Slope length = (half span + overhang) ÷ cos(pitch). A 12 × 8 m building at 15° with 0.3 m overhangs has 4.45 m slopes. With 1.0 m cover sheets 3 m long and 150 mm end laps, each slope needs 12 sheets across in 2 courses (48 sheets) plus 10 purlins of 12 m and about 360 fasteners.
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Purlins per slope = ⌈slope ÷ 1.20⌉ + 1; fasteners = sheets across × purlins × 3 × 2 slopes
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Formulas
Slope length = (span ÷ 2 + overhang) ÷ cos(pitch)Sheets across = ⌈ridge length ÷ cover width⌉Courses = ⌈(slope − end lap) ÷ (sheet length − end lap)⌉Purlins/slope = ⌈slope ÷ spacing⌉ + 1Fasteners = sheets across × purlins × fixings per crossing × 2
Worked example
12 m long, 8 m span, 15°, 0.3 m overhang, 1.0 m cover, 3 m sheets, 150 mm lap, purlins at 1.2 m:
Slope = 4.3 ÷ cos 15° = 4.45 m; rise 1.07 m
Sheets = 12 across × 2 courses × 2 slopes = 48
Purlins = 10 × 12 m; ridge cap 12.6 m; fasteners 360
Frequently Asked Questions
How many roof sheets do I need?
Sheets across each slope = ridge length ÷ cover width; courses up the slope = slope length less one lap, divided by sheet length less one lap. Multiply by 2 slopes for a gable roof.
What is the minimum pitch for metal roof sheets?
It depends on the profile, often around 5–10° for trapezoidal sheets with sealed laps, and lower for standing seam. Check the manufacturer's minimum.
How far apart are purlins?
Usually 1.0–1.8 m depending on the sheet profile, gauge and wind or snow load. The sheet maker's span tables give the limit.
