Rule of thumb. Most codes assume residential floor loads to be 50 pounds per sq. ft. — 40 pounds live load plus 10 pounds dead load. If load conditions do not exceed this value, you can use the following rule of thumb for sizing uniformly loaded residential floor joists:
Half the Span + 2 = Joist Depth
First, round the clear span of the floor joist up to the nearest foot and divide by 2. Then add 2 to the answer. For example: For a clear span of 15 ft. 6 in., round up to 16 and divide this span by 2, giving you 8. Next, add 2 to get the required depth in inches (8 + 2 = 10). It’s important to use the actual lumber dimensions, not the nominal dimensions, so a 2x12 floor joist will be required.
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