Determine the width, length and pitch or slope of the roof, the desired spacing of joists, and the type of roof material. Figure dead load based on the weight per square foot of the roofing material, such as rafters and shingles; use lumber industry tables or Internet searches to get weights. Find live loads and wind loads from U.S. Department of Agriculture or other tables, showing weather information by locale. Adapt the pitch to the load; steep roofs shed snow loads better, but flat roofs are less subject to wind load.
Refer to basic industry sources, such as the American Wood Council; Structural Building Components Association; educational institutions, like the University of Massachusetts or University of Florida; or industry groups, like Universal Forest Products. Use their tables or free calculation programs to determine roof joist size and spacing.
Enter the pertinent information, such as type and size of lumber, roof pitch, spacing of joists and load factors and the program will calculate the maximum span for joists. Adjust the size of lumber or spacing as required to meet the required span or width of the joist from wall to wall. Adapt the calculation program by entering the required span first; a program will calculate spacing and lumber size.