How this calculator works
Roof area = building footprint area x pitch multiplier (a standard rise:run geometric factor, e.g. 1.118 for 6:12). Purchase area = roof area x (1 + waste / 100). Squares = purchase area / 9.29 m2 (1 square = 100 sq ft). Bundles = round up(purchase area / (9.29 / bundles per square)).
Worked example
A 12 m x 9 m footprint (108 m2) at a 6:12 pitch (1.118 multiplier) gives a 120.7 m2 sloped roof area - with 10% waste, about 13 squares and 40 bundles at 3 bundles/square.
Assumptions to check
- This is a MATERIAL QUANTITY ESTIMATE ONLY - it does not model rafters, trusses, decking/sheathing, structural loads, fastening or nailing patterns, underlayment installation, or working-at-height safety. Roof design, structural sizing, and installation must come from a qualified, insured roofing contractor and local building code.
- The pitch multiplier is a simple geometric shortcut (sloped area = footprint area / cos(pitch angle)) used for material estimating, not a structural roof design calculation.
- Hips, valleys, dormers, skylights, and roof penetrations are approximated only by the waste allowance, not modeled individually - complex roofs need a higher waste allowance or a site measurement.
- Ridge caps, starter strips, and underlayment are not included - only field shingle bundles.