During a residential inspection in Michigan, you discover that the contractor installed 2x8 floor joists at 24 inches on center spanning 12 feet. The building official questions whether this meets code requirements for a 40 psf live load. What should be your primary concern?
Correct Answer
A) Both spacing and span exceed recommended limits
2x8 joists at 24" o.c. spanning 12 feet exceeds the recommended span limits for 40 psf live load. The combination of wide spacing and long span creates structural concerns.
Why This Is the Correct Answer
The primary concern is that BOTH the spacing (24 inches on center) and the span (12 feet) together exceed recommended limits for 2x8 joists under a 40 psf live load. Span tables in the IRC show that 2x8 joists at 24" o.c. typically have a maximum allowable span of approximately 10–11 feet for a 40 psf live load, depending on wood species and grade. At 12 feet, the span exceeds those limits. The combination of maximum spacing and excessive span creates a cumulative structural deficiency.
Why the Other Options Are Wrong
Option B: The joist depth is insufficient for the load
While joist depth affects span capacity, the depth (2x8 = 7.25 inches) is not itself insufficient as an isolated concern — the problem is the combination of wide spacing and long span. Framing the issue solely as 'depth is insufficient' misidentifies which variable is the root cause.
Option C: The spacing is too wide for the span length
While the 24" spacing is too wide for this span, selecting only the spacing as the concern is incomplete. The question asks for the primary concern, which encompasses both factors together — the span at 12 feet also independently exceeds code limits for this joist at this spacing.
Option D: The span is acceptable for the joist size
The 12-foot span is NOT acceptable for 2x8 joists at 24" o.c. under 40 psf loading. Span tables confirm that 12 feet exceeds the allowable span for this combination. This answer is factually incorrect.
Memory Technique
Remember: 'Two strikes, you're out.' When both spacing (24" o.c. is the widest common residential spacing) and span (12 feet) are at or beyond their individual maximums, the combination fails. Either factor alone might be borderline — together, they definitely exceed code.
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