How do you use a new roof estimate calculator, and how accurate is it?
Short answer. A roof estimate calculator multiplies your roof's area, adjusted for pitch, by an average price for the material you pick, so the result is only as good as the area you feed it and the price it assumes. You can do the same math yourself: footprint plus overhangs, times a pitch factor, divided by 100 for squares, plus 10 to 15 percent waste, then place the result in your local price range instead of using a national per-square figure.
The math, step by step
- Footprint. Measure the house and garage outline and add the roof overhangs on each side.
- Pitch factor. Multiply by the factor for your slope: about 1.05 for 4 in 12, 1.12 for 6 in 12, 1.20 for 8 in 12, 1.30 for 10 in 12, and 1.41 for 12 in 12.
- Squares. Divide by 100.
- Waste. Add about 10 percent for simple gable roofs and 15 percent or more for roofs with many hips and valleys.
Worked example
A house and garage with a 1,800 square foot footprint including overhangs, on a 6 in 12 roof: 1,800 times 1.12 is about 2,016 square feet, or 20.2 squares. With 12 percent waste, plan on roughly 22.5 squares. A moderately pitched, single-layer roof of that size usually falls within North Georgia's typical range of roughly $8,000 to $16,000.
What no calculator can see
- How many layers are on the roof now.
- Soft, rotted or plank decking.
- Flashing condition at chimneys, walls and skylights.
- Ventilation that needs correcting.
- Access, height and safety setup.
- Storm damage that could make the replacement an insurance claim.
Use the calculator to check whether a written quote is in the right neighborhood, not to set your budget. See how roofers measure a roof and roof replacement cost in Georgia.