How the metal roofing calculator works
Metal sheets are ordered as a number of panels, each cut to a length, rather than as an area. Panels run from eave to ridge in one piece whenever transport allows, so what matters is how many fit side by side along the roof and how long each one must be. The count uses the cover width, the strip each sheet adds once it overlaps its neighbour by a rib.
Panels per slope = roof length ÷ cover width (rounded up) · Cut length = slope length + eave overhangFasteners are estimated at four screws per panel on every purlin line, including the first at the eave and the last at the ridge, which is typical for exposed-fastener profiles about 1 m or 36 in wide. The ridge cap is counted in pieces with a lap already deducted: 2 m pieces lapped 10 cm, or 10 ft 6 in pieces lapped 6 in. On a lean-to, the same line gives the top flashing against the wall.
Worked example
A garage roof is 10 m long with two slopes, each 5 m from eave to ridge. Trapezoidal sheets with a 1.0 m cover width need 10 per slope, 20 in total, each cut to 5.05 m to leave 5 cm hanging into the gutter. With purlins every 60 cm, a 5.05 m sheet crosses 10 purlin lines, so 20 × 10 × 4 = 800 screws: four bags of 200 or so, plus spares for the trim. The ridge takes six pieces. The roof surface is 100 m² and the sheets ordered cover 101 m², because the overhang adds a strip along both eaves.
Common panel profiles
| Profile | Cover width | Fixing | Usual minimum pitch |
|---|---|---|---|
| US exposed-fastener rib panel (R/PBR, ag panel) | 36 in (0.91 m) | Screws through the panel | about 3:12 (14°) |
| Corrugated 2.5 in | about 24–25 in | Screws on the crown or in the valley, per maker | about 3:12 |
| European trapezoidal (35–45 mm ribs) | 1.0–1.1 m | Screws through the panel | about 5–10°, per maker |
| Standing seam | 12–18 in (30–45 cm) | Hidden clips | often 3° or lower for mechanically seamed |
| Metal tile-effect sheet | about 1.1 m | Screws in the step | about 12–14° |
Pitch limits differ between manufacturers; read the installation guide for your profile, since a panel laid too flat can let wind-driven rain back up through the laps.
Thickness and structure
US panels are sold by gauge: 29 gauge (about 0.36 mm) for sheds and farm buildings, 26 or 24 gauge for homes and heavier snow. In Europe, steel sheets are typically 0.5–0.7 mm thick. Purlin spacing depends on the profile, the thickness and your snow and wind loads, so take it from the manufacturer's span table; for new roof framing, extra snow load or any doubt about the structure, ask an engineer or follow the local building code.
Mistakes that cost money
- Dividing by the full sheet width. A sheet 1.1 m wide may cover only 1.0 m after the side lap, which adds one sheet in every ten.
- Over-driving screws. The rubber washer should just bulge at the edge; crushed washers leak and loose ones rattle.
- Skipping condensation control. Bare steel over a heated or humid space drips underneath; use a breathable underlay or anti-condensation backing.
- Leaving swarf on the roof. Drill and cut filings rust within days and stain the coating; sweep them off the same day and cut with snips, not an abrasive disc.
Frequently asked questions
How many metal roof panels do I need for a 30 x 40 roof?
Along a 40 ft eave, 36 in cover panels take 40 ÷ 3 = 13.3, so 14 per slope, 28 for a gable roof, each cut to the slope length plus the eave overhang.
How many screws per sheet of metal roofing?
For a 36 in exposed-fastener panel, allow about 4 screws at every purlin, roughly 80 screws per 100 ft² when purlins are 24 in apart.
How long should metal roof panels overhang?
Let panels run 1½ to 3 in (4–7 cm) past the eave so water drops into the gutter rather than running back under the sheet.
What is the cover width of a metal roof panel?
It is the width each panel adds after overlapping its neighbour: 36 in for most US rib panels and about 1.0–1.1 m for European trapezoidal sheets. Use it, not the overall sheet width.
Can I put metal roofing on a low-pitch roof?
Exposed-fastener panels generally need about 3:12 or more; mechanically seamed standing seam can go much flatter. Check the minimum pitch in the manufacturer's installation guide.
Last reviewed: 2026-09-27 · How we calculate