A drywall contractor is installing 5/8-inch drywall on a ceiling with joists 24 inches on center. What is the maximum allowable deflection of the joists to prevent cracking?
Correct Answer
C) L/240
For drywall ceilings, the maximum allowable deflection is typically L/240 to prevent cracking of the drywall and tape joints. This is more restrictive than floor deflection limits due to the brittle nature of drywall.
Why This Is the Correct Answer
L/240 is the correct maximum allowable deflection for drywall ceilings because drywall is a brittle material that cracks easily under structural movement. This deflection limit is specifically established to prevent visible cracking in drywall and tape joints. The L/240 standard applies regardless of joist spacing (16" or 24" O.C.) and is more restrictive than typical floor deflection requirements due to the fragile nature of gypsum board.
Why the Other Options Are Wrong
Option A: L/120
L/120 is too lenient for drywall applications and would allow excessive deflection that would cause visible cracking in the drywall and separation of tape joints.
Option B: L/180
L/180 is still too lenient for drywall ceilings and would not provide adequate protection against cracking of the brittle gypsum board material.
Option D: L/360
L/360 is overly conservative and typically reserved for plastered ceilings or special applications where extremely minimal deflection is required, making it unnecessarily restrictive for standard drywall installations.
Memory Technique
Remember '240 for drywall' - the number 240 sounds like 'dry wall' when you say 'two-forty dry-wall' quickly, helping you associate L/240 with drywall applications.
Reference Hint
Florida Building Code, Chapter 16 - Structural Design, Table 1604.3 for deflection limits, or Chapter 25 - Gypsum Board and Plaster
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