What Size Rafters Do You Use in a Framed House?

Rafters Bear Three Loads

• Rafters must bear three types of load: dead, live and wind. Dead load is the weight of the roof itself, live load additional natural weights such as snow and ice and wind is the force of wind hitting the roof. These will vary with the width or span of the roof, its pitch or angle of slope and locale. A flat or low-slope roof will have the lightest dead loads and wind loads, but the largest live loads because snow and ice pile up on them. Steep roofs have more material and thus more dead load and are more subject to wind force but shed snow and ice best.

Roofing Affects Rafters

• The roof covering affects rafter sizing. Heavy material such as clay tile will require stronger rafters. Textured roof materials, like composite shingles, retain snow and ice more than slick surfaces -- e.g., metal. Plywood sheathing will be heavier than oriented strand board (OSB) and solid sheathing will be heavier than "strip" sheathing, typically 1-by-4 strips nailed across rafters and used most often for cedar shakes or similar individual shingles.

Run and Pitch Affect Size

• Two key figures in calculating rafter size are run and pitch. The run is the distance each rafter must support from a peak to a wall, which will vary with the size of the building and the style of roof. Hip rafters, for instance, run from a peak to a house corner and must be longer than common rafters, which go straight from a peak to a wall. The pitch is measured in inches per foot -- for example, 5/12, which indicates a 5-inch rise every foot of width.

Pitch Plays Major Role

• Pitch has a major bearing on rafter sizing. A 2-by-4 rafter on a steeper roof, greater than 3 inches of rise per foot, will span 8 feet 6 inches on a light load and 5 feet 11 inches on a heavy load. Lumber tables do not recommend this size for low-slope roofs; those tables start with 2-by-6 rafters. A 2-by-6 under light load at 24-inch spacing will span 12 feet 8 inches on a low-slope but 13 feet 4 inches on a pitch of more than 3 inches per foot.

Spacing Influences Size

• Spacing also affects rafter sizes. Most roofs are framed with 24-inch rafter spaces. That spacing will allow a 2-by-6 maximum rafter span of 12 feet 8 inches on a low load and 9 feet 9 inches on a heavy load roof. Change the spacing to 16 inches and those distances increase to 14 feet 8 inches and 11 feet 9 inches. Move to a 2-by-8 rafter and maximums on 24-inch spacing are 16 feet 8 inches and 12 feet 11 inches.

The Effect of Wood

• The type of wood also will affect rafter sizing. No. 1 yellow pine is the strongest and most commonly used. Its maximum spans are consistently a foot or two longer than No. 2 pine or fir or spruce, which also are used for rafter framing. Lumber industry tables will show maximum spans for all types of rafter lumber.