Wood Roof Rafter Specs

Run, Pitch, Spacing

• The run of a rafter, or the distance it must span between peak and wall; the pitch, or slope; and spacing are the key elements in calculating rafter specifications. Low-pitch roofs, rising 3 inches per foot or less, require stronger rafters than high-pitched roofs because they are more subject to live load. Rafters spaced 16 inches apart will have different spans and strength requirements than those spaced 24 inches apart.

Table Guidelines

• Rafter tables developed by lumber industry and construction organizations provide guidelines for rafter specifications. They typically are divided between low slope --- 3 inches per foot or less --- and high slope --- more than 3 inches per foot --- and load categories, or light and heavy. Light roofs are those with live loads of 20 lbs. per foot or less, while heavy loads must support 40 or more lbs. per foot. Weather Bureau statistics provide typical live loads by region; areas with heavy winter snowfalls have high loads, and those in milder climates have less.

Spacing

• Virtually all rafters use 2-inch dimension lumber --- actually, 1-1/2 inches wide --- but the width varies according to strength required. The type of wood also is important. A 2-inch by 4-inch pine rafter on a steep roof with a light load can span 8 feet, 6 inches in 24-inch spacing without additional support. That increases to 10 feet, 4 inches with 16-inch spacing. Comparable numbers using fir wood are 7 feet, 9 inches and 9 feet, 6 inches.

Changing Size

• Increasing the size of the rafter to 6 inches with 24-inch spacing changes those spans to 13 feet, 4 inches for pine and 11 feet, 5 inches for fir. Lower the spacing to 16 inches, and the spans increase to 16 feet, 4 inches for pine and 14 feet for fir. A 2-inch by 10-inch rafter at 24 inches will span 22 feet, 5 inches in pine or 19 feet, 3 inches in fir.

Use Length and Pitch

• Plan a roof starting with its length and pitch. Figure 24-inch spacing, which is most common in house construction, and select a type of lumber; pine is the strongest, but fir is less subject to warping and bending after installation. Next, use the width of the roof, the live load and the pitch to select the proper size of rafter from a table. Remember, you can add internal braces to increase the width a rafter will support; roof trusses, for example, are rafters built with internal braces so smaller lumber can span greater distances. Rafter specifications usually are set by architects, engineers or other designers of the structure.