Formula and input notes
Roof area = plan length × plan width × slope factor × (1 + waste ÷ 100); roofing squares = area ÷ 100
| Input | Practical note |
|---|---|
| Slope factor | The slope factor converts horizontal plan area to sloped surface area. Derive it from the actual pitch rather than choosing it from appearance. |
Worked gambrel roof example
Gambrel Roof Calculator example: 46.8 ft × 23.5 ft × 1.12 × 1.1 = 1,354.95 ft², or 13.55 roofing squares.
Read squares and sloped area
Use summed roofing squares for budgeting, then prepare plane-by-plane field material and transition-flashing quantities.
Geometry and accessories outside the total
Knee locations, unequal slopes, end walls, transitions, valleys, overhangs, trim, and panel or shingle layout make one factor for the whole footprint inaccurate.
Questions specific to this calculation
Can one slope factor represent both gambrel roof segments?
Break each side of a gambrel into its upper and lower roof planes and calculate their plan dimensions and slope factors separately before adding them.