Roofing Calculator: Roof Area and Number of Sheets
Enter the building length and width, the overhang, the roof pitch and the effective size of one sheet. You get the roof surface area and the number of sheets to order.
Estimates only. Check important results before you rely on them.
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From related categoriesHow to use this calculator
- Enter the building length and width and the overhang.
- Enter the roof pitch in degrees.
- Enter the effective width and length of one sheet after overlap.
- Read the roof area and the sheets needed.
The formula
Worked example
A 30 ft × 20 ft building with a 2 ft overhang has a plan area of 75.8 m². At a 25° pitch the slope factor is 1.103, so the roof area is 83.6 m² (900 sq ft). With 10% wastage and sheets of 1,050 × 2,400 mm effective size you need 37 sheets.
Understanding your result
A steeper roof has more surface, so the sheets increase with pitch. Use the effective (covered) size of the sheet, not its full size, as overlaps are lost.
When to use it
- Order roofing sheets or tiles.
- Compare a flat and a pitched roof.
- Estimate the paint or waterproofing area.
Assumptions and limits
These are planning estimates. Real use depends on material quality, site conditions and workmanship. Confirm quantities with your engineer or contractor before ordering. The tool treats the roof as a simple pitched surface. Hips, valleys, ridge caps, gutters and purlins are not included.
Frequently asked questions
How do I calculate roof area from the floor area?
Divide the plan area by the cosine of the roof pitch angle.
What is the effective size of a roofing sheet?
The area it covers after the overlap with the next sheet is taken off.
What roof pitch is common?
Between 15° and 35° for many sheet and tile roofs. Check the manufacturer's minimum.
How much wastage should I allow?
About 10%, more for complex roofs with hips and valleys.
Does overhang matter?
Yes. It adds to the plan area and so to the sheets.