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 roofing example
Roofing Calculator example: 54.8 ft × 27.6 ft × 1.12 × 1.1 = 1,863.38 ft², or 18.63 roofing squares.
Read squares and sloped area
Use roofing squares for broad comparison, then prepare separate counts for field material, starter, ridge, underlayment, flashing, ventilation, and fasteners.
Geometry and accessories outside the total
Eaves, rakes, overbuilds, dormers, valleys, waste pattern, tear-off layers, and roof access change material and labor quantities.
Questions specific to this calculation
When should I use roof area instead of roof pitch?
Measure the horizontal plan footprint represented by one roof section and use a slope factor that matches that section; add hips, valleys, and dissimilar slopes separately.