Section R602 of the IRC provides span tables for roof rafters used in residential construction. Rafter sizing is based on the span (measured horizontally from the plate line to the ridge), spacing, lumber species and grade, and roof load. Roof live load is typically 20 pounds per square foot for normal-slope roofs (less than 10 degrees pitch) or 15 pounds per square foot for steeper roofs. Snow load is combined with live load in certain climates, with ground snow load varying from zero in warm regions to 100+ pounds per square foot in snowy areas. Span tables accommodate various rafter dimensions (2x4, 2x6, 2x8, 2x10, 2x12) and on-center spacings. Rafters must be supported at the ridge by either a ridge board or by birds-mouth cuts where rafters are cut and notched to sit on the plate line at the top of exterior walls. Collar ties must be installed in the upper third of the rafter span to prevent roof spread, typically one collar tie for every other rafter pair. Collar tie sizing is specified in tables and is generally smaller than rafter size. Purlins (intermediate supports running perpendicular to rafters) are required where rafter span exceeds table limits. Roof sheathing is typically ½-inch or ⅝-inch plywood or oriented strand board with nailing patterns specified by roof type. Roof decking must resist wind uplift forces, with nailing spacing reduced near roof edges and gables. Ventilation requirements specify minimum 1 inch clearance between sheathing and insulation to permit air flow.