What Is the Standard Spacing of a Roof Rafter?

24-Inch Spacing

• Perhaps the most common spacing of rafters (and trusses, which are rafters built with internal bracing and installed as a unit) is 24 inches. That dimension conforms to the 4-foot by 8-foot size of plywood or oriented strand board (OSB) wood commonly installed as decking on a roof. That makes planning and installing roof decking easier.

Roof Pitch

• The steepness of a roof affects rafter spacing. A roof with a slope of more than 3 inches per foot can use 24-inch spacing with 2-inch by 4-inch rafter boards on a roof about 16 to 18 feet wide, depending on the type of lumber. On a low-slope roof, a pitch of less than 3 inches per foot, 24-inch spacing requires 2-inch by 6-inch lumber for a comparable roof.

Three Basic Options

• Rafter tables generally offer three spacing options: 12, 16 or 24 inches. The spacing dramatically affects the distance a rafter can span. No. 1 southern yellow pine, a common rafter lumber, in 2-inch by 4-inch dimension can span 8 feet 5 inches with 12-inch spacing, 7 feet 3 inches with 16-inch but only 5 feet 11 inches with 24-inch. Span distances increase with size of lumber -- a 2-inch by 6-inch rafter, for instance, can span 9 feet 5 inches on 24-inch spacing, while a 2-by-10 at that spacing can support 15 feet 8 inches.

Center Spacing

• All rafter spacing is figured on centers, the middle of a 2-inch rafter board (which actually is 1 1/2 inches wide). Rafters with 24-inch spacing will start with the outside edge of the first rafter even with the end of the roof. The second rafter will be 23 1/4 inches from the end of the roof, so the center of the first rafter will be 24 inches from the center of the second rafter. Rafters are rarely made of lumber thicker than 2 inches; strength is added by increasing the width of the board, 4 to 6 to 8 inches or more.