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How to calculate roof sheet or sandwich panel quantity
Last reviewed:
Short answer
Divide the roof length (parallel to the purlins) by the panel's effective cover width to get the number of sheets per slope; sheet length = slope length from ridge to eave + overhang. Then sheets × length × cover width gives the area — add 5 – 10% for waste and laps.
Steps
- Measure the building length along the purlins (the direction sheets are laid side by side).
- Find the panel's effective cover width — not its full width — usually 1000 mm for sandwich panels and about 1000 – 1100 mm for corrugated sheet depending on the profile.
- Sheets per slope = building length ÷ cover width, rounded up.
- Sheet length = horizontal run from ridge to eave ÷ cos(pitch) + overhang (usually 150 – 300 mm).
- If the length is too long to transport or handle, split it into two sheets with a 150 – 200 mm end lap.
Worked example
A store 20 m long and 12 m wide with a two-slope gable roof, 10° pitch, 0.2 m overhang, using sandwich panels with a 1 m cover width.
| Item | Calculation | Result |
|---|---|---|
| Horizontal run per slope | 12 ÷ 2 | 6 m |
| Sheet length | 6 ÷ cos 10° + 0.2 | about 6.3 m |
| Sheets per slope | 20 ÷ 1 | 20 sheets |
| Total sheets | 20 × 2 | 40 sheets × 6.3 m |
| Area | 40 × 6.3 × 1 | 252 m² |
| With 5% allowance | 252 × 1.05 | about 265 m² |
Common mistakes
- Using the full sheet width instead of the cover width — the order comes up short.
- Forgetting the pitch: the sloped area is larger than the plan area.
- Forgetting accessories: ridge caps, flashings and sealed screws.
- A pitch below the manufacturer's minimum (often around 5°) causes leaks.
Send roof dimensions or drawings with your quotation request and the quantity will be reviewed with you before supply.
FAQ
Should I order by m² or by number of sheets?
Ordering the number of sheets and each length is most accurate, since sheets are cut to length — pricing is usually per m².
How many screws per m²?
It depends on purlin spacing and wind load — roughly 4 to 6 screws per m² on typical roofs; follow the manufacturer's instructions.
General guidance — not an offer or commitment by BUILDORA.