A 40 × 30 ft gable roof at 4/12 with 12 in overhangs takes 28 exposed-fastener panels of 36 in coverage, each cut to 16 ft 10⅜ in, for 1,417 sq ft (14.17 squares) of roof. With 10% extra that is 1,247 screws, 5 pieces of ridge cap, 9 of eave trim, 7 of rake trim, a foam closure per panel at the eave and at the ridge, and 2 rolls of synthetic underlayment. The same roof in 16 in standing seam is 64 panels.
How metal roof panels are calculated
Metal panels run in one piece from eave to ridge, so each roof plane is a row of panels side by side. The count is the plane's width along the eave divided by the panel's net coverage (not its full sheet width, since the edges overlap), rounded up. The panel length is the sloped distance from the eave edge to the ridge.
Panels per plane = width along the eave ÷ coverage, rounded up · Panel length = (half the width + eave overhang) × √(1 + (pitch ÷ 12)²)
Worked example with the defaults: each side is 40 + 2 × 1 = 42 ft along the eave. 42 ft ÷ 36 in = 14 panels, times 2 sides = 28. The run from eave to ridge is 30 ÷ 2 + 1 = 16 ft, times 1.0541 at 4/12, so each panel is cut to 16 ft 10⅜ in. Panels are ordered cut to length, so the waste allowance only applies to screws and underlayment.
Runs longer than 45 ft, the maximum length Metal Sales recommends, are split into two panels with a 6 in end lap, which the calculator shows as "end lapped".
Panel types: R-panel, corrugated and standing seam
| Panel | Coverage | Fastening | Screws per square | Minimum slope |
|---|---|---|---|---|
| Exposed fastener, 36 in (R-panel, Classic Rib) | 36 in | through the panel | about 80 | 3/12 |
| Exposed fastener, 24 in (Delta Rib, corrugated style) | 24 in | through the panel | about 40 | 3/12 |
| Standing seam (Vertical Seam) | 12, 16 or 18 in | hidden clips | none through the panel | 3/12 open framing, 1/12 solid deck |
The coverages, slopes and screw counts are from Metal Sales' Post-Frame Installation Guide and Residential Design Manual, which puts typical fastening at 40 screws per square for 2 ft wide panels and 80 for 3 ft wide panels. On the default roof the 24 in panel needs 42 panels and 624 screws. Standing seam clip spacing depends on your framing and wind uplift, so take it from the panel's load tables rather than a rule of thumb.
Steel, tin or metal roof calculator?
"Tin roof" is the everyday name for a metal panel roof. The panels here are steel with a Galvalume or galvanized coating, as Metal Sales lists them, so the numbers apply whichever name you use. Always enter the panel's net coverage width from the supplier, not the width of the sheet, because the side laps overlap.
Minimum slope for metal roofing
The IRC (R905.10.2) allows lapped, non-soldered panels without sealant down to 3/12, the same panels with lap sealant down to ½ in 12, and standing seam down to ¼ in 12. Manufacturers may ask for more: Metal Sales wants 3/12 for Classic Rib and 1/12 for Vertical Seam over a solid deck. The slope check under the calculator tests both. Below 3/12, choose standing seam.
Trim, closures and underlayment
- Trim comes in 10 ft 2 in pieces in Metal Sales' PBR guide; lapping 2 in leaves 10 ft of cover each. The default roof has 42 ft of ridge (5 pieces), 84 ft of eaves (9 pieces) and 67.5 ft of rakes (7 pieces).
- Closures are foam strips shaped to the panel ribs. Inside closures go under the panels at the eave, outside closures on top of the panels under the ridge cap: one of each per panel, 36 in long for a 36 in panel.
- Underlayment goes over the whole deck. TAMKO's Synthetic Guard covers 905.67 sq ft per roll after laps, so 1,417 sq ft plus 10% is 2 rolls. Add an ice barrier at the eaves where your code requires it; the roofing calculator sizes it.
Hip roofs are not laid out from the footprint here, because every panel meeting a hip is cut on an angle. Enter each plane under "Roof planes" at its widest width and treat the extra as waste.
Next steps
Check the pitch with the roof pitch calculator before choosing a panel. The roof replacement cost calculator compares corrugated and standing seam prices with shingles for your state, and the shingle calculator gives the shingle side of the comparison. New roof, new gutters: the gutter slope calculator lays out the hangers and downspouts.