Loads, Span and Width

• Rafter sizes are determined primarily by three types of roof load, or weight, on the roof. Dead load is the weight of the roofing material itself -- the rafters, decking and roofing. Live load is the external weight -- primarily snow and ice accumulations. Wind load is the force of the wind against a roof surface. Other factors which affect rafter size are the span or width of the roof and its pitch or slope.
  
  

Wood and Spacing

• Most rafters and truss chords are made with 2-by-4-inch framing lumber. Properly braced, that will handle roof loads in most sections of the United States. Extreme northern and mountainous regions subject to heavy snows and deep accumulations may require 2-by-6-inch rafters. Spacing also affects size; most rafters and trusses are spaced 24 inches apart, but 16-inch spacing may allow use of thinner rafters.
  
  

Codes and Tables

• Building codes specify rafter board dimensions in most locales, but maps are available from government and industry organizations showing average loads and permissible spans. The American Wood Council and several universities also offer online rafter tables and calculators that provide the maximum spans for rafters of various woods with different pitches.
  
  

Pitch Effect

• In general, steeper roofs have longer allowable rafter spans -- the distance from a peak to a supporting wall -- for a 2-by-4 rafter because those shed snow more efficiently. Flat or low slope roofs retain snow load but are less subject to wind load. Roofing material also is an influence. Metal panels, for instance, shed show better than rougher, asphalt shingles.
  
  

Lengths by Pitch

• Specific lengths of rafters vary with the roof pitch. A framing square has tables for the length of rafters according to pitch, which is expressed as inches of rise per foot of run from the peak to the wall, such as 5/12 for a 5-inch rise per foot. A 5/12 pitch, for example, requires 13 inches of rafter for every foot of run, while a 9/12 roof requires 15 inches per foot differential. A 24-foot-wide roof with a 12-foot run requires a 13-foot rafter at a 5/12 pitch.