About this calculator
Enter the roof size, slope, sheet size and overlap.
How to use it
- Enter roof length and plan width of one side.
- Enter the slope and number of slopes.
- Enter sheet size and end overlap.
The formula
Sloped length = plan width ÷ cos(slope); sheets = columns × rows × slopes
- cover width
- sheet width after side overlap
Worked example
A 30 × 12 ft two-slope roof at 15°, 3 m sheets
- Sloped length = 3.66 ÷ cos 15° = 3.79 m.
- 10 sheets across × 2 rows × 2 slopes = 40 sheets.
What the result means
Order ridge caps and fasteners too.
Assumptions
- Simple gable or lean-to roof.
Limitations
- Hips, valleys and skylights need separate measurement.
Frequently asked questions
How do I measure roof area?
Plan area ÷ cos of the slope, for each sloped side.
What overlap do sheets need?
Usually 150 mm at the ends.
What is cover width?
The width a sheet covers after side overlap, often about 1 m.
Last reviewed on 3 October 2026. Found a mistake? Tell us.