Once roof surface area is known (accounting for pitch — the same pitch-multiplier math used for sheathing applies here too), the roofing material takeoff breaks into several distinct line items beyond just the shingles themselves.
Shingles
Shingles are sold by the square (100 sq ft covered) and by bundle (typically 3 bundles per square for standard architectural shingles) — waste runs 10% for a simple gable roof, higher for hip roofs, steep pitches, or roofs with lots of valleys and penetrations.
Underlayment
Underlayment is calculated by roof area plus overlap allowance (synthetic underlayment rolls typically require a few inches of overlap at each course) — don't forget ice-and-water shield in valleys and along eaves where code requires it, which is priced and ordered separately from the field underlayment.
Flashing
Flashing is counted by location, not area: valley flashing by linear foot of valley, step flashing by piece count along walls/chimneys, and drip edge by linear foot of eave and rake — each of these is a distinct material line item that a square-footage-only estimate will miss entirely.
Ridge cap and ventilation
Ridge cap and ventilation (ridge vent, off-ridge vents) round out the takeoff — measured by linear footage of ridge, and easy to forget since it's a small percentage of total material cost but a code-required component on most jobs.