Understanding your result
The headline is the screws to buy, split in the subtitle into panel screws, stitch screws and the trim allowance, with the bags. The stats give the screws per panel, the screws per square, the stitch screws and the bags. The breakdown shows how many fastening rows a panel has, how many screws each row takes, the stitch screws per lap and the totals.
The number to compare with other estimates is the screws per square. A supplier’s rule of thumb of “80 per square” assumes a particular panel, purlin spacing and pattern; if yours comes out at 129, the difference is in the edge rows and the stitch screws, which the table shows. The per panel figure is what you count against when the roof is half done.
The panel count and length come from the metal roof panel calculator (or the shed roof calculator for a small corrugated roof, which counts its own screws), and the trim screws are in the trim calculator; the 10 percent here is a cushion for them.
How we calculate this
The pattern is the non-obvious part. Manufacturers’ installation guides fasten ribbed panels on every purlin with a screw beside each major rib (five across a 36-inch five-rib panel), and at the eave and the ridge, where uplift is highest, with a screw at every rib (nine). The calculator treats the first and last rows as edge rows and every row between as a field row; a panel too short to have field rows still gets its edge rows. Purlin spacing sets how many rows there are: 24 inches is standard for 29- and 26-gauge panels on a residential roof; high-wind zones use 12 to 18. The panel maths guide draws the pattern.
Stitch screws depend on the lap length, which is the panel length, and the number of laps, which is one fewer than the number of panels on each plane; the calculator uses panels minus one across the whole roof, which is within a lap or two of the exact figure.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Screws per panel per field row | 5 | Manufacturers’ installation guides for 36-in five-rib panels (PBR, R-panel): one each side of each major rib |
| Screws per panel at eave and ridge rows | 9 | Same guides: every rib at the eave and ridge |
| Fastening row spacing | 24 in | Typical purlin spacing for 29- and 26-gauge residential panels; the panel’s span table governs |
| Stitch screw spacing | 18 in | Manufacturers’ guides: 12 to 24 in along side laps |
| Panel length and coverage | 18 ft, 36 in | The site’s example roof |
| Bag size | 250 | Common packaging (250 and 500) |
| Extra for trim and flashing | 10% | Estimating practice |
Assumptions last reviewed October 7, 2026.
The calculator does not count the clips and clip screws of a standing seam roof (the standing seam calculator does), the trim screws exactly (the trim calculator does), or the longer screws some makers specify at the eave. It does not know your wind zone; the manufacturer’s uplift table may require more screws per row at edges and corners. The standing seam versus exposed fastener guide covers what the screws mean for the roof’s life.
Two worked examples
A gable roof in 36-inch panels
15.7 squares of 36-inch panels 18 feet long, purlins at 24 inches, 5 screws per field row, 9 at the edge rows, stitch screws every 18 inches, 10% extra, bags of 250.
- Panels: 1,570 ÷ (3 ft × 18 ft) = 29
- Rows per panel: floor(216 ÷ 24) + 1 = 10 (8 field rows plus eave and ridge)
- Screws per panel: 8 × 5 + 2 × 9 = 58; panel screws: 29 × 58 = 1,682
- Stitch per lap: 216 ÷ 18 = 12; stitch screws: 28 × 12 = 336
- Total: (1,682 + 336) × 1.10 = 2,220 screws, 9 bags of 250; 129 per square
Nine bags, of which the stitch screws are one and a half: order them as a separate, shorter screw.
A lean-to in corrugated, counted by panels
Twelve 24-inch corrugated panels 12 feet long, purlins at 24 inches, 4 screws per field row, 6 at the edge rows, stitch every 18 inches, no extra.
- Rows per panel: floor(144 ÷ 24) + 1 = 7 (5 field rows)
- Screws per panel: 5 × 4 + 2 × 6 = 32; panel screws: 12 × 32 = 384
- Stitch per lap: 144 ÷ 18 = 8; stitch screws: 11 × 8 = 88
- Total: 472 screws, 2 bags; 164 per square
Corrugated sheet runs higher per square than ribbed panels because the panels are narrower (more laps per square) and the crests are closer together.
Where to find your inputs
Squares or panels. From the panel calculator: the squares of panel area, or the panel count and length. Counting by panels is more exact.
Row spacing. The purlin spacing on the plans or on the roof, or the batten spacing over an old roof; 24 inches unless the span table says otherwise.
Screws per row. From the panel maker’s installation sheet: the diagram shows a screw beside each rib. Count them across one panel for the field rows and for the eave row.
Stitch spacing. Also on the installation sheet: 12 inches in high wind, 18 to 24 otherwise.
Common mistakes
- Using a flat number per square. The pattern and the purlin spacing change the count by half. Build it from the panel.
- Forgetting the edge rows. The eave and ridge rows take nearly twice as many screws as a field row.
- One screw length for everything. Panel screws go into the purlin; stitch screws go only through two sheets of metal. Buy both.
- Screws through the rib. On ribbed panels the screw sits in the flat beside the rib; on corrugated it sits on the crest. Follow the maker’s sheet.
- Over-driving. A crushed washer leaks. Set the clutch and check the first dozen.
- No spare. A gun misfires and a screw strips now and then. Keep the 10 percent.
Questions people ask
- How many screws per square of metal roofing?
- About 80 to 130 for ribbed panels on purlins at 24 inches, depending on the pattern. The calculator's default roof, 36-inch panels 18 feet long with five screws per panel at each 24-inch row, nine at the eave and ridge rows and stitch screws every 18 inches along the laps, comes to 129 per square before the trim allowance. Closer purlins or high-wind patterns push it past 150.
- Where do the screws go on a ribbed metal panel?
- In the flat of the panel beside each major rib, not through the rib, on every purlin or fastening row: five across a 36-inch PBR or R-panel in the field, and one at every rib (nine) at the eave and the ridge where the wind load is highest. Corrugated sheet is the exception: it is fastened through the crest of the corrugations with longer screws.
- What are stitch screws?
- Short screws (usually ¾ inch) with a sealing washer, driven through the side lap where one panel overlaps the next, between the purlin rows, to hold the two panels together and keep the lap closed. They go every 12 to 24 inches along each lap; 18 inches on an 18-foot panel is 12 per lap, 336 for 28 laps.
- What length of screw for metal roofing?
- The screw must go through the panel and at least 1 inch into the wood. For ribbed panels on 1× battens or plywood, 1 inch; on 2× purlins, 1½ inches; for corrugated sheet fastened on the crest, add the corrugation height, so 2 inches or more. Metal-to-metal (panels on steel purlins) uses self-drilling screws sized to the purlin thickness.
- How many screws are in a bag?
- Common bags hold 250 or 500 screws; a 1-pound box of 1-inch roofing screws holds about 90 to 110. The calculator's default roof needs 2,220 screws including 10 percent for trim, which is nine bags of 250. Buy the panel screws and the stitch screws separately; they are different lengths.
- Why do metal roof screws leak?
- Over-driving crushes the rubber washer and splits it; under-driving leaves it loose; driving at an angle seats it on one side only. Set the drill clutch so the washer just flattens, drive square, and expect the washers to age: a fastener check at 10 to 15 years, replacing any with cracked washers, is part of owning an exposed-fastener roof.