Under the NC Building Code, what is the maximum allowable deflection for floor joists under live load?
Correct Answer
C) L/360
The NC Building Code specifies L/360 as the maximum allowable deflection for floor joists under live load conditions.
Why This Is the Correct Answer
The correct answer is C (L/360). The NC Building Code (consistent with the IRC) limits floor joist deflection under live load to L/360, where L is the span length. This ratio ensures floors feel stiff underfoot and protects finishes like tile or drywall from cracking.
Why the Other Options Are Wrong
Option A: L/180
L/180 is incorrect. This is a less restrictive limit sometimes applied to roof rafters under live load only. Using L/180 for floors would allow excessive bounce and could damage floor finishes. It represents a much looser standard than required for floor joists.
Option B: L/300
L/300 is incorrect. L/300 is sometimes cited for total load (live + dead) on floor members or for certain ceiling applications, but the live-load-only limit for floor joists is the stricter L/360. Choosing L/300 confuses the total-load limit with the live-load limit.
Option D: L/240
L/240 is incorrect. L/240 is the deflection limit commonly applied to floor joists under total load (live + dead combined), not live load alone. The live-load-only deflection limit is the more restrictive L/360.
Memory Technique
Think of the deflection limits as a ladder: L/180 (roof rafters, live only) → L/240 (floors, total load) → L/360 (floors, live load only). The denominator gets bigger (stricter) as you go from roof to floor and from total to live load. '360 for floor live load' — the most common floor deflection value on exams.
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