Roofing Sheet Calculator
Calculate how many roofing sheets you need for a shed or roof, allowing for overlap, plus purlins and cost.
Inputs
Effective cover after side overlap
Formula
- Sheets across = roof length ÷ effective sheet width
- Rows = roof width ÷ (sheet length − end overlap)
- Total sheets = sheets across × rows
Roofing sheets overlap on all sides, so the covered area is always less than the sheet's actual size. Work out how many sheets fit across the roof using the effective cover width, then how many rows are needed up the slope after subtracting the end overlap. Round both up to whole sheets.
Step-by-step calculation
- 1
Sheets across
length ÷ cover width
9
- 2
Rows
width ÷ (sheet length − overlap)
3
- 3
Total
across × rows
27 sheets
Worked example
- 1A 30 ft × 20 ft roof with 3.5 ft cover sheets → 30 ÷ 3.5 = 9 sheets across.
- 210 ft sheets with 6 in overlap cover 9.5 ft → 20 ÷ 9.5 = 3 rows.
- 3Total = 9 × 3 = 27 sheets.
Tips
- Lay sheets against the prevailing wind so rain is not driven into the side laps.
- Use J-hooks with EPDM washers at every second corrugation on the purlin.
Frequently asked questions
How much overlap do roofing sheets need?
One and a half corrugations at the sides and 150–200 mm at the ends on low-slope roofs.
What slope should a sheet roof have?
At least 1 in 12 (about 5°); 1 in 6 sheds water far better in heavy rain.
How many purlins do I need?
One every 1.2–1.5 m along the rafter — the result shows the estimated purlin length.
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