Roofing Guide: Everything You Need to Estimate
To estimate roofing, multiply your home's footprint (length × width) by a pitch factor, then divide by 100 to get squares. Add 10–15% for waste. Shingles come 3 bundles per square. A 40×30 ft home with a 6:12 roof needs about 14.8 squares and costs roughly $5,900–$10,300 installed.
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Use the free Roofing Calculator to estimate your project.
How to estimate roofing
Formula: Squares = Footprint × pitch factor × (1 + waste %) ÷ 100; Bundles = squares × 3
- Measure the footprint. Use the building's length × width at ground level.
- Apply the pitch factor. A steeper roof has more surface — e.g. 6:12 multiplies area by 1.118.
- Convert to squares. Divide total roof area by 100. One square = 100 sq ft.
- Add waste and bundles. Add 10–15%, then multiply squares by 3 for shingle bundles.
Example: A 40 ft × 30 ft home = 1,200 ft² footprint × 1.118 (6:12) × 1.10 waste ≈ 1,476 ft² = 14.8 squares ≈ 45 bundles.
Common mistakes to avoid
Ignoring roof pitch
Using the footprint alone underestimates a sloped roof's true surface area. Always apply a pitch multiplier.
Skipping the waste factor
Hips, valleys, and starter/ridge courses create offcuts. Add 10% for simple roofs and 15% for cut-up roofs.
Forgetting accessories
Underlayment, drip edge, ridge cap, and flashing aren't counted in shingle bundles — budget for them separately.
Measuring from the ground
Eaves and overhangs extend past the walls. Measure the actual roof plane, not the building's footprint, when you can.
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