11.2 Gypsum Board, Plaster, and Framing
Key Takeaways
- Type X 5/8-inch gypsum board is the standard board for 1-hour rated assemblies; green board is water-resistant but not waterproof and not a wet-area tile backer.
- ASTM C840 caps single-layer screw spacing at 16 inches on walls and 12 inches on ceilings; listed fire-rated designs override general spacing.
- GA-214 defines six finish levels (0-5); Level 5 is required under gloss paint or critical raking light to prevent joint photographing.
- Three-coat plaster (scratch, brown, finish) is about 7/8 inch over metal lath; veneer plaster is a thin skim over blue board.
- Estimate 16-inch o.c. studs with (length x 0.75) + 1, then add for corners and openings; steel studs are coded by web, flange, and mil thickness.
Gypsum Board, Plaster, and Framing
CSI Division 09 — Finishes opens with interior framing and wall surfacing. The exam pairs gypsum board (drywall) installation with IBC Chapter 25, ASTM C1396 (board), ASTM C840 (application), and light-gauge steel and wood framing rules. Open-book strategy: tab the gypsum board chapter and the fastener-spacing tables in your reference.
Gypsum board types and uses
- Regular board: 1/2 inch and 5/8 inch most common.
- Type X: fire-rated, 5/8 inch, contains glass fibers; required in 1-hour rated assemblies.
- Type C: enhanced fire resistance for some 2-hour assemblies.
- Water-resistant (green board): ASTM C1396 backing in damp areas — not allowed as tile backer over wet showers; use cement board (ASTM C1325) there.
- Abuse/impact-resistant: ASTM C1629 for corridors.
A trap: green board is not waterproof and is prohibited on ceilings at wide framing spacing because it sags.
Fastener spacing (ASTM C840)
Maximum fastener spacing depends on whether a wall or ceiling and whether nails or screws are used:
| Location | Screws (single layer) | Nails |
|---|---|---|
| Walls | 16 inches o.c. | 8 inches o.c. (edges/field) |
| Ceilings | 12 inches o.c. | 7 inches o.c. |
For fire-rated assemblies, follow the listed (UL) design spacing exactly — it overrides general tables. Set screw heads slightly below the paper without breaking it; broken paper reduces holding power.
Finishing levels (GA-214)
The Gypsum Association GA-214 standard defines six finish levels, 0 through 5:
- Level 0: no taping/finishing.
- Level 1: tape embedded; plenum/concealed areas.
- Level 2: tape plus one coat; garages, behind tile.
- Level 3: two coats; medium/heavy textures.
- Level 4: three coats; flat paints, light textures.
- Level 5: skim coat over Level 4; gloss paints and critical lighting.
Exam trap: specifying Level 5 under gloss or raking light prevents joint photographing — choosing Level 4 there is the wrong answer.
Plaster systems
Three-coat plaster = scratch, brown, and finish coats over metal lath. Total thickness over metal lath is about 7/8 inch. Veneer plaster is a thin (about 1/16 to 1/8 inch) skim over blue board. Plaster lath and accessories follow ASTM C841/C842. Mixing water must be potable; over-troweling drives fines to the surface and causes crazing cracks.
Framing and a takeoff scenario
For wood studs at 16 inches on center (o.c.), estimate studs for a wall as: (wall length in feet x 0.75) + 1, then add for corners and openings. For a 40-foot wall: 40 x 0.75 = 30, + 1 = 31 studs, plus typically 3 per corner and 2 per opening.
Light-gauge steel studs are specified by web depth, flange, mil thickness, and steel grade (for example 362S162-33, meaning 3-5/8 inch web, 1-5/8 inch flange, 33 mil). Drywall screws into steel must be self-drilling when the stud exceeds 33 mil (20-gauge structural).
A specification calls for a single layer of gypsum board on a ceiling fastened with screws. What is the maximum allowable screw spacing under ASTM C840 for a non-rated assembly?
An interior wall is 32 feet long with studs at 16 inches on center. Using the standard estimating rule, approximately how many field studs are required before adding corners and openings?
Gypsum Board Types and Thickness
Know the board types: regular, Type X (5/8-in fire-rated — the workhorse of rated assemblies), moisture/mold-resistant ("green/purple") for wet areas, cement board as a tile backer in showers (never regular gypsum behind tile in wet areas). Common thicknesses 1/2 in (walls) and 5/8 in (ceilings, fire-rated). A 1-hour wall is commonly one layer of 5/8 Type X each side of studs — a frequently tested UL assembly concept.
Fastening, Joints, and Finish Levels
Fasten with drywall screws at 12–16 in o.c. to studs (16 in walls, 12 in ceilings), set just below the surface without tearing the paper. Tape and finish joints in coats; the GA finish levels run 0–5: Level 4 for flat paint/light texture, Level 5 (skim coat) for gloss paint/critical lighting. Control joints in long runs (≥30 ft) prevent cracking; corners use corner bead.
Plaster and Lath
Plaster is a multi-coat wet system over metal or gypsum lath: scratch → brown → finish (3-coat) for cement plaster/stucco. Stucco over framing uses a WRB + lath + 3 coats, with a weep screed at the base. Plaster is more labor-intensive but harder than drywall; the exam tests the coat sequence and the difference between veneer plaster (thin, over special blueboard) and traditional 3-coat.
Common Exam Traps
- Trap: Regular gypsum behind shower tile. Use cement board/backer.
- Trap: A 1-hour wall uses 1/2-in board. Use 5/8 Type X.
- Trap: Mixing up the plaster coats — order is scratch → brown → finish.
- Trap: Over-driving screws and tearing the face paper (loses holding power).
What thickness and type of gypsum board is standard for a one-hour fire-rated wall assembly?
Sound, Fire Assemblies, and Drywall Takeoff
Rated and sound walls hinge on assembly details: resilient channel, staggered studs, or a double layer of board raises the STC (Sound Transmission Class), while penetrations must be firestopped to keep a rated wall's rating. Estimate drywall by wall/ceiling area in sheets: a 4x8 sheet = 32 SF, a 4x12 = 48 SF. Worked example: 1,280 SF of wall / 32 SF per 4x8 sheet = 40 sheets, plus ~10% waste = about 44 sheets, then add joint compound (roughly one box per ~100-150 SF) and tape.
Interior Framing Layout and Backing
Non-load-bearing interior partitions are framed at 16 or 24 in o.c. to match the board edges; openings get headers and full-height jamb studs. Install blocking/backing in the wall before drywall for anything that hangs later — grab bars, cabinets, handrails, TVs, and toilet accessories — because adding it after closing the wall means demolition. Coordinating this backing list from the drawings is a recurring real-world miss the exam frames as a callback/rework cost.