For full-length panels on a rectangular roof face, divide the eave length by the panel's net coverage width and round up for that face. Repeat for every roof face, then add the counts. Panel length must also cover the slope with the allowances specified for your roof system.
Worked example: a simple gable roof
Suppose each of two rectangular roof faces measures 31 ft along the eave and needs a panel length of 12 ft down the slope, including the specified allowances. With 36 in (3 ft) net coverage, each face needs 31 ÷ 3 = 10.33, rounded up to 11 panels. Order 22 panels at the required length before allowing for spares or additional cuts.
Dividing the total area by one panel's area gives 744 ÷ 36 = 20.67, or 21 panels rounded up. That is one short for this layout unless a suitable cut piece can actually be reused. Rounding each face separately avoids assuming that every offcut fits elsewhere.
Panel length, overlaps and waste
Measure the sloping distance, not just the horizontal footprint. For a roof with pitch expressed as rise in 12, slope length = horizontal run × √(rise² + 12²) ÷ 12. Confirm the required ridge clearance, eave projection and panel length with your supplier before ordering.
Net coverage width already accounts for the side lap; do not subtract it twice. If more than one panel is needed down the slope, end laps reduce the combined covered length. Use the panel maker's permitted lap and fastening details. Hips, valleys and penetrations need a cut layout; a blanket waste percentage cannot determine which panel lengths to buy.
Coverage width, not overall width
Panels overlap, so the coverage (net) width is less than the panel’s overall width — use the coverage figure or you come up short.
| Profile | Coverage width |
|---|---|
| Ribbed (R-panel, PBR) | ~36 in (3 ft) |
| 5V-crimp | ~24 in |
| Standing seam | ~16 in (12–19 in) |
Metal Roof Calculator (Tin & Steel)
Enter your footprint, pitch and panel width for the panels, screws and cost — exposed-fastener or standing seam.
Exposed fastener vs standing seam
Exposed-fastener panels (corrugated, R-panel) screw through the face — cheaper and faster, but the exposed screws and their gaskets are the long-term wear point. Standing seam hides the fasteners under interlocking seams — pricier but longer-lived. The type changes the coverage width and the fastener count.
Fasteners, trim and cost
Price the actual panel count and lengths, plus ridge caps, edge trim, closures, underlayment, fasteners and labor. Fastener spacing depends on the panel system, substrate and design loads; use the manufacturer's fastening schedule instead of treating a screws-per-square allowance as an installation specification. The calculator provides a budget estimate to refine with a supplier quote.
Check the exact product before ordering: Union Corrugating's panel profiles list different coverage widths and minimum slopes. The widths above are examples, not interchangeable specifications for all brands.
Steel, tin or aluminum?
The panel count is the same whatever the metal — only the price and longevity change. “Tin roof” today usually means galvanized or Galvalume steel; aluminum resists coastal corrosion; both come in the same corrugated and standing-seam profiles. Size the panels here, then price them for your chosen metal.