Use the span tables in the links below to determine the maximum allowable lengths of joists and rafters. These tables provide maximum spans for the #2 grades of four common species of lumber (Southern Pine, Douglas Fir, Hem-fir, and Spruce-pine-fir (SPF).
Determine Maximum Joist and Rafter Spans
Floor Joist Span Tables - Use these tables to determine the maximum span of a floor joist, given the size, spacing, species, and grade of lumber.
Ceiling Joist Span Tables - Use these tables to determine the maximum span of a ceiling joist, given the size, spacing, species and grade of lumber being used.
Rafter Span Tables - Use these tables to determine the maximum span of a rafter, given the size, spacing, species and grade of lumber.
Before you can determine the maximum spans for joists and rafters, you must know the load requirements for your given area. For example, some regions get large amounts of snow, whereas others do not get any. Some other regions have special wind and seizmic requirements that should be considered when designing wood structures. Contact your local building code officials to determine the building code for your area.
Considerations when using the Rafter and Joist Span Tables
The lengths and sizes of joists vary depending on the species and quality of the lumber used. For example, you can span a #1 grade (no knots) southern yellow pine joist a greater distance than a #2 grade of the same species (with knots).
The #1 grade material is more expensive than the #2 grade, but if you can use fewer or smaller #1 grade joists or rafters in your design, you may actually save money.
Depending on the availability and cost of various size framing lumber, it might be more economical to use a larger size joist or rafter in your design, because you can typically use fewer of them.
When calculating the span of joists or rafters, use the horizontal distance between to vertical supports.
You can break up a long span of rafters by installing a purlin and bracing on the underside. With the purlin/bracing installed, the long rafter span would be broken up into two shorter spans, thus allowing the rafter to carry additional load.
Before you make your final decision on lumber sizing and span, consult your local building code officials to determine design loads for your area.