Whether you are building a new home or already own one, understanding the function of your floor joists is critical. The code also identifies joist sizing and spans for various joist spacing – from 12” up to 24” apart. It outlines max span lengths for wood joists of all sizes – from 2×6 to 2×12 and greater. Proper floor joist sizing is specified by the International Residential Code – the IRC. I then mentioned that I could prove my point because there are specific load tables outlined by the building code that dictate floor joist sizing. I took a look and said they looked fine but just needed some added support. He claimed his floor joists were too small for his home, which made his floor wobbly. To calculate maximum spans of species not shown above, use the Span Calculator or the Span Tables for Joists and Rafters on the American Wood Council website.I had a good debate with a friend the other day. To calculate maximum joist spans using different design criteria (loading, spacing, grade, etc.), for these common lumber species, see the International Residential Code (IRC). The above spans are based on American Softwood Lumber standard sizes. Uninhabitable attics with limited storage, a live load of 20 psf, a dead load of 10 psf, and a deflection limit of L/240 ** Span is greater than 26 feet in length.
Ceiling Joist Span Tables Uninhabitable attics without storage, a live load of 10 psf, a dead load of 5 psf, and a deflection limit of L/240 This is based on the design requirement where your uninhabitable attic with limited storage has a live load of 20 psf, a dead load of 10 psf, and a deflection limit of L/240 (span in inches divided by 240). Example: The highlighted cell (below) shows that a 2" x 6" Douglas Fir joist, with a grade of #2, spaced 24"Īpart, can have a maximum span of 10 feet - 8 inches (10-8).
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