Under Oregon building codes, what is the maximum allowable spacing for floor joists when using 2x10 Douglas Fir lumber in residential construction with a 12-foot span?
Correct Answer
D) 16 inches on center
For a 12-foot span using 2x10 Douglas Fir joists in residential construction, the maximum spacing is 16 inches on center per Oregon building code span tables.
Why This Is the Correct Answer
16 inches on center is correct. For a 12-foot span using 2x10 Douglas Fir lumber in residential construction, Oregon's building code span tables (based on the IRC) allow a maximum spacing of 16 inches on center. This spacing provides adequate structural performance while meeting deflection and load requirements for standard residential floor systems.
Why the Other Options Are Wrong
Option A: 12 inches on center
12 inches on center is incorrect. While 12" OC would be structurally stronger (closer spacing = more joists = more support), it is not the maximum allowed — it is more conservative than required. The question asks for maximum allowable spacing, not minimum.
Option B: 19.2 inches on center
19.2 inches on center is incorrect. For a 12-foot span with 2x10 Douglas Fir, 19.2" OC spacing exceeds what the span tables allow. This spacing is used in some other applications (like 5-ply plywood divisions) but is not the maximum for this specific combination.
Option C: 24 inches on center
24 inches on center is incorrect. For a 12-foot span with 2x10 Douglas Fir, 24" OC would result in excessive deflection and likely fail code span table requirements. This spacing would require a larger lumber size or shorter span to be compliant.
Memory Technique
For exam purposes, remember the standard residential framing spacing hierarchy: 12", 16", 19.2", and 24" OC. For 2x10 Doug Fir at a 12-foot span, 16" OC is the sweet spot — not too tight, not too loose. '12-foot span = 16 on center' is a testable pairing.
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