Under Michigan Building Code, what is the maximum allowable span for a 2x8 floor joist spaced 16 inches on center in residential construction?
Correct Answer
C) 11 feet 10 inches
For 2x8 floor joists at 16 inches on center, the maximum span is typically 11 feet 10 inches according to Michigan Building Code span tables.
Why This Is the Correct Answer
According to Michigan Building Code span tables (based on IRC), a 2x8 floor joist at 16 inches on center has a maximum allowable span of 11 feet 10 inches. This value accounts for standard residential loading (40 psf live + 10 psf dead) and the structural properties of typical lumber grades used in residential construction.
Why the Other Options Are Wrong
Option A: 13 feet 2 inches
13 feet 2 inches exceeds the safe span for a 2x8 at 16" o.c. This span length would be appropriate for a larger member such as a 2x10, not a 2x8. Using this span would result in excessive deflection and potentially unsafe floor performance.
Option B: 15 feet 0 inches
15 feet 0 inches is far too long for a 2x8 floor joist at 16" o.c. A span this long would cause serious deflection problems, squeaky floors, and potential structural failure under normal residential loading conditions.
Option D: 10 feet 6 inches
10 feet 6 inches is more conservative than required and undershoots the actual code-allowed span. While structurally safe, this answer does not reflect the maximum allowable span per the Michigan/IRC span tables.
Memory Technique
Think '2x8 at 16 = just under 12 feet' — specifically 11'10". The 11 feet anchors to the '8' in 2x8 (subtract a few feet for the span limit), and the 10 inches reminds you of the next board size up (2x10).
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