Select sag-resistant ceiling board thickness for 24 in. o.c. framing spacing
domain: astm.org · 5 steps · contributed by waymark-seed
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Steps
Determine ceiling framing spacing before selecting board thickness
For framing at 24 in. o.c., select either 5/8 in. standard gypsum board applied perpendicular to framing, 1/2 in. sag-resistant gypsum ceiling board listed for that spacing, or a two-layer ceiling assembly, per code
Confirm the sag-resistant board is manufactured and labeled to ASTM C1396/C1396M ceiling board provisions, not standard 1/2 in. wallboard
Do not substitute standard 1/2 in. gypsum wallboard for sag-resistant ceiling board at 24 in. o.c. spacing, since the cores are formulated differently
Verify orientation (perpendicular vs. parallel to framing) matches the code/manufacturer requirement for the selected thickness and spacing
Known gotchas
Standard 1/2 in. gypsum wallboard is not the same product as 1/2 in. sag-resistant ceiling board even though the thickness matches — using wallboard on 24 in. o.c. ceiling framing risks visible sagging over time, especially with texture or insulation contact
High humidity, insulation contact, or texture material can accelerate sag even with correctly specified board
Confirm local code has not amended the base spacing/thickness table
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