How do you calculate roof square footage with a roof pitch calculator?
Short answer. Measure the roof's footprint including overhangs, multiply it by the pitch factor for your slope, and divide by 100 to get squares. For example, a 1,344 square foot footprint at 6/12 times 1.118 is about 1,503 square feet, or 15 squares, before adding waste.
Worked example: a two-story house with a porch
- Main roof footprint: the house is 40 by 30 feet. Add 1 foot of overhang on every side: 42 by 32 = 1,344 square feet.
- Main roof pitch: 6/12, factor 1.118. 1,344 × 1.118 = 1,503 square feet.
- Front porch: 24 by 8 feet at 3/12, factor 1.031. 192 × 1.031 = 198 square feet.
- Total roof area: 1,503 + 198 = 1,701 square feet, or 17.0 squares.
- Waste: for a simple gable, about 10 percent, so plan on roughly 18.7 squares of shingles.
Where homemade numbers go wrong
- Using the house's living area. A two-story house with 2,800 square feet of living space sits on a footprint of about 1,400, so its roof is far smaller than the living area suggests.
- Forgetting overhangs. On a 40 by 30 house, a 1 foot overhang adds about 150 square feet.
- Missing the garage and porches. Measure every roofed area.
- One pitch for everything. Steep front gables and low porches need their own factors.
- Counting ridge cap and starter as area. These are ordered by linear feet, not squares.
Checking a quote
Your calculation should land within a few percent of the roofer's measured squares before waste. If it is far off, ask how they measured; aerial reports and on-roof measurements are both normal. For the cost side, see roof replacement cost in Georgia and how roofers measure a roof.