Rust on a Galvalume Metal Roof: Cut Edges, Scratches, Filings and Fasteners
How Galvalume Protects Itself
U.S. Steel describes Galvalume sheet as steel with a coating of "nominally 55% aluminum, 43% zinc alloy" that "combines the galvanic corrosion protection of zinc with the passivating barrier protection of aluminum." The aluminum forms a tough barrier. The zinc sacrifices itself to protect nearby bare steel, which is how the coating helps protect small scratches and cut edges.
That protection has limits. Steel that is exposed over a larger area, kept wet, or touching the wrong materials can still rust. On painted Galvalume, U.S. Steel's exposure testing found that corrosion creeping in from a cut edge "is slight and stops increasing after 2 years." So a little edge discoloration on a painted panel is not, by itself, a sign of failure.
What Causes Rust on a Galvalume Roof
Metal filings (swarf). Cutting panels on or near the roof throws off fine steel particles. Star Building Systems, a metal building manufacturer, warns that this swarf "will be hot, will embed into the panel coating, causing the area to rust." U.S. Steel lists "iron debris and saw fines left over from cutting panels" among things that should not touch Galvalume. The result is a scatter of small orange specks, usually near where panels were cut or trimmed.
The wrong cutting tool. Star Building Systems says electric shears "will also wipe the Galvalume® coating over the cut edges to offer protection," and that "saws, abrasive blades, grinders or torches should never be used." Rough, saw-cut edges with no coating left on them rust faster than sheared ones.
Copper, lead and treated lumber. U.S. Steel's bulletin on corrosive conditions tells installers to keep Galvalume away from "Copper, including copper bearing chemicals from pressure treated lumber, and water drainage from copper pipes such as air conditioning units," and from lead. Lead pipe flashings, copper flashing, a copper condensate line dripping onto the roof, or treated wood in contact with panels can all cause localized corrosion.
Pencil marks. The same bulletin lists graphite, "including pencil marks," as something to keep off the coating.
Wet debris and standing water. Zinc and aluminum protect themselves with an oxide film that needs air. U.S. Steel lists "piles of leaves, ash, dust, or other organic debris" that stay wet, and "Ponding water or maintaining wet conditions for extended periods of time," among conditions that break that film down. It also states that its warranty on bare Galvalume roof panels is voided at slopes under 1/4 in 12.
Fasteners. U.S. Steel says fasteners "should be chosen that have a life expectancy equivalent to GALVALUME Sheet Steel." When they do not, the screws can rust long before the panels do, leaving streaks down the roof. Unless the manufacturer specifies otherwise, the 2024 International Residential Code calls for galvanized fasteners on steel roofs and accepts stainless steel (R905.10.4).
Harsh chemicals. Muriatic acid used to clean brick and concrete is "very corrosive" to Galvalume, and the bulletin warns against contact with uncured concrete. Keep masonry cleaning runoff off any metal roof below it.
Reading the Rust Pattern
| What you see | Likely cause |
|---|---|
| Scattered tiny orange dots, often near cuts or trim | Embedded filings from cutting |
| Rust along a cut edge, rough or jagged | Panel cut with a saw or grinder |
| Streaks running downhill from screw heads | Fasteners corroding |
| Corrosion below a lead pipe flashing, copper line or AC unit | Lead or copper contact or drainage |
| Rust under leaves, in a valley, or behind a chimney | Debris or water holding moisture against the metal |
| Straight gray or dark lines along panels | Pencil marks left from layout |
When Rust Matters and When It Is Cosmetic
Surface specks from filings and minor edge staining are mostly cosmetic, but filings should be removed early, before the rust spreads into the coating around them. Rust that is growing, rust at fasteners, rust under debris, and anything that has gone soft or through the metal are not cosmetic. Through-rust means the panel or section needs replacing.
Fastener rust in particular should be dealt with before the washer and the hole around it wear out. Repairs to rusted screws, worn washers and pipe flashings on metal roofs are covered on our metal roof repair page.
Keeping a Galvalume Roof From Rusting
- Have panels cut with shears, away from the roof surface, and sweep or blow off filings the same day.
- Keep copper and lead off the roof: use compatible pipe flashings, and redirect copper condensate lines so they do not drip onto the panels.
- Clear leaves and debris from valleys and behind chimneys and walls.
- Use fasteners rated to last as long as the panels, and neutral-cure sealants, which U.S. Steel specifies.
- Keep acidic masonry cleaners and wet concrete off the roof.
If rust is widespread or the coating has failed across large areas, a metal roof replacement may make more sense than spot repairs.
Sources: U.S. Steel Technical Bulletin, "GALVALUME Sheet Steel" (TBP 12.6) and "Conditions Corrosive to Hot-Dip Galvanized and GALVALUME Coated Steel Sheet" (TBP 2005.18), both via McElroy Metal; Star Building Systems, "Best Practices for Cutting Metal Panels On-Site"; International Residential Code (2024) R905.10.4.
Frequently Asked Questions
Does Galvalume rust?
It is very rust resistant, but it can rust where filings were left on it, where it was cut with abrasive tools, where it touches copper or lead, where wet debris sits on it, or where fasteners corrode.
Is rust on cut edges of a metal roof normal?
Some edge discoloration can occur. U.S. Steel found that edge corrosion on painted Galvalume is slight and stops increasing after about two years. Sheared edges hold up better than saw-cut ones.
What are the little rust spots on my new metal roof?
Most often embedded steel filings from cutting panels during installation. They should be removed early before they stain the coating around them.
Can copper touch a Galvalume roof?
No. U.S. Steel says to avoid contact with copper, including runoff from copper air-conditioning lines and copper-bearing chemicals from treated lumber.
Why are the screws on my metal roof rusting?
Usually because the fasteners do not last as long as the panels. U.S. Steel recommends fasteners with a life expectancy equal to the Galvalume. Rusting screws should be replaced before the holes and washers fail.
