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A rough guess is fine — the contractor will measure.

• Footprint: measure your building's length × width (or use the house footprint from your property records), and add a little for the overhangs.
• Adjust for pitch (rough multipliers): flat/low slope ≈ 1.0–1.1 · medium ≈ 1.15–1.25 · steep ≈ 1.3–1.4.
• Or measure from above: free satellite tools like Google Earth's measure tool can trace the roof outline.
• Roofers often talk in “squares”: 1 roofing square = 100 sq ft.

Example: 40 ft × 30 ft footprint = 1,200 sq ft; with a medium pitch × ~1.2 ≈ 1,440 sq ft.

Approximate only — the installer will measure.

• 1-car: roughly 200–300 sq ft
• 2-car: roughly 400–600 sq ft
• 3-car: roughly 600–900 sq ft

Or measure length × width of the floor.