A Specifier's Guide to Galvalume (ASTM A792): Performance, Limitations, and Best Practices

What is Galvalume?

Galvalume is a continuously hot-dip coated sheet steel governed by ASTM A792. The coating is precisely defined as 55% aluminum, 1.6% silicon, and the balance (43.4%) zinc by weight. The combination is engineered to deliver the best characteristics of both parent metals: the long-term barrier protection of aluminum, and the sacrificial (galvanic) protection of zinc at cut edges and scratches. In most atmospheric exposures, it outlasts a comparable galvanized panel by a wide margin.

What is the difference between AZ50 and AZ55?

ASTM A792 specifies coating weight through designations such as AZ50 and AZ55. The number signifies the minimum total coating weight on both sides of the sheet in hundredths of an ounce per square foot — AZ55 specifies a minimum of 0.55 oz/ft². AZ50 is the standard specification for most building products. AZ55 is required where the warranty calls for it, typically for unpainted Galvalume roofing exposed long-term to weather.

Where should you NOT use Galvalume?

Galvalume's electrochemistry makes it highly effective in some applications and inappropriate in others. Avoid the following situations:

• Dissimilar metals — direct contact with copper (pipes, AC condensate, copper flashings) or lead creates a galvanic cell that rapidly corrodes the Galvalume
• Alkaline environments — wet, uncured concrete or mortar chemically attacks the aluminum-zinc coating
• Treated lumber — many modern pressure-treated woods contain copper-based preservatives that aggressively corrode Galvalume in direct contact
• Severe coastal environments — high chloride concentrations in salt spray break down the protective oxide layer within a few hundred yards of breaking surf

When is galvanized (G90/G185) or painted Galvalume the better call?

When the detail puts the panel in contact with concrete, treated lumber, or dissimilar metals, switch to G90 or G185 galvanized — zinc tolerates those environments better. When salt-air exposure is the issue, painted Galvalume (or a different coating system entirely) is the right call: the paint film is the working barrier, with the Galvalume substrate as backup. When in doubt, send your environment and detail conditions to Metal Master with your quote request, and we will walk through the substrate decision with you.

Frequently asked questions

What is Galvalume made of?

The Galvalume coating is 55% aluminum, 1.6% silicon, and 43.4% zinc by weight, applied by continuous hot-dip process to a steel substrate. The specification is ASTM A792.

What is the difference between AZ50 and AZ55?

AZ50 and AZ55 refer to minimum total coating weight on both sides of the sheet, in hundredths of an ounce per square foot. AZ55 (0.55 oz/ft² minimum) is heavier and is typically required for unpainted Galvalume roofing warranties; AZ50 is the standard specification for most building products.

Can Galvalume touch pressure-treated wood?

No. Most modern pressure-treated lumber contains copper-based preservatives that aggressively corrode Galvalume in direct contact. Use a separator membrane, galvanized fasteners, or a different substrate where contact is unavoidable.

Is Galvalume good for coastal applications?

Not within a few hundred yards of breaking surf. The chloride concentration in salt spray breaks down the protective oxide layer on the aluminum-zinc coating. Painted Galvalume or a different coating system is the correct call in severe coastal environments.

When should you use galvanized (G90) instead of Galvalume?

Choose galvanized G90 or G185 when the panel will contact concrete, mortar, copper, lead, or treated lumber — zinc tolerates those environments better than the aluminum-zinc Galvalume coating.