CSLB General Building (B)Finishes and Building EnvelopeHard
A drywall contractor is hanging gypsum board on a corridor ceiling that runs 62 feet in a straight, unbroken line with no doors or perimeter relief joints. Per ASTM C840 requirements, no single field of gypsum board should exceed 30 feet without a control joint. What is the minimum number of control joints required for this run?
- A2
- B3
- C1
- D4
Show answer & explanationAnswer & explanation
Correct answer: A. 2
A single control joint would divide the 62-foot run into two segments of 31 feet each (62 / 2 = 31), which still exceeds the 30-foot maximum. Two control joints divide the run into three segments of about 20.7 feet each (62 / 3 = 20.67), each safely under 30 feet, satisfying the requirement with the minimum number of joints.
Why the other options are wrong
- B. Three joints work but are more than the minimum required.
- C. One joint creates two 31-foot segments, still exceeding the 30-foot limit.
- D. Four joints far exceed the minimum needed to meet the 30-foot rule.
Drywall Control Joint Spacing
ASTM C840 requires control joints in large gypsum board ceiling or wall runs so no single field exceeds 30 feet in either direction, accommodating structural movement and preventing cracking.
- Maximum unbroken run is 30 feet per ASTM C840
- Divide total length by segments to verify each is ≤30 ft
- Perimeter relief or doorway breaks can also satisfy the requirement
Memory trick: 'Divide till under thirty' — keep dividing the run until each piece fits under 30 feet.