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Span Tables


Use the span tables below to determine allowable lengths of joists and rafters, based on size and standard design loads.  You can also use the Wood Beam Calculator from the American Wood Council website to determine maximum rafter and joist lengths.



Rafter and Joist Span FAQ

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 a greater distance than a #2 grade (with knots) southern yellow pine.

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.

Rafter spans can be extended slightly beyond what the rafter tables suggest, when there is a cantelever extending beyond the supporting wall. 


Ceiling Joist Span
Use this table to determine the maximum lengths of ceiling joists based on species of lumber, joist spacing, and joist size.

Floor Joist Span
Use this table to determine the maximum lengths of floor joists based on species of lumber, joist spacing, and joist size.

Roof Rafter Span
Use this table to determine the maximum lengths of roof rafters based on species and grade of lumber, spacing, dimensions, load, and slope.


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