Drywall (C-9) · Lesson 5 of 5

Gypsum Board Diaphragm Ceilings: Nails, Spacing, and Capacity

"Gypsum Board Diaphragm Ceilings: Nails, Spacing, and Capacity" is lesson 5 of 5 in the Drywall (C-9) Learning Hub. It opens with "When the Ceiling Board Is Structural" and works through 5 short steps, then a worked example you can check yourself against.

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Steps in this lesson

  1. When the Ceiling Board Is Structural
  2. What Table 2508.6 Prints
  3. Reductions in the Notes
  4. Layout and Fastening Rules
  5. Two Books, Two Wordings for Edge Distance

The worked example in the course applies this lesson to a realistic job in 6 steps, from "Check the proportions" through "Shear values with the reductions in the notes".

What you'll be able to do

  • Explain the main idea of "Gypsum Board Diaphragm Ceilings: Nails, Spacing, and Capacity" in plain language you can use on the exam.
  • Apply this step with confidence: "Reductions in the Notes".
  • Apply this step with confidence: "Layout and Fastening Rules".
  • Apply this step with confidence: "Two Books, Two Wordings for Edge Distance".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
Gypsum Board Diaphragm Ceiling: Fasteners and CapacityPLAN: 1/2 in. BOARD ON JOISTS 16 in. o.c.24 ft16 ft19 joist lines; end lines are the blockingFASTENER COUNTShape: 24 / 16 = 1.5 (limit 1-1/2:1)Joist lines: 288 / 16 = 18 spaces= 19 lines (ends = blocking)Each line 192 in.: 192 / 7 = 27.4round up: 28 spaces + 1 = 29 nails19 x 29 = 551 nails (5d, 1-5/8 in.)At 24 in. o.c.: 288 / 24 = 12 spaces= 13 lines; 13 x 29 = 377 nailsJoists (1/2 in. board)Table 2508.6 plfNormal loading (-25%)SDC D, E, F (-50%)16 in. o.c.9067.54524 in. o.c.7052.53516 ft x 24 ft ceiling: 19 joist lines x 29 nails = 551 nails; 90 plf short-termSources: 2025 CBC Table 2508.6 and 2508.6.1-2508.6.4; 2025 CRC Table R702.3.6.Each reduction is shown alone; the designer decides how they combine. Nail count is an estimating method.CC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: When the Ceiling Board Is Structural

2025 CBC 2508.6 permits gypsum board on wood joists to form a horizontal diaphragm ceiling in accordance with Table 2508.6; 2025 CRC R702.3.6 and Table R702.3.6 say the same for houses. In that case the board and its fasteners resist wind or seismic force, so spacing and layout are no longer just a finishing preference. The plans or engineer will say a ceiling is a diaphragm.

Step 2: What Table 2508.6 Prints

Gypsum panel product at least 1/2 in. thick on framing 16 in. o.c. has a shear value of 90 plf of ceiling; at 24 in. o.c. it is 70 plf. The fastener is a 5d cooler or wallboard nail, 1-5/8 in. long, 0.086-in. shank, 15/64-in. head. Note c allows 1-1/4 in. No. 6 Type S or W screws in place of the nails. These values are not cumulative with other horizontal diaphragm values.

The remaining 3 steps, the worked example and the quiz are in the course. This is a preview. The full lesson, with the step-by-step explanations, a worked example and a quick-check quiz, is inside the classroom.

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