How do you estimate roof replacement, and are estimate calculators accurate?
Short answer. A roof estimate starts with the roof's surface area in squares: the footprint plus overhangs, times a pitch multiplier, plus 10 to 15 percent for waste. Online calculators can give a ballpark, but they cannot see your decking, layers, flashing, or ventilation, which is where real quotes differ.
Do the math yourself in four steps
- Footprint: measure the outside of the house and garage, then add the overhangs, often 1 to 2 feet on each side.
- Pitch multiplier: multiply by the factor for your pitch: about 1.054 for 4/12, 1.118 for 6/12, 1.202 for 8/12, 1.302 for 10/12, and 1.414 for 12/12.
- Waste: add about 10 percent for a simple gable roof and 15 percent or more for hips, valleys, and dormers.
- Squares: divide by 100. One square is 100 square feet of roof.
Example: a footprint of 2,000 square feet including overhangs, at 6/12, is about 2,236 square feet of roof. Add 10 percent waste and you are ordering roughly 25 squares of shingles.
What calculators miss
- Decking: rotten or delaminated sheets only show up during tear-off or a careful inspection.
- Layers: a hidden second layer doubles tear-off labor and disposal.
- Flashing and penetrations: chimneys, skylights, and wall intersections add hours no formula sees.
- Ventilation: correcting poor attic venting is a real cost and a warranty requirement.
- Access and steepness: steep slopes and tight lots add labor.
Use your squares to sanity-check quotes, not to replace them. Then compare against realistic local ranges in roof replacement cost in Georgia, and see how roofers measure a roof to understand where professional numbers come from.