Roof Area Calculator
The surface area of a gable, hip, shed or gambrel roof from the building’s footprint, overhang and pitch, in square feet or metres and roofing squares.
Result
1,408.72 ft²
Surface area before waste: add your own allowance when ordering materials.
- Roofing squares
- 15
- Slope factor
- 1.118
- Footprint with the overhang
- 1,260 ft² (117.06 m²)
- Pitch angle
- 26.57 °
- Ridge length
- 42 ft (12.8 m)
https://calcgallery.com/roofing/roof-area-calculator/
Results are estimates for planning only. Actual needs vary with site conditions, products and workmanship. Always confirm quantities with your supplier and follow local building codes and manufacturer instructions. Read the full disclaimer
How to use the Roof Area Calculator
- Enter the building's length and width, and how far the eaves overhang the walls.
- Choose the roof type and enter the pitch, as a rise in 12 or in degrees.
- Read the roof's surface area and the roofing squares, 100 square feet each, rounded up.
The slope factor
A pitch of 6 in 12 rises 6 inches for every 12 inches of level run. The sloping length along that run is, by Pythagoras,
slope factor = √(rise² + 12²) ÷ 12 = √(1 + (rise ÷ 12)²)roof area = (length + 2 × overhang) × (width + 2 × overhang) × slope factorA 12 in 12 roof is 45°, and its factor is exactly √2, 1.4142. This is the same factor the roof pitch calculator gives for a rafter.
Other roof shapes
A shed roof is a single plane: the same formula. A gambrel, the barn shape, has a steep lower slope and a shallower upper one on each side, so its area weighs the two slope factors by how much of the span each covers. The page does not address structure, loads or codes; it measures area.
With your numbers
- Roof area = footprint with overhang × √(1 + (rise ÷ 12)²)
Quick reference
Surface area before waste: add your own allowance when ordering materials.
| Job | Roof surface area |
|---|---|
| 40 × 28 ft, 6 in 12, no overhang | 1,252.2 ft² |
| 40 × 28 ft, 4 in 12 | 1,180.58 ft² |
| 40 × 28 ft, 12 in 12 | 1,583.92 ft² |
| Hip, 40 × 28 ft, 6 in 12 | 1,252.2 ft² |
| 40 × 28 ft, 12 in overhang | 1,408.72 ft² |
| Gambrel, 12 and 4 in 12 | 1,382.25 ft² |
Common questions about the Roof Area Calculator
- How do I work out the area of a roof?
- Multiply the footprint the roof covers by its slope factor. The footprint is the building plus the overhang on every side; the slope factor is how much longer a sloping line is than its level run. A 6 in 12 pitch has a factor of 1.118, so a roof over 1,120 square feet of footprint is about 1,252 square feet.
- Does a hip roof have more area than a gable roof?
- No: over the same footprint at the same pitch, they have exactly the same area. A hip roof has four planes instead of two, but every plane slopes at the same pitch and together they cover the same footprint, so the total is the footprint times the same slope factor. The shapes differ; the area does not.
- What is a roofing square?
- A hundred square feet of roof, the unit shingles and other roofing are sold by. The page rounds squares up, since you cannot buy part of one, and the area is before any waste allowance for cuts, hips and valleys.
- Should I include the overhang?
- Yes. The roof covers the eaves as well as the walls. A 12-inch overhang all round turns a 40 by 28 foot house into a 42 by 30 foot roof footprint, 12.5% more roof.