01
Convert length to exposure
Subtract the required headlap from 16 inches and divide by two for the nominal field exposure. Verify that pitch and system guidance permit the selected lap.
02
Calculate pieces from coverage
A full field piece covers width multiplied by exposure. Divide 14,400 square inches by that coverage for a theoretical pieces-per-square figure, then add project-specific allowances.
03
Detail the roof edges separately
Starter pieces, ridge or top courses, hips, valleys and rake cuts are not captured by simple field coverage. Measure those conditions and retain replacement stock.