Understanding your result
The headline is the sheets to buy, with the sheets across, the sheets up, the number of slopes and the screws beneath it. The stats show the sheets across, the slope length, the screws and the ridge cap or wall flashing pieces. The breakdown walks through the eave with its overhangs, the slope, the sheets across and up, the purlin rows, the screws, the ridge or wall pieces and the closures.
The row to check is sheets up each slope. One sheet up is the normal shed; two or three means end laps, each of which needs a purlin under it and sealant on a low pitch, and it is often cheaper to buy a longer sheet than to lap two. The slope length is the sheet length to ask for if the supplier cuts to length.
For a larger roof in wider ribbed panels, the metal roof panel calculator does the same count by coverage width, and the screws calculator handles patterns other than the corrugated default. The trim calculator adds rake trim and eave trim if the shed gets them.
How we calculate this
The slope on a lean-to includes the overhang at both the low eave and the high end against the wall; on a gable it includes the eave overhang only, since the sheet stops under the ridge cap. The pitch factor converts the plan run to the sloped length; the pitch calculator gives it for a pitch measured in degrees or from a level.
Sheets stacked up the slope overlap by the end lap, so each extra sheet adds its length less the lap; the calculator reports how far the stack reaches so you can see the spare. Screws are counted from the purlin rows: every other crest in the field rows, every crest at the eave and ridge, which the panel maths guide illustrates.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Sheet | 26 in × 10 ft corrugated, 24 in coverage | The common US shed sheet (also 8 and 12 ft); 29 gauge |
| Pitch | 4/12 | A typical shed pitch; 3/12 is the minimum for standard laps |
| Overhang | 6 in | Common on sheds; 12 in on houses |
| End lap | 6 in | Manufacturers’ guides for pitches of 4/12 and up; 8 to 12 in below |
| Purlin spacing | 24 in | Span tables for 29-gauge corrugated on sheds |
| Screws per sheet per purlin | 5 (every other crest); 9 at the eave and ridge (every crest) | Manufacturers’ fastening patterns for 2½-in corrugated |
| Closures | 1 per sheet at each eave and ridge | Manufacturers’ guides |
| Waste | 5% | Estimating practice |
Assumptions last reviewed October 7, 2026.
The calculator does not count the purlins themselves, the rake trim or eave trim (sheds often have none), gutters or underlayment, and it uses one pitch for both slopes of a gable. For a shed with a loft or a steeper roof, the pitch guide covers what the pitch does to the sheet length and the laps.
Two worked examples
A 12 by 10 foot gable shed
Length 12 feet, width 10 feet, 4/12, 6-inch overhangs, 26 × 10 foot sheets, purlins at 24 inches, 5 screws per purlin, 5% waste.
- Eave: 12 + 1 = 13 ft; sheets across: 156 ÷ 24 = 6.5, so 7
- Run: 5 + 0.5 = 5.5 ft; slope: 5.5 × 1.054 = 5 ft 10 in, one 10-ft sheet up
- Sheets: 7 × 1 × 2 = 14; with waste 15 sheets
- Purlin rows: 3; screws: 14 × (1 × 5 + 2 × 9) × 1.05 = 338
- Ridge cap: 2 pieces; closures: 28
Fifteen 10-foot sheets, with 4 feet of each one over the ridge as offcut: 8-foot sheets would do this shed with 2 feet to spare and a third less waste.
A 20 by 16 foot lean-to at 2/12 in 8-foot sheets
Lean-to, length 20 feet, width 16 feet, 2/12, 6-inch overhangs at both ends, 26 × 8 foot sheets.
- Eave: 21 ft; sheets across: 252 ÷ 24 = 10.5, so 11
- Run: 16 + 1 = 17 ft; slope: 17 × 1.014 = 17 ft 3 in; sheets up: 1 + round up ((17.23 − 8) ÷ 7.5) = 3, covering 23 ft
- Sheets: 11 × 3 = 33; with waste 35
- Purlin rows: 9; screws: 612; wall flashing: 3 pieces; closures: 22
Three sheets up with two end laps on a 2/12 roof means sealant in every lap. A single 18-foot sheet cut to length would remove both laps and 6 feet of overlap from the order.
Where to find your inputs
Length and width. Outside of the shed walls: length along the ridge (or along the low eave of a lean-to), width across the slope.
Pitch. From the plans or a level on a rafter; most sheds are 3/12 to 6/12.
Sheet size. From the supplier’s shelf tag: width, coverage and length. The coverage is the width less one corrugation.
Purlin spacing. From the sheet maker’s span table for the gauge: 24 inches for 29-gauge on a shed.
Common mistakes
- Counting by the sheet width. A 26-inch sheet covers 24 inches. Count by the coverage.
- Forgetting the overhang in the eave. Six inches each end adds half a sheet across on a small roof.
- Stacking sheets without a purlin under the lap. Every end lap must land on a purlin and be screwed through both sheets.
- Short screws. Corrugated is fastened on the crest; the screw needs the crest height plus 1 inch into the wood.
- A flat lean-to. Below 3/12 the laps need sealant; below 2/12 the roof holds water. Steepen it if you can.
- No closures. Open corrugations at the eave and ridge let in rain, wind and wasps. One per sheet, both ends.
Questions people ask
- How many corrugated sheets do I need for a shed roof?
- Count across the eave and up the slope for each roof plane. A 12 by 10 foot gable shed with 6-inch overhangs has 13-foot eaves: 13 × 12 ÷ 24 inches of coverage = 6.5, so 7 sheets across each slope; the slope is 5 feet 10 inches, so one 10-foot sheet up. That is 7 × 1 × 2 = 14 sheets, 15 with 5 percent waste.
- What size are corrugated roofing sheets?
- The common US sheet is 26 inches wide and covers 24 inches after one corrugation of side lap, in 8, 10 and 12 foot lengths; 29 gauge is the usual thickness for sheds. Larger ribbed panels cover 36 inches and are cut to length. In metric markets sheets are 762 or 1,000 mm wide; enter yours under "other sheet size".
- How much overlap between sheets up the slope?
- Six inches of end lap on pitches of 4/12 and steeper, 8 to 12 inches on shallower roofs, always over a purlin so both sheets are screwed through the lap. Side laps are one corrugation (one and a half in windy sites). The calculator stacks sheets up the slope with the end lap you set when the slope is longer than one sheet.
- How many screws per corrugated sheet?
- On every purlin, a screw on every other crest across the sheet (about 5 on a 26-inch sheet), and on every crest at the eave and the ridge (about 9). A 10-foot sheet on three purlin rows takes 5 + 9 + 9 = 23 screws; the default 12 by 10 shed takes about 340 with 5 percent spare. Corrugated is fastened on the crest, so the screws are longer than for ribbed panels.
- What pitch does a shed roof need for corrugated metal?
- Three in twelve or steeper for standard laps. Between 2/12 and 3/12 the side laps need sealant and the end laps grow to 8 to 12 inches; below 2/12 water sits in the corrugations and corrugated sheet is not recommended. A lean-to against a wall often ends up shallow; give it 3/12 if the headroom allows.
- What goes at the ridge and the eaves of a corrugated roof?
- Foam closure strips cut to the corrugation profile under the sheets at the eave and on top of them at the ridge, one per sheet, then a ridge cap over the closures on a gable roof, or wall (headwall) flashing tucked under the siding on a lean-to. The default gable shed takes 28 closures and 2 pieces of 10-foot ridge cap.