Galvalume, Galvanized, PVDF and SMP: The Layers on a Metal Roof

Two metal roofing quotes for the same house can look alike until you reach the material line. One says 26-gauge Galvalume with a Kynar finish. The other says galvanized with an SMP paint. The color chips look the same. The difference is in what sits under and inside that color.

This guide works through the layers from the steel outward. Gauge, the thickness of the steel itself, is a separate subject covered in what metal roof gauge means. Here the subject is the coatings, because they do most of the work of keeping steel from rusting and keeping the color you chose.

Anderson Metal Roofing and Shingles installs residential metal roofs and commercial metal roofing and wall panels in several gauges, finishes and protective coatings. The words below apply to both kinds of work.

Bare steel rusts. So the steel coil that becomes a roof panel runs through a bath of molten metal that bonds to both faces before any paint is applied. The Metal Roofing Alliance puts it simply: all steel roofing is treated with a metallic coating layer to help prevent rust.

Galvanized steel is coated with zinc. The standard that covers it is ASTM A653, Standard Specification for Steel Sheet, Zinc-Coated (Galvanized) or Zinc-Iron Alloy-Coated (Galvannealed) by the Hot-Dip Process. The site's commercial page describes it the same way: a steel sheet dipped in molten zinc.

Galvalume is a trade name for steel coated with an aluminum-zinc alloy. Its standard is ASTM A792, Standard Specification for Steel Sheet, 55 % Aluminum-Zinc Alloy-Coated by the Hot-Dip Process. The 55 percent in the title is the aluminum share of the coating; the balance is zinc, with silicon in Type 1 coatings and added magnesium in Type 2. The standard covers what the coating is made of and how it is tested, not how long a roof lasts. Your quote should say which coating you are getting.

Neither coating makes a panel rust-proof. The National Park Service's roofing brief makes a point that still holds for new roofs: a plating or coating protects only as long as it remains intact, and once it is worn or damaged the exposed iron will rust. That is why paint layers and edge details matter.

Paint on a factory-finished panel is not a single coat of color. The coil is cleaned, pretreated, given a thin primer, then a finish coat that carries the color, all baked on in a coil-coating line before the sheet is formed into a panel. The U.S. Department of Energy's cool roof guidelines describe metal roofs as often coated with fluoropolymer- or silicone-polyester based paints. Those two families are what the resin names on your quote refer to.

The Metal Roofing Alliance lists why the paint is there: it adds a protective layer that can help prohibit rust, corrosion, UV damage and weather damage, and it can reflect sunlight.

Field paint is different from a factory finish. The site's post on touch-up paint for metal roofing explains that touch-up fades faster than a baked-on finish and is meant for small scratches, not for recoloring a roof.

The finish coat is named for its resin, the binder that holds the pigment. Two resin families cover nearly every painted steel roof you will be quoted in the Upstate.

• PVDF stands for polyvinylidene fluoride, the fluoropolymer family in the DOE's description. Kynar 500 and Hylar 5000 are resin trade names you will see on spec sheets; the paint maker buys the resin and blends it with pigments. PVDF finishes are generally sold on color retention and chalk resistance and are usually the higher-priced finish.
• SMP stands for silicone-modified polyester, the silicone-polyester family in the DOE's description. SMP finishes are common on agricultural panels, shops and many homes, and are usually the lower-priced finish.
• Plain polyester finishes also exist, mostly on budget panels. Ask whether a low quote uses one.
• The warranty tells you which you have. Manufacturers publish separate finish warranties for PVDF and SMP, with different chalk and fade terms. If the quote does not name the resin, ask for the product data sheet and the finish warranty for your exact color.
• Ask about installation scratches. Ask the installer how each finish handles handling and cutting on the job, and what touch-up the manufacturer supplies.

Every painted roof changes with sun and rain. Chalk is a powdery film that forms as the resin at the surface breaks down. Fade is a shift in color. Finish warranties limit how much of each is allowed over the warranty period, and they measure both with ASTM test methods rather than by eye.

ASTM D4214, Standard Test Methods for Evaluating the Degree of Chalking of Exterior Paint Films, rates chalk by rubbing the surface and comparing the transfer to a reference scale. ASTM D2244, Standard Practice for Calculation of Color Tolerances and Color Differences from Instrumentally Measured Color Coordinates, measures color change with an instrument. When a warranty says a finish will not chalk or fade beyond a certain rating, those are the methods behind the number.

Read the exclusions. Finish warranties may exclude certain colors, coastal exposure, low slopes, field-cut edges and damage from contact with other materials. The DOE guidelines note that lower surface temperatures reduce color fading, which is part of why the color you choose can change the warranty terms you are offered. Ask for the warranty document for the exact color, not the brochure.

The coatings protect the faces of the sheet. Three situations put bare or weakened steel in contact with things that attack it.

• Cut edges. Where a panel is cut to length, the steel core is exposed in a thin line. Factory cuts and hemmed edges limit this. Field cuts with abrasive blades also throw hot filings that rust on the finish. The site's post on protecting panels from rust covers clearing metal shavings and using the right cutting tools.
• Dissimilar metals. The National Park Service brief explains galvanic action: when dissimilar metals such as copper and iron are in direct contact, one corrodes the other, and the brief notes the metals do not have to touch, since rainwater carrying from one to the other is enough. On a steel roof that means no copper or lead flashing draining onto panels, and fasteners of a compatible metal.
• Treated lumber, mortar and concrete. The 2021 International Residential Code, in force in South Carolina since January 1, 2023, requires fasteners in contact with preservative-treated wood to be hot-dipped galvanized steel, stainless steel, silicon bronze or copper (section R317.3.1), a rule that exists because those treatments are corrosive to plain steel. Panel manufacturers' installation guides generally also say to keep panels out of direct contact with treated lumber and wet mortar or concrete. The Park Service brief lists alkalis from mortar and portland cement washing onto a roof as a cause of sheet-metal pitting.

If your roof is already leaking, the coating is probably not the cause. Leaks on metal roofs usually trace to fasteners, flashing, penetrations and panel ends. That is a repair question, and the roof repair and maintenance page is the place to start.

If you are choosing between a painted finish and leaving Galvalume bare, the next guide, painted vs bare Galvalume, lays out that decision. The metal roof maintenance schedule covers keeping debris and other metals off it.

When you request a quote, ask for four things in writing: the metallic coating standard (A653 or A792), the resin family (PVDF or SMP), the paint maker's finish warranty for your exact color, and how field cuts and treated-wood contact will be handled. Anderson Metal Roofing and Shingles gives free quotes and verifies measurements on site, and you can start through the quote form.