Sheathing covers the whole roof
Sheathing (decking) goes over the rafters before any shingle, so its area is the full sloped roof: footprint × pitch multiplier. Sheets are <strong>4×8 = 32 square feet</strong>, installed with 1/8-inch gaps and H-clips, and the calculator adds the standard 10% waste for rakes, hips and offcuts.
Thickness and spacing go together
Half-inch OSB or plywood spans rafters 24 inches on center; 5/8-inch is the upgrade for 24-inch spans with heavy shingles, and any rafter spacing over 24 inches needs thicker decking. Roofers sheathe from the bottom edge up, starting with a starter row cut to keep seams staggered.