Galvalume vs Galvanized Steel: Coating Chemistry and Where Each One Fails
Both coatings protect steel, but they do it by different mechanisms — and they fail in opposite environments. Here is the chemistry, and how to pick the right substrate for an Ontario roof.
Almost every steel roofing panel sold in Ontario sits on one of two metallic-coated substrates: hot-dip galvanized steel, or Galvalume. They look nearly identical on a sample chip and the paint system on top is often the same. They are not interchangeable. The two coatings protect the steel underneath by different mechanisms, and because the mechanisms differ, they fail in almost opposite conditions.
If you are specifying a roof, an eavestrough, a flashing detail, or a set of trims that will sit against masonry, this is the decision that quietly determines whether the assembly is still sound in thirty years.
What Is Actually in Each Coating
Galvanized steel is carbon steel sheet run through a molten bath of essentially pure zinc in a continuous hot-dip line, then cooled. The coating is zinc, with a thin iron-zinc intermetallic layer where it bonded to the steel. In North America it is specified under ASTM A653 with a G designation such as G90; in metric, Z275.
Galvalume is an alloy coating, nominally 55% aluminum, 43.4% zinc and 1.6% silicon by weight, applied the same way and specified under ASTM A792 with an AZ designation such as AZ50 or AZ55. That weight breakdown is misleading if you picture it, though. Aluminum is far less dense than zinc, so by volume the coating is roughly 80% aluminum. When the alloy solidifies it forms an aluminum-rich dendritic structure — think of overlapping crystal branches — with zinc concentrated in the spaces between the branches. That microstructure is the whole story.
Two Different Protection Mechanisms
Zinc protects steel two ways at once. First, it is a physical barrier. Second, and more importantly, zinc is electrochemically more active than steel, so when both are wetted by the same film of water, the zinc corrodes preferentially and the steel does not. That is sacrificial, or cathodic, protection. It is why a scratch through a galvanized coating does not immediately bloom into red rust: the surrounding zinc keeps protecting the exposed steel across a short distance.
Galvalume is predominantly a barrier coating. The aluminum-rich portion forms a tight, self-repairing oxide film that is chemically very stable and corrodes extremely slowly in ordinary atmospheric conditions. The zinc trapped between the aluminum dendrites still provides sacrificial protection, but only a limited amount, because there is less of it and it is distributed through the coating rather than being the whole coating.
This is the practical trade-off in one sentence: Galvalume lasts much longer on the open field of a panel, and galvanized protects a cut edge or a scratch more aggressively. Everything else follows from that.
Why Cut Edges Behave Differently
Every panel has cut ends. Every hole drilled for a fastener, every field cut around a vent, every snipped hem exposes bare steel at the edge of the coating. What happens next depends on how far the coating can throw its protection.
On galvanized steel, zinc dissolving from the edge of the coating deposits corrosion products across the exposed steel and keeps it cathodically polarized. The protected distance is measured in a couple of millimetres in a thin atmospheric moisture film, which is enough for ordinary sheet thicknesses. On Galvalume, the available zinc at the edge is a fraction of that, so the throwing distance is shorter. On thin gauges the aluminum-rich coating on the two faces is close enough together that the edge still stays passive. On heavier gauges, in wet or chloride-laden conditions, you can eventually see edge creep — a fine line of rust working in from the cut.
This is why good metal roofing practice hems exposed edges back on themselves rather than leaving a raw cut facing weather, why factory-cut lengths beat field cuts wherever the geometry allows, and why the eave edge — the wettest edge on the roof — gets a hem or a cleat rather than a bare snip.
Where Galvalume Fails
Galvalume's weakness is alkalinity. The aluminum oxide film that makes it so durable is stable in a narrow pH band, and strong alkali dissolves it. That gives a short list of conditions to avoid, and they come up more often than people expect:
- ✓Direct contact with wet concrete or fresh mortar. Cement paste is strongly alkaline. A Galvalume flashing tucked into a fresh mortar joint or bedded on green concrete is a known failure.
- ✓Runoff and drainage from masonry, stucco, or freshly poured concrete onto a Galvalume surface.
- ✓Animal confinement buildings. Ammonia from manure creates an alkaline, humid atmosphere. This is why the agricultural market often specifies galvanized or heavily coated product for barn interiors even where Galvalume is used outside.
- ✓Trapped, non-draining moisture. Anywhere water sits rather than drains — a flat lap, an unvented cavity, a bundle of sheets stored wet — behaves far worse than an exposed sloped surface.
- ✓Severe forming. The alloy coating is harder and less ductile than pure zinc and can microcrack on very tight bends, which opens the barrier at exactly the point the part is stressed.
Where Galvanized Fails
Galvanized steel's weakness is that its protection is consumptive. The zinc is used up. In a clean rural atmosphere that consumption is slow, but the rate rises sharply with acidity, with sulphur compounds, with chloride, and above all with time of wetness. A galvanized surface that stays damp — a north slope that never dries, a low-slope section, a run of trim behind a shrub — burns through its coating far faster than the same coating on a well-drained south face.
In practice, on an exposed unpainted roof surface in Southern Ontario, Galvalume outlasts an equivalent weight of galvanized coating by a wide margin. That is why bare mill-finish agricultural and industrial roofing shifted to Galvalume decades ago and largely stayed there.
How This Plays Out in a Real Specification
For painted architectural roofing — standing seam, hidden-fastener panels, most residential work in Southern Ontario — Galvalume is the default and the right default. The paint system provides the barrier duty on the exposed face and the Galvalume handles what gets through, plus the whole back face. Painted Galvalume with a quality primer and topcoat is the standard architectural substrate for good reason.
Galvanized still earns its place, and specifying it selectively is a sign of a contractor who is paying attention:
- ✓Flashings, counterflashings and reglets that will be built into masonry or bedded in mortar.
- ✓Interior liner panels and structural components in livestock buildings.
- ✓Parts that require severe forming — tight-radius bends, deep-drawn accessories, heavily worked trims.
- ✓Structural steel components of the assembly: purlins, hat channel, clip bases and framing, where cathodic protection at drill points and cut ends matters more than surface longevity.
A Note on Mixing Them
Galvalume and galvanized steel are close enough electrochemically that putting them in contact is not the disaster that copper-on-aluminum is. Zinc-coated fasteners in a Galvalume panel, or a galvanized flashing lapped under a Galvalume panel, are routine. The pairs that cause real trouble are copper, lead, and bare graphite against any zinc- or aluminum-bearing coating, and that is worth understanding separately before you finalize flashing and fastener details.
One more housekeeping item that costs roofs their coating every year: swarf. Cutting panels with an abrasive chop saw throws hot steel filings across the roof surface. Those particles embed in the coating, rust, and stain — and each one is a tiny site of bare steel sitting on the finish. Shear, nibble, or use a fine-tooth metal blade, cut away from finished surfaces, and sweep the roof clean at the end of every day.
Frequently Asked Questions
Is Galvalume always better than galvanized for a roof?
For an exposed, well-drained roof surface, Galvalume generally provides longer service from the same coating weight. It is not better everywhere. In alkaline exposure — mortar contact, concrete runoff, livestock ammonia — galvanized is the more appropriate choice, and in severely formed parts the more ductile zinc coating handles the bend better.
Can I tell which one I have by looking at it?
Not reliably once it is painted. Unpainted Galvalume has a fine, small-spangle, slightly matte silver appearance while traditional galvanized often shows a visible crystalline spangle pattern, but modern minimized-spangle galvanized looks similar. The certain answer is on the sheet stencil or the mill certificate, which carries the ASTM designation — A792 with an AZ number for Galvalume, A653 with a G number for galvanized.
Does a painted panel still need a good metallic coating underneath?
Yes. The paint film is thin — typically around one thousandth of an inch on the exposed face — and it will eventually be scratched, chipped at a fastener, or cut at a panel end. The metallic coating is what stops a scratch from becoming a rust streak, and it is the only protection at every cut edge on the roof.
What about cut edges on my finished roof — do they need to be touched up?
Field-cut edges in visible or wet locations should be detailed properly rather than painted over. Hemming the edge, turning it out of the water path, or covering it with trim is far more durable than touch-up paint, which is a cosmetic repair and not a coating replacement. Touch-up has its place on scratches in the field of a panel, applied thinly.
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