How do you calculate roof area?
A pitched roof covers more than the footprint beneath it, so multiply the plan footprint by the slope factor — √(rise² + 12²) ⁄ 12 — to get the actual surface area. A 6:12 roof has a factor of about 1.118, so a 1,200 sq ft footprint becomes ~1,342 sq ft of roof. Divide the result by 100 to get roofing squares.
Source: Slope-factor geometry · Verify against your AHJ and the adopted code edition.
The footprint under a roof isn’t the roof’s area — the pitch stretches it. Scale the footprint up by the slope factor:
slope factor = √(rise² + 12²) ⁄ 12
roof area = footprint × slope factor
For a 6:12 roof: √(6² + 12²) ⁄ 12 = √180 ⁄ 12 ≈ 1.118. A 1,200 sq ft footprint becomes ≈ 1,342 sq ft of roof, or ~13.4 squares (÷ 100).
The steeper the roof, the bigger the factor: 4:12 ≈ 1.054, 6:12 ≈ 1.118, 9:12 ≈ 1.25, 12:12 ≈ 1.414.
This covers a simple gable or hip over a rectangle. Dormers, valleys, and overhangs beyond the footprint add area — measure them or add margin — and order 5–15% extra for waste.
Work it out: the roofing calculator takes a footprint and pitch, applies the slope factor, and returns the roof area, squares, and a shingle-bundle estimate with waste.