CSLB General Building (B)Framing and CarpentryMedium
A designer specifies a 2x12 floor joist spanning 18 feet, and the applicable code requires that live load deflection not exceed L/360. What is the maximum allowable deflection for this joist?
- A0.60 inch
- B0.90 inch
- C0.30 inch
- D1.20 inches
Show answer & explanationAnswer & explanation
Correct answer: A. 0.60 inch
Convert span to inches: 18 ft × 12 in/ft = 216 inches. Divide by 360: 216 ÷ 360 = 0.60 inch. This is the maximum permitted deflection under live load for a floor member with an L/360 serviceability limit.
Why the other options are wrong
- B. 0.90 inch would require an L/240 limit, which is less restrictive than specified.
- C. 0.30 inch would correspond to a span of only 9 feet, not 18 feet.
- D. 1.20 inches is double the correct value and does not match L/360 for this span.
L/360 Deflection Limit
A common serviceability limit for floor joists under live load, meaning maximum deflection equals span length (in inches) divided by 360.
- Convert span to inches before dividing
- L/360 is typical for floors with plaster or brittle finishes
- L/240 is a less strict limit sometimes used for roofs
Memory trick: Span in inches ÷ 360 = how much sag is allowed.