Residential Building Inspector B1Floor ConstructionMedium
An inspector is reviewing plans for a new single-family dwelling. The floor joists are 2x10 lumber spaced at 16 inches on center. What is the maximum clear span allowed for these joists when supporting a live load of 40 psf and a dead load of 10 psf, assuming No. 2 Spruce-Pine-Fir (SPF) grade lumber?
- A17 feet 6 inches
- B13 feet 1 inch
- C16 feet 2 inches
- D14 feet 9 inches
Show answer & explanationAnswer & explanation
Correct answer: D. 14 feet 9 inches
According to IRC Table R507.3.1(1) for 2x10 floor joists with No. 2 SPF lumber, a live load of 40 psf, a dead load of 10 psf, and spacing at 16 inches on center, the maximum allowable clear span is 14 feet 9 inches.
Why the other options are wrong
- A. This span exceeds the allowable limits for the specified joist size and grade.
- B. This span is for a lighter live load or closer spacing for 2x10s.
- C. This span is for a different joist size or grade, or a lighter load condition.
Floor Joist Span
The maximum distance a floor joist can safely extend between supports without excessive deflection or failure, determined by material, size, spacing, and applied loads.
- Span limits are found in IRC tables.
- Factors include joist species/grade, dimensions, spacing, and live/dead loads.
- Exceeding span limits leads to structural failure or excessive deflection.
Memory trick: Joist Spans Require Careful Table Lookups.