CSLB General Building (B)Framing and CarpentryMedium
A builder compares two floor framing layouts using the same Douglas fir-larch No. 2, 2x10 joists and the same live load. One layout spaces joists at 16 inches on center; the other spaces them at 24 inches on center. What effect does increasing the spacing from 16 to 24 inches on center have on the maximum allowable span per standard joist span tables?
- AThe allowable span stays the same since joist size and grade are unchanged
- BThe allowable span increases because fewer joists are needed
- CThe allowable span decreases because each joist carries a larger tributary load
- DThe allowable span becomes unpredictable and must be field-tested
Show answer & explanationAnswer & explanation
Correct answer: C. The allowable span decreases because each joist carries a larger tributary load
Increasing joist spacing increases the tributary floor area (and thus the load) each individual joist must support. Since the joist size, species, and grade remain the same, standard span tables show a reduced allowable span at wider spacing to keep deflection and bending stress within limits.
Why the other options are wrong
- A. Span tables clearly show span decreases as spacing increases for a fixed joist size.
- B. Fewer joists does not increase capacity per joist; it increases the load each one carries.
- D. Span tables provide engineered, predictable values; no field testing is needed.
Joist Spacing vs. Span
For a fixed joist size and grade, increasing on-center spacing increases the tributary load per joist, which decreases the maximum allowable span shown in span tables.
- Tributary load per joist = spacing x load per sq ft
- Wider spacing = more load per joist = shorter allowable span
- Span tables list multiple spacing options (12, 16, 19.2, 24 in o.c.)
Memory trick: 'Spread joists wide, span must slide (shorter).'