Gauge and substrate get bought as one line item, but they answer two different questions. Gauge is how stiff the panel is. Substrate is how the steel fights rust once the paint or a cut edge gives up.
What the gauge number actually means
Gauge is a nominal number. The decimal behind it is what counts. In the metal roofing trade, 24 gauge is carried as 0.0239 in. nominal base steel and 26 gauge as 0.0179 in. Galvanized sheet gauge tables run a little heavier because they include the coating. Either way, 24 gauge is about a third thicker than 26.
That third is worth more than it sounds. Bending stiffness goes with thickness cubed, so a flat in 24 gauge is roughly 2.4 times stiffer than the same flat in 26, all else equal. Coil is ordered to a decimal with a tolerance, not a gauge, and the decimal behind "24 gauge" varies by producer, so ask for it.
Thickness is a stiffness question
Oil canning. Thinner steel shows waviness sooner: on wider flats, low slopes, dark colors, and in low-angle light. Going to 24 gauge helps, but it does not cure it. Striations or pencil ribs in the pan, a narrower pan, a flat deck, and clips that are not overdriven matter as much. The Metal Construction Association calls oil canning an inherent characteristic of light-gauge panels, not a cause for rejection, so set expectations before the panels go up.
Hail. This is North Texas. Thicker steel takes a smaller dent from the same stone, and 24 gauge plainly out-dents 26. But an impact class under UL 2218 belongs to a tested panel assembly, not to the coil. Check the listing for the exact profile, gauge, and substrate before you promise anyone a Class 4 roof.
Foot traffic. 24 gauge over solid deck walks fine. 26 gauge over purlins on 5 ft centers will crease if you step in the flat between supports. Walk the purlin lines.
One thing the gauge number does not tell you is steel grade. Most 26 gauge exposed-fastener coil is Grade 80 full-hard: it resists a permanent dent, but it is no stiffer, and a tight hem cracks the paint and sometimes the steel. 24 gauge standing seam coil is usually Grade 50 so it seams and hems cleanly.
Substrate is a corrosion question
G90 galvanized is hot-dip zinc on steel per ASTM A653. G90 means 0.90 oz of zinc per square foot, total of both sides. Zinc protects sacrificially: it corrodes in preference to the steel, so a scratch or a cut edge is covered by the zinc next to it. The trade-off is that zinc weathers away steadily, so life is mostly how much you started with.
Galvalume is 55% aluminum, 43.4% zinc, 1.6% silicon by weight, per ASTM A792. AZ50 is 0.50 oz/sq ft both sides and is the standard under paint. AZ55 is 0.55 oz/sq ft and is what you get bare, usually with a clear acrylic coat. The aluminum forms a barrier that does not weather away the way zinc does, and the zinc in the alloy still gives some sacrificial protection. Galvalume outlasts G90 in most atmospheric exposures at comparable coating thickness (AZ55 and G90 are close; the alloy is lighter). Long-term exposure data puts it at two to four times, depending on the environment. Most painted roofing coil in the US is on Galvalume, and the mills' 25-year substrate perforation warranties are written around it.
Where G90 is the better pick:
- Anything that touches concrete, mortar, or wet green lumber. Galvalume does not like wet alkaline contact.
- Animal confinement and other ammonia-heavy interiors.
- Anything you plan to paint in the field (see bonderized, below).
Keep Galvalume away from copper, lead, and copper-treated lumber, and do not let copper runoff drain onto it. That goes for G90 too. Isolate with a membrane or a painted surface.
Cut edges and bonderized
Every panel and trim piece has a cut edge with bare steel showing. On G90 the surrounding zinc sacrifices and the edge stays clean a long time. On Galvalume you may see a little edge rust stain early, and it generally stops. The thicker the sheet, the more bare edge to protect, which is one reason Galvalume is rarely specified much heavier than 22 gauge. Hem drip edges where you can, and keep cut edges out of standing water.
Bonderized, also sold as paint-grip, is galvanized (usually G90, sometimes G60) with a phosphate conversion treatment. It comes dull gray and takes paint well. It is not a finish. Left bare it stains and white-rusts, worse if it sits wet in a bundle. Prime and paint it after install, and keep it dry. There is no bonderized Galvalume; field-painted trim means galvanized.
Which to spec
| Application | Gauge | Substrate |
|---|---|---|
| Mechanically seamed standing seam, wide pans, low slope | 24 | Galvalume AZ50, PVDF paint |
| Snap-lock standing seam, narrow striated pan, steep slope over deck | 24 preferred, 26 workable | Galvalume AZ50 |
| Exposed fastener (R/PBR, 5V, corrugated) over purlins | 26 typical, 24 for heavier duty | Galvalume AZ50 painted, AZ55 bare |
| Trim and flashing to match a panel | Match the panel; go one heavier on wide fascia | Same as the panel |
| Trim to be painted in the field | 24 or 26 | G90 bonderized |
The short version: standing seam wants 24 gauge because the pans are wide and the clips are hidden, so stiffness is doing the work. Exposed fastener gets away with 26 because the ribs are tall and the screws pin it down. Either way, put it on Galvalume unless you have one of the G90 reasons above.
Frequently asked
How much stiffer is 24 gauge than 26 gauge metal roofing?
24 gauge is about a third thicker than 26 gauge (roughly 0.0239 in. versus 0.0179 in. nominal base steel), and because bending stiffness scales with thickness cubed, a 24 gauge flat is roughly 2.4 times stiffer. That is why standing seam with wide pans is normally spec'd in 24 gauge.
Is Galvalume better than G90 galvanized for a metal roof?
In most open-air exposures Galvalume (55% aluminum-zinc, ASTM A792) outlasts G90 galvanized (ASTM A653) at comparable coating thickness, by roughly two to four times in long-term exposure data, and most painted roofing coil is on Galvalume. G90 is the better pick where the steel touches concrete, mortar, or wet green lumber, in animal confinement or other ammonia-heavy interiors, or when trim will be painted in the field.
What is bonderized or paint-grip steel?
Bonderized (paint-grip) is galvanized steel, usually G90, with a phosphate conversion treatment that gives it a dull gray surface that takes paint well. It is not a finish on its own; it must be primed and painted after installation or it will stain and white-rust.