AZ50 vs AZ55: What the Galvalume Coating Number Actually Means
AZ50 and AZ55 are coating weights, not grades — and the difference between them is smaller than the difference between painted and unpainted product. Here is how the designation works.
AZ50 and AZ55 appear on nearly every steel roofing submittal in Canada, and they are routinely misread as quality grades — as if AZ55 were a better product and AZ50 a cheaper one. They are neither. They are coating weights defined by a standard, and the correct one to specify depends almost entirely on one question: is the panel painted or not?
What the Designation Encodes
The system comes from ASTM A792, the standard for steel sheet with 55% aluminum-zinc alloy coating. The AZ prefix identifies the alloy coating. The number is the minimum total coating mass, expressed in hundredths of an ounce per square foot of sheet — counting both surfaces combined.
- ✓AZ50 — a minimum of 0.50 oz/ft² of coating, total across both faces
- ✓AZ55 — a minimum of 0.55 oz/ft² of coating, total across both faces
Two details are worth pinning down because they are the source of most confusion. First, it is both sides combined, not each side. AZ50 puts roughly a quarter of an ounce per square foot on each face. Second, the number is a triple-spot test minimum — the average of three specified sample locations. The standard also sets a lower single-spot minimum, because coating weight varies slightly across a coil and a single low reading is not a rejection on its own.
The Metric Equivalents
ASTM A792M is the metric companion, and its AZM designations are given in grams per square metre, again both sides combined. In Canadian specifications you will see both systems, sometimes on the same document.
- ✓AZM150 — 150 g/m², the metric near-equivalent of AZ50
- ✓AZM165 — 165 g/m², the metric near-equivalent of AZ55
- ✓AZM180 — 180 g/m², a heavier coating for aggressive exposure
- ✓AZM100 — a light coating, appropriate for interior or protected applications, not exposed roofing
Because aluminum is much less dense than zinc, an aluminum-zinc coating is physically thicker than a pure zinc coating of the same mass. An AZ50 coating is on the order of 20 µm per side — a little under one thousandth of an inch. That thickness advantage per unit of coating weight is one reason Galvalume coating numbers look lower than galvanized G-numbers without meaning less protection.
Why Painted Product Uses AZ50 and Bare Product Uses AZ55
This is the actual decision rule, and it is not arbitrary.
On a painted panel, the paint system is doing barrier duty on the exposed face. A pretreatment, a primer and a topcoat together form a continuous film that keeps water and oxygen off the metallic coating. The Galvalume underneath is a second line of defence — it handles what gets through at a scratch, a fastener penetration or a cut edge, and it protects the back face. Because the paint is carrying the first line, AZ50 is the conventional and adequate specification for painted product, and it is what most painted architectural and residential panel is built on.
On an unpainted panel — mill finish Galvalume, typically shipped with a thin acrylic coating that is a handling and forming aid rather than a protective finish — there is no paint film. The metallic coating is the only protection there is, on both faces, for the life of the roof. That is why AZ55 is the standard specification for bare exposed product, and why you see it on agricultural, industrial and utility roofing where mill finish is the norm.
So the honest answer to "should I upgrade from AZ50 to AZ55?" on a painted architectural panel is that it is a marginal 10% increase in coating mass, and it will matter far less to the roof's service life than the paint system, the gauge, the edge detailing and the fastener selection. Those are the levers worth pulling.
When a Heavier Coating Is Genuinely Warranted
Heavier designations exist because some exposures justify them. Consider going above the standard weight where:
- ✓The product is unpainted and permanently exposed, particularly on low slopes where water lingers.
- ✓The building is agricultural with high humidity, though note that ammonia-rich livestock atmospheres are alkaline and are a case for galvanized rather than heavier Galvalume.
- ✓The location sees continuous chloride exposure — direct road salt splash or spray.
- ✓The panels are near industrial emissions with significant sulphur or acid content.
- ✓The assembly includes areas that will stay wet and cannot dry, which is a design problem worth fixing directly rather than compensating for with coating weight.
AZ Numbers Are Not G Numbers
The numbering convention is identical — hundredths of an ounce per square foot, both sides — but the alloys are completely different, so the numbers are not comparable as performance. G90 galvanized carries 0.90 oz/ft² of zinc; AZ50 carries 0.50 oz/ft² of aluminum-zinc alloy. That looks like a large deficit until you account for the fact that the alloy coating corrodes far more slowly per unit mass in ordinary atmospheric exposure, and that its lower density makes it thicker per unit mass. In exposed atmospheric service, AZ50 typically outlasts G90 by a wide margin.
Where the ranking flips is at cut edges, where galvanized's stronger sacrificial action reaches further, and in alkaline exposure, where the aluminum-rich coating is chemically vulnerable and zinc is not.
How to Verify What You Are Getting
The designation should appear in three places, and a specifier should be able to see at least one of them:
- ✓The mill certificate for the coil, which states the ASTM standard, the coating designation, the steel grade and the base metal thickness.
- ✓The stencil printed on the underside of unpainted sheet, which typically carries the coating designation directly.
- ✓The manufacturer's written product data sheet and the project submittal.
A quote that says "Galvalume" with no designation is incomplete in the same way a quote that says "metal" with no gauge is incomplete. Ask for AZ50 or AZ55 in writing alongside the gauge, the steel grade and the paint system, and compare quotes on all four.
Frequently Asked Questions
Is AZ55 worth paying extra for on a painted roof?
Usually not, as a priority. It is roughly 10% more coating mass sitting under a paint system that is already providing the primary barrier. On a painted panel, spending the same effort on gauge, on a PVDF rather than SMP topcoat, on hemmed edges and on correct fasteners returns considerably more service life.
Does AZ50 mean 50% aluminum?
No — that is the most common misreading. The 55% aluminum figure describes the alloy composition and applies to both AZ50 and AZ55. The AZ number is a coating weight in hundredths of an ounce per square foot across both faces, entirely separate from the composition.
Is the coating measured per side or total?
Total, both surfaces combined. AZ50 means 0.50 oz/ft² across both faces, so roughly half that on each side. This trips people up when comparing coating weights between products and standards, so it is worth confirming explicitly.
What should I specify for an unpainted agricultural roof?
AZ55 or its metric equivalent AZM165 is the conventional specification for bare exposed Galvalume, and heavier designations are available where exposure is more aggressive. If the building houses livestock, the interior ammonia atmosphere is alkaline and a zinc-coated product for interior liner and structural components is the better choice regardless of what is on the exterior.
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