallurgical bond of the original coating, so a repaired area is a maintenance point, not an equivalent surface. Design cuts and penetrations out of wet locations where you can.
Does white rust mean the material is ruined?
Not necessarily. Light white rust is a cosmetic and coating-loss issue; the sheet may still be serviceable with reduced remaining life. Heavy white rust indicates significant zinc consumption and the material should be assessed rather than installed on faith. Either way it is preventable with dry, ventilated storage.
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