Question 19
Domain 3: Floor Construction and Supporting FramingPer the IRC, what is the maximum allowable deflection for a floor joist supporting a living area under live load?
Correct answer: C
Explanation
The IRC limits floor joist deflection for a living area under live load to L/360, meaning the joist may deflect no more than span divided by 360. This standard controls serviceability so floors do not feel excessively bouncy or sag under normal occupancy.
Why each option is right or wrong
A. L/180
B. L/240
C. L/360
IRC Section R301.7 (Deflection) sets the serviceability limit for floor members supporting live loads in habitable areas at L/360. For a joist spanning any length L, the maximum permitted midspan deflection under live load is therefore one three-hundred-sixtieth of that span, which is the governing criterion for a living-area floor.
D. L/480