Metal, tile & flat
Metal roof panel maths: coverage width, laps and cut lists
Metal panels are counted along the eave and sized up the slope. Get coverage width and panel length right, and the rest of the order follows.
Two directions, two numbers
A metal roof is a row of long panels laid side by side along the eave, each running from the eave up to the ridge. That gives the order two independent numbers:
- How many panels: the eave length divided by the width each panel covers.
- How long each panel is: the slope length plus the overhang at the eave.
Shingle roofs mix the two into an area; metal roofs keep them apart, which is why the order is a list of lengths rather than a number of squares.
Coverage width, not sheet width
Exposed-fastener panels overlap at their edges: the last rib of one sits over the first rib of the next. A panel sold as “36-inch” is about 38 to 39 inches wide overall and covers 36 inches of eave; 24-inch corrugated is about 26 inches wide. The data sheet calls the useful figure coverage or net coverage, and it is the only width to count with. Standing seam is different: its edges stand up into the seam instead of lying over each other, so the seam spacing is the coverage.
On a 40-foot eave with 36-inch coverage, 480 ÷ 36 = 13.3, so 14 panels, the last cut to 12 inches. A 12-inch last panel is easy to blow off and looks wrong; shifting the start so the two end panels are each about 24 inches wide is the installer’s fix.
Panel length
The run here is measured from the ridge centreline to the drip edge in plan, overhang included; the eave overhang is how far the panel extends past the drip edge, usually 1 to 2 inches, or up to 3 into a gutter.
A 40-foot gable roof at 6/12
Eave 40 ft per side, run 16 ft, 36-inch coverage, 2-inch overhang, 3% waste.
- Slope: 16 × 1.118 = 17 ft 11 in; panel: 18 ft 1 in
- Panels: 14 per side, 28 for two planes, 29 with waste
- Area: 29 × 3 × 18.08 = 1,571 sq ft
The metal roof panel calculator gives exactly these figures.
Long slopes and end laps
Panels can be made very long, but shipping, handling and thermal movement set practical limits. Over about 40 feet, suppliers either roll-form on site or split the slope into two panels end-lapped 6 inches over a purlin with sealant between them. Steel grows roughly ¾ inch per 100 feet for a 100 °F temperature change, which is why long panels need fasteners or clips that let them move.
Small sheds hit the opposite problem: stock sheets in 8, 10 and 12 foot lengths rarely match the slope, and the choice is between offcut and laps. The corrugated shed roof calculator stacks sheets with end laps when the slope is longer than one sheet.
Several sections: the cut list
Most roofs are not one rectangle. A main roof, a porch at a different pitch and two dormer faces each need their own panel length. The way to order them is a cut list: one line per length, with the count.
| Section | Eave | Run | Pitch | Panel length | Panels |
|---|---|---|---|---|---|
| Main roof, front | 40 ft | 16 ft | 6/12 | 18 ft 1 in | 14 |
| Main roof, back | 40 ft | 16 ft | 6/12 | 18 ft 1 in | 14 |
| Porch, left | 20 ft | 8 ft | 4/12 | 8 ft 8 in | 7 |
| Porch, right | 20 ft | 8 ft | 4/12 | 8 ft 8 in | 7 |
Ordered cut to length, that is 28 × 18 ft 1 in and 14 × 8 ft 8 in. Ordered in stock lengths it becomes 28 × 20 ft and 14 × 10 ft, with 72 feet of offcut. The cut list calculator builds the list for up to four sections either way.
Waste where metal meets hips and valleys
Plain gables waste almost nothing: 2 or 3 percent for a damaged panel. Every panel that meets a hip or a valley is cut on the diagonal, and the triangle that comes off rarely fits anywhere else. Allow 10 to 15 percent on those sections. It is one of the reasons metal costs more on a hip roof than its area suggests.
Trim and fasteners finish the count
The panels are about half of the material order. The rest is trim (ridge cap, eave trim, rake trim, valley and wall flashing, each in 10-foot sticks that lap 2 to 6 inches), closure strips cut to the panel profile at the eave and ridge, butyl tape and sealant, and the screws: on exposed-fastener panels, a screw beside each major rib at every purlin row, plus stitch screws along the side laps. The trim calculator and the screws calculator take their lengths and areas from the same measurements.
Measuring for metal
The measurements are the ones in measuring a roof from the ground, with more care at the edges: panel lengths are ordered to the inch, so measure each section’s run and confirm the pitch, which roof pitch explained covers. Measure eaves from the outside of the rake trim to the outside of the rake trim. For the choice between exposed-fastener and standing seam panels, see standing seam vs exposed fastener.