How do you calculate roof square footage for a cost calculator?
Short answer. Measure each roof plane's horizontal length and run, multiply the run by your pitch factor to get the slope length, and multiply length by slope length for each plane, then add the planes together. A simple 40 by 30 foot gable house with 1 foot overhangs and a 6/12 pitch works out to about 1,500 square feet, or 15 squares.
Step by step
- Sketch the roof from above and label each plane.
- Measure the eave length of each plane, including overhangs.
- Measure the run, the horizontal distance from the eave edge to the ridge.
- Multiply the run by the pitch factor to get the slope length. Factors: 4/12 is 1.054, 6/12 is 1.118, 8/12 is 1.202, 10/12 is 1.302, 12/12 is 1.414.
- Find each plane's area. Rectangles are length times slope length. Triangular hip ends are half the base times slope length. Trapezoids are the average of the two parallel edges times slope length.
- Add the planes and divide by 100 for squares.
Worked example
A 40 by 30 foot house with a gable roof and 1 foot overhangs on every side measures 42 by 32 feet. Each roof plane is 42 feet long with a run of 16 feet. At 6/12, the slope length is 16 times 1.118, about 17.9 feet. One plane is 42 times 17.9, about 751 square feet. Two planes total about 1,503 square feet, or 15 squares. Add about 10 percent waste for ordering.
Where homeowners go wrong
- Forgetting overhangs.
- Using floor area instead of footprint.
- Skipping porch and garage roofs.
- Guessing pitch instead of measuring it.
Once you have the area, use North Georgia whole roof ranges rather than a per square foot price; see roof replacement cost in Georgia and the professional version of this math in how roofers measure a roof.