Understanding your result
The headline is the panels to order, with the length, the coverage width and the count per side beneath it. The stats show the panels per side, the panel length, the area covered and the linear feet. The breakdown goes from the slope to the panel length, from the eave to the panels per side, flags the width of the last panel, and then adds the planes and the waste.
The number to remember is the panel length to order: panels are made to it, and an inch short at the eave is a leak at the gutter. The second is the last panel row. A last panel of a few inches is a nuisance to fasten and looks wrong; shifting the start so the two end panels are each about half a panel wide fixes it, and the row tells you when to think about it.
The area feeds the screws calculator, the eave and ridge lengths feed the trim calculator, and a roof with several sections of different slope goes to the cut list calculator, which runs this arithmetic for each section.
How we calculate this
The run here includes the eave overhang in plan, because the panel must reach the fascia: from the ridge centreline to the drip edge, horizontally. The slope is that run times the pitch factor, the same multiplier the roof area calculator uses, and the pitch calculator gives it for a pitch measured in degrees. The panel then overhangs the drip edge by an inch or two so water drips clear, which is the eave overhang field. At the ridge the panel stops an inch or two short under the ridge cap; that gap is small and is covered by rounding the length up to the next inch.
The count ignores the rake overhang only if you did; the eave length should include it, since the panels run to the end of the rake trim. The metal roof panel guide works through coverage width, side laps, end laps and the cut list for a roof with a valley.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Coverage width | 36 in | The standard coverage of PBR, R-panel and 5-rib exposed-fastener panels (manufacturers’ data sheets); corrugated is 24 in |
| Eave overhang | 2 in | Manufacturers’ installation guides: 1 to 2 in past the drip edge, up to 3 in into a gutter |
| Waste | 3% | Estimating practice for a plain roof; 10 to 15% with hips, valleys or dormers |
| Run | includes the eave overhang in plan | The panel runs to the fascia |
| Maximum convenient panel length | 40 ft | Shipping and handling practice; longer slopes are lapped or site-formed |
| Length rounding | up to the next inch | Panels are cut to the inch |
Assumptions last reviewed October 7, 2026.
The calculator does not count hips, valleys or dormers as separate sections (use the cut list calculator), does not include the trim, closures or screws (their own calculators), and does not know the panel gauge or weight. The standing seam versus exposed fastener guide compares the two systems on quantities, cost and life.
Two worked examples
A gable roof with 36-inch panels
Eave 40 feet per side, two planes, run 16 feet at 6/12, 36-inch coverage, 2-inch eave overhang, 3% waste.
- Slope: 16 × 1.118 = 17.89 ft; panel length: 17.89 ft + 2 in = 18 ft 1 in
- Panels per side: 480 in ÷ 36 in = 13.3, so 14 (the last one cut to 12 in)
- Two planes: 28; with 3% waste: 29 panels
- Area: 29 × 3 ft × 18.08 ft = 1,571 sq ft (15.7 squares); 524 linear ft of panel
The 12-inch last panel is worth avoiding: start with a half-width panel at each rake and the two end panels are 24 inches each.
A lean-to in corrugated
One plane, eave 24 feet, slope measured at 12 feet, 24-inch corrugated, 2-inch overhang, no waste.
- Panel length: 12 ft + 2 in = 12 ft 2 in
- Panels: 288 in ÷ 24 in = 12 panels, the eave dividing evenly
- Area: 12 × 2 × 12.17 = 292 sq ft (2.9 squares); 146 linear ft
A plain shed roof with an eave that is a whole number of panels: no cut panels, no waste, twelve sheets and the screws.
Where to find your inputs
Eave length. Along the bottom edge of one plane, from rake trim to rake trim, including the rake overhangs. For a hip roof, the eave of each plane at the drip edge (the hips then need the cut list calculator).
Run. From the ridge centreline to the drip edge in plan: half the building width plus the eave overhang on a symmetrical gable.
Coverage width. On the panel’s data sheet, as “coverage” or “net coverage”: 36 inches for most ribbed panels, 24 for corrugated, 12 to 18 for standing seam (which has its own calculator).
Eave overhang. The manufacturer’s figure: 1 to 2 inches past the drip edge, or whatever reaches the centre of the gutter.
Common mistakes
- Counting by the sheet width. A 38-inch sheet covers 36 inches. The count uses the coverage.
- Ordering the run as the length. The panel runs up the slope, which is longer than the run by the pitch factor, and then overhangs the eave.
- Forgetting the rake overhang in the eave. The panels run to the end of the rake trim, a foot past the wall at each end.
- Zero waste with a valley. Every panel that meets a valley is cut on the diagonal. Allow 10 to 15 percent on that section.
- A sliver for the last panel. Shift the start so both end panels are about half a panel wide.
- Starting into the wind. Lay the first panel at the end away from the prevailing wind so the side laps face away from the weather.
Questions people ask
- How many metal roofing panels do I need?
- Divide the eave length of each roof plane by the panel's coverage width and round up, then add the planes. A 40-foot eave with 36-inch coverage panels needs 480 ÷ 36 = 13.3, so 14 panels per side and 28 for a gable roof. The last panel on each side is cut to width; on this roof it is 12 inches wide.
- How long should metal roofing panels be?
- The slope length from the ridge to the eave plus the overhang past the fascia, usually 1 to 2 inches, or up to 3 inches into a gutter. A 16-foot run at 6/12 is 16 × 1.118 = 17.9 feet of slope; with a 2-inch overhang the panel is 18 feet 1 inch. Panels are cut to order to the inch, so give the supplier the exact figure.
- What is coverage width versus panel width?
- A ribbed panel is wider than the strip it covers because its edge rib laps over the next panel's edge. A standard 36-inch-coverage panel (PBR, R-panel, 5-rib and similar) is about 38 to 39 inches wide overall; 24-inch corrugated is about 26 inches wide. Always count with the coverage width, which the data sheet states.
- How much waste for a metal roof?
- Almost none on a plain gable or shed roof, because panels are cut to length; 2 to 3 percent covers a damaged panel and a cut on the last one. Hips, valleys and dormers are different: every panel that meets them is cut on the diagonal, and the offcuts rarely fit elsewhere, so allow 10 to 15 percent on those sections. The cut list calculator handles several sections.
- Can metal panels be too long?
- Over about 40 feet they are hard to ship on a standard trailer and awkward to lift, and they expand noticeably in the sun (steel grows about ¾ inch per 100 feet per 100 °F). Suppliers usually split long slopes into two panels end-lapped 6 inches over a purlin, or roll-form on site. The calculator notes it when a panel passes 40 feet.
- How many square feet does a metal panel cover?
- Coverage width times length: a 36-inch panel 18 feet 1 inch long covers 3 × 18.08 = 54 square feet. Twenty-nine of them cover about 1,571 square feet, 15.7 squares, which is the figure the screws calculator uses for the fastener count.