Galvalume is steel sheet coated with an alloy of about 55% aluminum, 43.5% zinc and 1.5% silicon, applied by hot-dipping. It is the standard bare and pre-painted sheet for metal building roofs and walls because, in long-term atmospheric tests, the coating corroded several times more slowly than a galvanized (pure zinc) coating of the same thickness.
This guide covers what the coating is made of, why it lasts, how it compares with galvanized sheet, the coating weights you will see on an order, and the two places it should not be used. Figures come from the zinc industry’s GalvInfoNote 1.4, 55% Aluminum-Zinc Alloy-Coated Steel Sheet (Rev 1.2, December 2017; checked October 2026) unless noted.
What Galvalume Is
Galvalume is a trade name, not a generic term. It is a registered trademark of BIEC International and is used by licensed sheet producers. The same product is sold as ZINCALUME by Steelscape and under other names in other markets. The generic description is “55% aluminum-zinc alloy-coated steel sheet,” often shortened to 55% Al-Zn or AZ sheet.
The product was introduced in 1972 and was taken up first for metal building roofing. GalvInfo describes it as an excellent long-life cladding, especially for low-slope roofs on industrial buildings, where it is often used bare with the coating exposed directly to the weather. It is also widely used as the base sheet for pre-painted panels.
The ASTM specifications
Bare 55% Al-Zn sheet is bought to ASTM A792/A792M, Standard Specification for Steel Sheet, 55% Aluminum-Zinc Alloy-Coated by the Hot-Dip Process (current edition A792/A792M-25a; scope checked October 2026). The scope says the sheet is intended for uses that need corrosion resistance, heat resistance or both, and it is offered in commercial, forming, drawing, high-temperature and structural steel designations. Pre-painted sheet on this substrate is covered by ASTM A755/A755M.
What the Coating Looks Like Inside
The reason Galvalume outlasts galvanized sheet is its microstructure. As the coating cools after dipping, aluminum-rich crystals (dendrites) grow first. The zinc that is left fills the spaces between them, forming a connected network of zinc-rich areas. Small needles of silicon sit in the coating, and a thin, hard intermetallic layer bonds it to the steel.
Each part has a job:
- Aluminum-rich dendrites behave like a barrier coating and are largely unaffected by most environments in the early years.
- Zinc-rich areas corrode first and provide galvanic (sacrificial) protection, which limits rust staining at sheared edges and scratches where bare steel is exposed.
- Silicon is there for manufacturing, not corrosion. It holds back growth of the brittle intermetallic layer so the sheet can be roll-formed into panels without the coating cracking off.
As the zinc-rich areas corrode, their corrosion products plug the spaces between the dendrites and the corrosion rate slows. GalvInfo describes the resulting rate curve as flattening over time, in contrast with the steady, linear thinning of a zinc coating. That is the main reason for the long life of bare Galvalume roofs.
Galvalume vs Galvanized
Galvanized sheet is coated with zinc only. If you need the background on that coating, see our guide to what galvanized steel is and how it protects. In outdoor exposure tests, the 55% Al-Zn coating corroded much more slowly than zinc in every type of atmosphere tested:
| Test site | Environment | Galvanized corrosion rate divided by 55% Al-Zn rate |
|---|---|---|
| Kure Beach, NC, 25 m from ocean | Severe marine | 3.8 |
| Kure Beach, NC, 250 m from ocean | Moderate marine | 8.2 |
| Bethlehem, PA | Industrial | 6.4 |
| Saylorsburg, PA | Rural | 18.7 |
Source: Table 1 of GalvInfoNote 1.4, comparing G90 galvanized with AZ50 Galvalume of roughly the same coating thickness (about 20 µm). A ratio of 3.8 means the zinc coating lost thickness 3.8 times as fast. GalvInfo’s own summary is more conservative for service life, saying the 55% Al-Zn coating has proven to outlast galvanized steel by two to four times, depending on the environment.
| Galvalume (55% Al-Zn) | Galvanized (zinc) | |
|---|---|---|
| Sheet specification | ASTM A792/A792M | ASTM A653/A653M |
| Common outdoor coating | AZ50, AZ55, AZ60 | G90 is the usual comparison |
| Corrosion behavior | Rate slows over time | Steady, linear thinning |
| Cut edges and scratches | Protected by the zinc-rich phase | Protected by zinc |
| Deep drawing | Coating adhesion is weaker | Usually preferred |
Galvanized products are not all the same thing: a hot-dip galvanized beam or bolt has a much thicker coating than galvanized sheet. Our article on whether galvanized steel rusts explains how zinc coatings wear away, and galvanized vs stainless steel covers the step up from coatings to stainless.
Coating Weights: AZ50, AZ55 and AZ60
The AZ number is the coating designation in ASTM A792, and the bracketed AZM number is the metric equivalent. GalvInfo lists these as the common designations for outdoor use:
- AZ50 [AZM150], about as thick as a G90 [Z275] galvanized coating. It is also the designation called for when 55% Al-Zn coating is used on cold-formed steel framing to ASTM A1003.
- AZ55 [AZM165], a heavier coating.
- AZ60 [AZM180], heavier again.
- AZ70 [AZM210] is used on corrugated steel pipe to ASTM A929.
Watch the units when comparing with galvanized sheet. The aluminum-zinc alloy is much less dense than zinc, so a 20 µm Galvalume coating weighs about 150 g/m² while a 20 µm zinc coating weighs about 275 g/m². A lower coating weight does not mean a thinner coating. The sheet thickness itself is a separate number, usually given as a gauge; our steel gauge thickness chart converts those to inches and millimeters.
Where Galvalume Should Not Be Used
GalvInfo names two notable exceptions to Galvalume’s better performance:
Animal confinement buildings
Barns housing swine, cattle and poultry produce waste decomposition by-products that can be extremely aggressive to the 55% Al-Zn coating. Some sheet producers neither recommend nor warrant bare Galvalume for these buildings. Ask the panel manufacturer what they warrant before ordering for a livestock building.
Contact with concrete
Wet concrete is highly alkaline and quickly attacks the aluminum in the coating, leaving it porous and prone to corrosion. GalvInfo advises against contact even with dry, fully cured concrete, giving the example of bottom tracks on exterior steel-framed walls: if the concrete gets wet, the alkaline water attacks the coating. Keep the sheet isolated from slabs, curbs and foundation walls.
Other Practical Points
Galvalume Plus and scuff marks
Bare 55% Al-Zn sheet marks easily. Forming rolls can leave black abrasion lines, and installers’ handprints and boot marks can turn dark and stay visible for years. Products sold as “Plus” (for example Galvalume Plus) have a thin clear resin coating that prevents these marks and also improves formability. If a bare roof will be seen, the resin-coated version is worth asking for.
Heat resistance
Because of its high aluminum content, the coated sheet can take surface temperatures up to 750°F (400°C) without discoloring and up to 1200°F (650°C) without heavy oxidation and scaling.
Where it fits in a metal building
In a typical pre-engineered metal building, Galvalume or pre-painted Galvalume is the roof and wall skin, fixed to cold-formed purlins and girts that sit on the hot-rolled steel frames. The coating protects the sheet only. Panel thickness, fastener spacing and the purlins that carry wind and snow loads are part of the building design, which must be prepared or checked by a licensed structural engineer.
Galvalume FAQ
Is Galvalume better than galvanized?
For bare roof and wall sheet in most atmospheres, yes. In GalvInfo’s exposure data, zinc corroded 3.8 to 18.7 times faster than 55% Al-Zn of similar thickness. The exceptions are animal confinement buildings and contact with concrete, and galvanized sheet is usually preferred for deep-drawn parts.
How long does a Galvalume roof last?
It depends on the coating weight, the climate, the roof slope and how it is installed. The industry’s own summary is that 55% Al-Zn outlasts galvanized sheet by two to four times, and GalvInfo notes more than 35 years of strong performance on unpainted low-slope roofs. For a specific roof, use the panel maker’s warranty terms.
What is Galvalume made of?
A steel sheet hot-dip coated with an alloy of 55% aluminum, 43.5% zinc and 1.5% silicon.
Can Galvalume touch concrete?
It should not. Concrete’s alkalinity attacks the aluminum in the coating, and the zinc industry advises against contact even with cured concrete that could get wet.
What does AZ55 mean?
It is a coating weight designation under ASTM A792. AZ50, AZ55 and AZ60 are the common outdoor grades, with AZ50 being about as thick as a G90 galvanized coating. The metric equivalent of AZ55 is AZM165.
Reference information only, not engineering advice. Disclaimer.