A standing seam roof is usually described by its panel profile, its gauge and its color. The metal under the paint gets a name on the mill paperwork, but it often gets only a passing word on the bid. For a St. Louis homeowner choosing a spec, that layer matters. The two substrates offered for steel panels in this area are galvanized steel and Galvalume, and they behave differently once a roof has been through several freeze thaw winters, a hot July and a spring of hail.
What the two coatings are
Both products start as cold rolled steel sheet. Galvanized steel is dipped in molten zinc, which leaves a bright zinc layer bonded to the steel. Galvalume is dipped in a metallic alloy of roughly 55 percent aluminum, about 43 percent zinc and a small amount of silicon. That alloy forms a dense oxide skin when it meets air. Galvanized sheet is commonly specified as G90 and Galvalume as AZ50. Those designations describe coating weight, not panel thickness, so a coating designation and a gauge are two separate numbers on the same spec. Read the designation on the paperwork, not the word on the sales sheet.
Cut edges and seams
Every panel has cut edges, and every standing seam has a bend that the paint has to survive. Zinc is sacrificial. When a cut edge is exposed, the zinc around it corrodes first and protects the steel beside it. That protection is useful, but it means the zinc is being spent at that spot. Galvalume protects cut edges too, through a different chemistry, and it is often chosen because its surface resists the white corrosion products that can show on bare galvanized. Neither coating is a guarantee against edge corrosion, and neither makes the paint unnecessary. A roofer should explain how the panels are cut and whether exposed edges are touched up in the field, because a cut edge is where the first rust tends to appear on a metal roof.
What St. Louis weather does to the substrate
A St. Louis roof sees a hard run of conditions. In winter, road salt and ice melt splash up from the drive and collect along the gutter line, and the freeze thaw cycle works at every seam and fastener. In spring, pollen, wet leaves and humid mornings keep the surface damp for hours. Summer heat expands and contracts the panel every afternoon. Hail season from spring into early summer can chip or scratch a finish. A scratch that cuts through the paint and into the coating is the spot where the substrate starts to matter, and it is worth photographing at the first inspection so you have a before picture.
The paint sits on top of the substrate
The paint finish is applied at the mill, to the coated steel coil, before the panels are formed. The substrate and the finish are therefore one spec, and changing either one changes the roof. A painted Galvalume panel and a painted galvanized panel can look identical from the ground. A color chart does not show which metal is underneath. The finish has its own warranty terms and its own failure modes, separate from the coating, so a bid that names the color and the gauge but not the coating has left part of the product unnamed. Ice at the eave adds one more load to that finish, because water that backs up under a bent edge sits against the metal through the cold months.
Reading the mill certificate
A mill certificate, sometimes called a mill test report, lists the steel grade, the thickness, the coating designation and an identifier for the coil the panels came from. Ask for the certificate that matches your job rather than a general sheet for the product line. A certificate for a different coil run tells you little about what is on your roof. If the roofer cannot produce one before the order goes in, that question should be settled before the metal is cut, not after the first delivery is on the drive.
What to ask for in writing
A written scope should name the substrate, not just the color. These are the items to request before you sign:
- The coating name and designation, such as G90 for galvanized or AZ50 for Galvalume, as shown on the mill certificate for the coil being run.
- The base metal gauge, on the same line as the substrate, so the thickness and the coating appear together.
- Whether exposed cut edges are touched up in the field, and with what material.
- The paint system and the coil it is applied to.
- A copy of the mill certificate for your job.
- The metal of the trim, flashing and fasteners, so you can see whether any of it differs from the panels.
Keys can put a written scope with photos and measurements in your hands, and the contractor attends the work while you keep your own records. If you want the spec for your house talked through before the order is placed, call the office at (314) 220-2333.
Before the order goes in
Check the substrate on the bid against the mill paperwork, then confirm that the trim, fasteners and flashing are the same family of metal or that any difference has been planned. Dissimilar metals at a seam can set up galvanic contact, and that is easier to prevent at the order stage than to repair on the roof. Choose the color last. The color chart is the easiest decision on this list, and it is the one least connected to how the metal will hold up.