US practice states a roof’s slope as inches of rise per 12 inches of horizontal run: a 6/12 roof climbs 6 inches per foot, which converts to atan(6/12) = 26.57° and a 50% slope (metric habit prefers an x/10 ratio for the same idea). The common classification bands run flat below 2/12, low-slope from 2/12 to 4/12, conventional from 4/12 to 9/12, and steep beyond — with a code hook attached, since IRC R905.2.2 sets 2:12 as the minimum slope for asphalt shingles.
One derived number does most of the practical work: the slope multiplier √(1 + (x/12)²), which stretches any plan dimension into its sloped equivalent. At 6/12 the multiplier is 1.1180, so a rafter with a 12 ft horizontal run to the ridge centre measures 12 × 1.1180 = 13.42 ft, and a 1500 ft² footprint carries 1500 × 1.1180 = 1677 ft² of actual roof surface.
Material ordering follows straight from the stretched area: 1677 ft² is 16.77 roofing squares (100 ft² each), and at the ARMA convention of 3 shingle bundles per square plus 10% waste the order rounds up to 56 bundles — the multiplier, not the footprint, is what the supplier gets paid on.