Metal roofing calculator

Panel count for PBR, 5V, and standing seam — counted across the eave by net coverage width, the way a supplier counts it, not by bolting a shingle waste factor onto an area. With the pitch factor, the end-lap, and the exposed-fastener screw estimate.

01 How the math works

A metal panel runs the full slope from eave to ridge in one cut-to-length piece, so you count panels across the eave, not by area. The count for one roof plane is the eave length divided by the panel's net coverage width, rounded up. Net coverage is the width one installed panel actually covers after the side rib laps the next one: 36 in (914 mm) on a PBR / R-panel, 24 in (610 mm) on a 5V-Crimp, and 16 in (406 mm) on the most common standing seam.

The pitch factor is the same geometry a shingle estimate needs: it turns the horizontal rafter run into the true slope length. At a 6/12 pitch the factor is 1.1180, so a 15 ft run is a 16.77 ft slope. You order each panel cut to that slope length plus the eave overhang, which is where metal parts company with shingles — there is no length-direction waste to pad.

One detail the spec sheet states quietly: net coverage is narrower than the steel the panel is formed from. A 36 in PBR panel is roll-formed from closer to 38–39 in of coil, and the two-odd inches that vanish under the next panel's rib are steel you bought and will never see again. That overlap is already inside the coverage-width count, so adding a shingle-style area waste factor on top counts it twice.

02 Worked example

Take the simple gable from our roofing scenarios: a 1,200 ft² (111.5 m²) footprint, two equal planes, a 6/12 pitch. Each plane has a 40 ft (12.19 m) eave and a 15 ft (4.57 m) rafter run.

At the 6/12 pitch factor of 1.1180, the slope length is 15 × 1.1180 = 16.77 ft (5.11 m). Add a 2 in (51 mm) eave overhang and each panel is cut to 16.94 ft (5.16 m) — under the ~24 ft single-panel limit, so no end-lap.

In 36 in (914 mm) PBR coverage, each plane needs ⌈40 ÷ 3⌉ = 14 panels, so 28 for both planes. A 5% cut allowance rounds that to 30 panels. Imperial and metric land on the same 30. The roof area is 1,341 ft² (124.6 m²) = 13.42 squares, so at roughly 80 screws per square that is about 1,074 exposed-fastener screws, plus 40 ft (12.19 m) of ridge cap.

03 When this calculator is wrong

A simple gable in cut-to-length metal runs about a 5% cut-and-damage allowance — below the 10% a shingle job carries — because the side-lap is already in the coverage width and the length is cut to fit. That number only holds for clean rectangular planes. Add hips, valleys, and dormers and it climbs to 10–15%, because every panel that dies into a hip or valley is cut on the roof angle and the offcut rarely feeds another run.

• Net coverage is a nominal figure. Manufacturers publish 36 in for PBR, but the panel you buy may cover slightly more or less. Over a 40 ft eave, a half-inch difference in coverage width is enough to swing the panel count by one. Confirm the exact net coverage on the product data sheet before ordering.
• The end-lap is real material a flat waste % won't predict. When the slope length runs past the longest panel you can ship and handle — commonly 24–40 ft (7.3–12.2 m) — panels lap end-to-end with a minimum 6 in (152 mm) head-lap. The calculator adds a second course and the lap steel once the slope exceeds your max-length input; set that input to what your supplier will actually deliver.
• The screw count is a planning figure, not a fastening schedule. The 75–80 screws per square figure covers ordering a box of fasteners. The required count and spacing come from the panel's wind-uplift rating for your design wind load, substrate, and gauge. Standing seam fastens with concealed clips, not field screws, so that line reads zero for it.
• Minimum slope rules out some low roofs. Exposed-fastener PBR and 5V are rated to a 3:12 minimum without side-lap sealant, or as low as 1/2:12 with applied lap sealant. A bare 5V roof at 2:12 is below spec regardless of what the panel count says. Check the product's approval and your local code.

04 What to do with the result

Order the rounded-up panel count, and order the panels cut to the slope-plus-overhang length the calculator shows. That cut-to-length order is the metal advantage — you pay for the exact run, not a bundle you break into and waste. Round up rather than down: a panel short mid-install means a reorder, a lead time, and a possible color-lot mismatch on the replacement.

Ridge and hip cap, eave and rake trim, closures, and sealant are separate line items, ordered by the linear foot to match the ridge and edge lengths — the panel count does not include them. Before you place the order, confirm two numbers with your supplier: the panel's exact net coverage width, and the longest single length they can deliver to your site. Those two set the panel count and whether you need an end-lap at all.

05 Common questions

How do I calculate how many metal roofing panels I need?

Divide each roof plane's eave length by the panel's net coverage width and round up: panels = ⌈eave ÷ coverage⌉. Multiply by the number of planes, then add a cut allowance — about 5% on a simple gable. A 40 ft eave in 36 in (3 ft) PBR coverage is ⌈40 ÷ 3⌉ = 14 panels per plane.

What is the coverage width of a metal roof panel?

Net coverage is 36 in for a PBR / R-panel, 24 in for a 5V-Crimp, and 12, 16, or 18 in for standing seam, with 16 in the most common. It is always narrower than the coil the panel is formed from, because the side rib laps the next panel.

How much waste should I add for a metal roof?

About 5% for a simple gable — less than the 10% a shingle roof carries, because panels are cut to length and the side-lap is already in the coverage width. Bump it to 10–15% for roofs with hips, valleys, and dormers, where panels are cut on the roof angle.

Do I use the pitch factor for metal roofing?

Yes. The pitch factor (√(run² + rise²) ÷ run) turns the horizontal rafter run into the true slope length, the same as for shingles. At 6/12 it is 1.1180; a flat footprint measurement understates a 6/12 roof by about 12%.

What is the minimum slope for a metal roof?

Exposed-fastener PBR and 5V panels are rated to 3:12 without side-lap sealant, or down to 1/2:12 with applied lap sealant. Standing seam varies — snap-lock around 3:12, mechanically-seamed lower. Confirm against the product approval and local code.