How the shingle calculator works
The bundle economy
Three bundles cover one square; bundles are bundled for a reason (one carrier-lift each). Order squares × 3 with the waste factor, then add the non-field lines: ridge cap bundles (33 lin ft each), starter strip (105 lin ft each), and underlayment rolls (400 sq ft overlapped). The cart that includes all four lines is the cart that finishes the roof.
The waste factor by geometry
Simple gables cut predictably: 10% covers starter cuts and errors. Hips and valleys generate diagonal cuts from every direction: 15%. Dormers and intersections push 20%+. The waste factor also prices difficulty — the same percentage that ruins extra shingles makes the installer's day longer, which is why complex roofs cost more per square installed.
Underlayment and the ice shield
Synthetic underlayment has largely replaced felt (lighter, stronger, 10-square rolls). Cold climates add ice-and-water shield along eaves, valleys, and penetrations — a self-adhering membrane that is cheap insurance against the ice-dam leak. The underlayment line is invisible in finished photos and decisive in year ten.