Metal Types
What is Galvalume steel and why is it the commercial default?
Galvalume is a steel substrate coated with an aluminum-zinc alloy (55% aluminum, 43.4% zinc, 1.6% silicon by weight). This alloy provides barrier corrosion protection from the aluminum and sacrificial protection from the zinc at cut edges and scratches. For commercial applications, Galvalume 24-gauge steel is the standard substrate because it combines structural rigidity with long-term corrosion resistance at a predictable cost per square foot.
When should a commercial project specify aluminum instead of steel?
Aluminum is required for any commercial structure located within the 1,500-foot coastal salt spray zone. The chloride concentration in airborne salt accelerates corrosion of the zinc component in Galvalume steel. Aluminum does not contain zinc and forms its own protective oxide layer that resists salt spray degradation. The trade-off is that aluminum costs approximately 40% more per square foot and requires different fastener specifications to prevent galvanic corrosion.
What is the 1,500-foot coastal exclusion zone?
Manufacturer warranty boundaries define specific distances from the ocean where Galvalume steel warranties are reduced or voided entirely. Most manufacturers set this exclusion boundary at 1,500 feet from the mean high tide line. Within this zone, only aluminum substrates with compatible PVDF coating systems carry the full warranty term.
How does salt spray resistance testing work?
ASTM B117 salt spray testing exposes coated metal samples to a continuous salt fog environment for thousands of hours. The test measures creep from scribe marks, blister formation, and coating adhesion loss. Galvalume steel shows visible corrosion at cut edges after approximately 1,000 hours. Aluminum substrates with proper coating systems can exceed 4,000 hours without measurable degradation.
What is galvanic corrosion and how does it affect metal selection?
Galvanic corrosion occurs when two dissimilar metals contact each other in the presence of an electrolyte like rainwater. If steel fasteners are used with aluminum panels, the steel will corrode the aluminum at the contact point. The reverse is also true: aluminum clips on Galvalume panels create a galvanic cell. Every metal contact point in the system must be evaluated for galvanic compatibility.
Applicable Engineering Certifications
UL 580
UL 90
UL 790 Class A
UL 2218 Class 4
ASTM E 2140
Both Galvalume steel and aluminum substrates used by The Metal Roof Pros carry current test reports for all listed certifications. Substrate selection is matched to the building's environmental exposure on every project.