Almost every painted metal roof on an American home is steel, and under the paint that steel is protected by one of two metallic coatings: galvanized, which is a zinc coating, or Galvalume, a coating of about 55 percent aluminum and 45 percent zinc. Galvalume lasts longer in most exposures and is the standard under premium standing seam. Galvanized steel still has its place, especially around livestock and concrete. This guide explains what each coating is, where each wins and what to ask for on a bid.
What steel roofing is
Bare steel rusts, so roofing steel is never bare. At the mill, a coil of sheet steel passes through a bath of molten metal that bonds to both sides and becomes the first line of defense. Paint goes on top of that metallic coating, not instead of it. When a bid says 24 gauge Kynar steel, the full story is steel of a certain thickness, coated with Galvalume or zinc, primed and painted with a PVDF finish. Each layer matters, and the metallic coating is the one homeowners hear least about.
Galvanized steel
Galvanized steel is coated with pure zinc. Zinc protects steel two ways: as a barrier, and by sacrificing itself at scratches and cut edges so the exposed steel does not rust. Coating weight is given as a G number. G90 means 0.90 ounce of zinc per square foot, counting both sides, and is the standard for painted roofing. G60 is lighter and common on budget agricultural panels. Galvanized has been the roofing standard for well over a century and is still widely used, especially in exposed-fastener panels and metal shingles.
Galvalume steel
Galvalume is coated with an alloy of about 55 percent aluminum, 43 percent zinc and a little silicon. The aluminum forms a tough, long-lasting barrier against moisture, while the zinc still gives some sacrificial protection at scratches. Coating weight is given as an AZ number. AZ50 is the grade used under paint, and AZ55 is slightly heavier and used for bare, unpainted panels. In most residential exposures Galvalume outlasts galvanized steel of the same gauge, which is why premium standing seam makers build on it. Bare Galvalume, usually with a thin clear acrylic coating from the mill, is also a popular finish in its own right, weathering to a matte spangled gray.
Both come in the same gauges: 24 gauge is the residential standing seam standard, 26 gauge the budget choice and a common weight for metal shingles and exposed-fastener panels, and 29 gauge the thin agricultural panel. See metal roof gauge explained for how thickness affects the roof.
How it is made and installed
Coated steel coil goes from the mill to a coil coater, where it is cleaned, primed and painted with PVDF or SMP paint on a continuous line, then baked. The painted coil goes to a panel manufacturer or an installer with a roll former, where it is formed into standing seam, ribbed or shingle profiles. From there, installation is the same for either coating and follows the style you chose; our guides to standing seam and exposed-fastener panels cover the steps.
Two installation habits matter more with steel than with aluminum. First, cut edges: every panel is cut somewhere, and a raw edge is where the steel is exposed. Good installers hem edges at the eave, tuck cuts under trim and avoid leaving raw cuts facing the weather, and they clean up metal shavings from the roof, because every shaving left behind rusts into a brown spot on the paint. Second, dissimilar metals: steel should not take runoff from copper or sit in contact with copper, and Galvalume in particular should not touch wet concrete, mortar or treated lumber without a barrier.
Where each one fits
| Situation | Galvalume | Galvanized |
|---|---|---|
| Typical house, inland or suburban | Preferred, longest life under paint | Good, slightly shorter life |
| Bare, unpainted roof | Popular matte gray look (AZ55) | Bright at first, weathers unevenly |
| Barn with livestock inside | Not recommended; ammonia attacks the coating | Preferred |
| Contact with concrete, mortar or stucco | Avoid direct contact | Tolerates it better |
| Within a few miles of the ocean | Limited or excluded warranty | Limited or excluded warranty; consider aluminum |
| Budget exposed-fastener panels | Common | Common |
Climate. In most of the country, Galvalume under a PVDF finish is the longest-lived painted steel you can buy. Neither coating is meant for the surf line; close to salt water, steel finish warranties are usually limited or excluded, and aluminum is the better metal. In snow, hail and high wind the coating makes no difference; gauge, profile and installation do.
Buildings with animals. Ammonia from livestock waste attacks the aluminum in the Galvalume coating from the inside of the building. Horse barns, dairy barns and poultry houses have long used galvanized steel for that reason, and manufacturers still say so.
House style. The coating does not change the look of a painted roof. Bare Galvalume is the exception: its soft gray spangle has become a signature of modern farmhouses, cabins and contemporary homes, and it costs less than painted steel.
Pros and cons of Galvalume
Pros
- Longer life than galvanized in most exposures
- Standard base for premium PVDF-painted standing seam
- Attractive bare finish that weathers to matte gray
- Same gauges, profiles and prices as galvanized
- Backed by long perforation warranties from many manufacturers
Cons
- Not for animal confinement buildings
- Should not touch wet concrete, mortar or treated lumber
- Protects cut edges a little less aggressively than zinc
- Still steel, so not the pick right at the ocean
- Bare Galvalume shows fingerprints and scuffs until it weathers
Approximate 2026 cost
The coating itself makes only a small difference in price; profile, gauge and paint system drive the cost of a steel roof. As broad national ranges installed in 2026, steel roofing runs roughly $6 to $12 per square foot for exposed-fastener panels, $10 to $18 for metal shingles and stone-coated steel, and $12 to $20 for standing seam, or about $600 to $2,000 per roofing square across the styles. Bare Galvalume standing seam usually comes in a bit under painted steel. A PVDF finish costs more than SMP.
These are approximate figures, not quotes. Prices vary by region, roof pitch and complexity, gauge, profile, paint, tear-off and deck repairs, and the time of year, and steel coil prices move month to month, which is why many bids are only good for a few weeks. Get three written bids that name the coating grade, gauge and paint. The metal roof cost guide covers the full picture.
What to ask the installer
The coating is easy to leave off a bid because most homeowners never ask. Ask.
Confirm licensing where your state requires it, liability and workers compensation insurance, a written contract that names the coating, gauge, paint, profile, color and payment schedule, and a permit where your town requires one. Our guide on metal roof warranties explains the difference between a paint warranty and a perforation warranty.
Common mistakes and red flags
None of these ruins a roof overnight. They show up as rust lines at an edge after ten years, a stained wall where runoff landed or a coating that chalks faster than the warranty allowed. The coating question takes thirty seconds to ask and belongs on every steel roof contract.
Related guides
The paint on top of the coating is the next decision; read PVDF vs SMP paint finishes. If you are near the water, compare aluminum roofing. For a side-by-side look at all the roofing metals, visit the roofing metals hub. Find installers and suppliers who work in steel at the steel directory or search by state at Find a Metal Roofer.
Frequently asked questions
What is Galvalume?
Galvalume is steel coated with an alloy of about 55 percent aluminum, 43 percent zinc and a little silicon. The aluminum gives a tough barrier against moisture and the zinc gives sacrificial protection at scratches and cut edges. It is the standard base metal under most painted standing seam roofing sold in the United States today and is also used bare, where its fine spangled gray finish is a popular look on its own.
Is Galvalume better than galvanized?
In most residential exposures, yes. Galvalume generally outlasts galvanized steel of the same gauge in rural, suburban and mild marine settings, and it is what premium panel makers use under PVDF paint. Galvanized does better in a few specific places: barns housing animals, where ammonia attacks the aluminum in the Galvalume coating, and anywhere the steel will touch wet concrete, mortar or other alkaline materials.
What do G90 and AZ50 mean?
They are coating weights. G90 galvanized steel carries 0.90 ounce of zinc per square foot counting both sides, the usual grade for painted roofing. G60 is lighter and shows up on budget agricultural panels. AZ50 Galvalume carries 0.50 ounce of aluminum-zinc alloy per square foot and is the grade for painted panels; AZ55 is slightly heavier and is used for bare, unpainted Galvalume. Ask for the grade in writing.
Can I use bare Galvalume on a house?
Yes. Bare Galvalume, usually with a thin clear acrylic coating from the mill to help handling and weathering, is a common choice for modern, farmhouse and rustic designs. It weathers to a soft matte gray over years and costs less than painted steel. It does not come with a paint finish warranty, and it is not the choice right at the coast or where runoff from copper or treated lumber will reach it.
Does steel roofing rust at the cut edges?
Cut edges are where the raw steel shows, and both coatings protect them in different ways. Zinc on galvanized steel sacrifices itself to shield the edge. Galvalume relies more on its aluminum barrier and protects cut edges a little less aggressively, which is why good installers hem or cover exposed edges and avoid leaving raw cuts facing the weather. A hairline of discoloration at a cut edge is normal; rust streaks down the panel are not.