How to calculate metal roof panels
Exposed-fastener panels are ordered two ways: how many (set by the coverage width) and how long (set by the slope). Count panels along the eave: the building length plus both gable overhangs, divided by the panel's coverage width, rounded up. Length is the horizontal run from the eave edge to the ridge — half the width plus the eave overhang on a gable — times the pitch multiplier.
Worked example — a 30 × 40 ft building, 4/12, 12-in overhangs, 36-in panels: 40 + 2 = 42 ft of eave ÷ 3 ft = 14 panels per side, 28 in all. The run is 15 + 1 = 16 ft × 1.0541 = 16' 11". Roof area is 1,417 sq ft, so 1,134 screws at 80 per square. Full breakdown: metal roof for a 30×40.
Coverage width is not panel width
Rib panels are a little wider than they cover, because the edge rib laps over the next panel. Always count with the coverage width: 36 in for R-panel, PBR-style and similar rib panels (Fabral Grandrib 3, Best Buy Metals Tuff-Rib, Metal Sales StrongPanel), 24 in for 5V crimp, and about 32 in for 7/8-in corrugated lapped two ribs on a roof.
| Profile | Coverage | Maker's minimum pitch | Stock lengths (store) |
|---|---|---|---|
| 36-in rib panel | 36 in | 2.5/12 | 8–20 ft, $30.99–$80.99 |
| 5V crimp | 24 in | 3/12 | 8–16 ft, $29.99–$53.99 |
Cut-to-length or stock panels?
Metal suppliers cut panels to the inch — Best Buy Metals from 3 to 45 ft, Fabral up to 40 — so every panel fits the slope exactly. Home centers and lumber yards stock fixed lengths, typically 8 to 20 ft for 36-in panels and 8 to 16 ft for 5V. With stock panels you buy the next length up and trim the extra, which wastes metal on most roofs: at about $4.05 per linear foot for 29-gauge, a foot of offcut on every panel adds up. On short slopes it is often cheaper to cut one longer panel in two — the calculator checks every stock length and does this when it saves money. Keep the factory end at the eave and put the cut end under the ridge cap. Runs longer than the longest stock panel need cut-to-length panels or an end lap.
Screws, trim and closures
- Screws: Fabral recommends 80 per square for its 36-in rib panel — at the eaves, ridge and end laps and every intermediate support. Use #14 × 1 in Woodtite screws into OSB or plywood, #10 × 1½ in into purlins.
- Ridge cap: a universal ridge cap is 10½ ft long; lapped 6 in, each piece covers 10 ft of ridge.
- Eave and rake trim close the panel edges along the gutters and gable ends. The calculator counts them in the same 10½-ft lengths — check what your supplier stocks.
- Foam closures seal the panel ribs: inside closures under the panels at the eave, outside closures on top of them under the ridge cap. They match the profile: 36-in strips for rib panels, 24-in for 5V.
- Underlayment: over a solid deck, a 10-square synthetic roll covers about 929 sq ft after laps. Panels screwed straight to purlins on a pole barn go without.
Minimum pitch
The IRC (R905.10.2) allows lapped, exposed-fastener metal panels at 3/12 and steeper, or down to ½/12 when every lap gets sealant. Standing seam can go to ¼/12. Makers rate 36-in rib panels like Grandrib 3 and Tuff-Rib from 2½/12; 5V crimp from 3/12. Below those, water backs up through the laps in heavy rain.
Weight and handling
29-gauge rib panels weigh about 0.7 lb per sq ft (26-gauge about 0.85) — roughly 70 lb per square, against about 220 lb for a square of architectural shingles. A 20-ft panel is still awkward: two people, and never in wind.
Common mistakes
- Counting panels by their full width instead of the coverage width — you end up a panel short on every side.
- Forgetting the gable overhangs in the eave length, or the eave overhang in the panel length.
- Measuring panel length along the wall height instead of along the slope.
- Skipping closures: open ribs at the eave let in wind-driven rain, birds and wasps.
Shingling the house instead? The roofing calculator handles squares, bundles and accessories, and the roof pitch calculator helps you measure the slope. Pouring the shop floor too? See the concrete slab calculator.