15.4 Drywall & Finishes: Gypsum Board, GA-214 Levels, Ceilings, Tile & Paint

Gypsum Board Products & Finishes

1. Gypsum Board (Drywall) Types

• Regular Gypsum Board (ASTM C1396): Standard $1/2\text{-inch}$ thickness for non-rated residential and light commercial interior partitions.
• Type X Gypsum Board ($5/8\ \text{inch}$ thick): Formulated with special core additives, including non-combustible glass fibers that maintain core cohesion when subjected to fire. One layer on each side of wood or steel studs is the basis of many 1-hour rated partitions, and two layers each side reach 2 hours. The rating belongs to the tested assembly (for example, a UL design number), not to the board alone.
• Type C Gypsum Board: Enhanced version of Type X containing expanded vermiculite, used for fire-rated horizontal floor-ceiling assemblies.
• Moisture-Resistant Gypsum Board ("Greenboard"): Water-repellent paper and core for areas with occasional moisture. The code does not allow water-resistant gypsum backing board where it will be directly exposed to water. Tile in tub and shower areas goes over cement, fiber-cement, or glass-mat gypsum backers (for example, ASTM C1325 cement board) installed to the manufacturer's instructions.
• Abuse- and Mold-Resistant Boards: Abuse-resistant and impact-resistant panels serve corridors and schools. Glass-mat (paperless) boards resist mold and serve as exterior sheathing.

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2. GA-214 Recommended Levels of Gypsum Board Finish

The Gypsum Association (GA-214) establishes six standardized finish levels:

                      GA-214 DRYWALL FINISH LEVELS

  LEVEL   TREATMENT DESCRIPTION               RECOMMENDED SERVICE ENVIRONMENT
  ─────────────────────────────────────────────────────────────────────────────
  LEVEL 0 No taping, finishing, or coating.   Temporary construction; dust barriers.
  ─────────────────────────────────────────────────────────────────────────────
  LEVEL 1 Tape embedded in compound; tool     Concealed spaces, plenums, elevator
          marks and ridges acceptable.        shafts, attics (provides "fire-taping").
  ─────────────────────────────────────────────────────────────────────────────
  LEVEL 2 Tape embedded and wiped; fastener  Water-resistant backer under tile;
          heads get one coat.                 garages, storage where looks are secondary.
  ─────────────────────────────────────────────────────────────────────────────
  LEVEL 3 Tape + one additional coat over     Medium or heavy textures applied
          joints; fasteners two coats.        before final painting.
  ─────────────────────────────────────────────────────────────────────────────
  LEVEL 4 Tape + two separate coats over flat Flat paints, light textures, and
          joints, one over angles; fasteners  wallcoverings.
          three coats; sanded smooth.
  ─────────────────────────────────────────────────────────────────────────────
  LEVEL 5 Level 4 PLUS a thin skim coat (or   Recommended under gloss, semi-gloss,
          equivalent material) over the       enamel, or untextured flat paint where
          ENTIRE surface.                     critical lighting exists.
  ─────────────────────────────────────────────────────────────────────────────

Exam Key Rule: Gloss or enamel paint over a Level 4 finish often shows joint banding (photographing): the different texture and porosity of paper and compound become visible in raking light. GA-214 recommends a Level 5 finish (skim coat) for gloss, semi-gloss, and enamel paints and for critical lighting. The finish level is a specification choice, not a building-code requirement, so it must be written into the contract documents.

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3. Installation Rules (Gypsum Construction Handbook / GA-216)

• Ceilings first: Hang ceiling board before walls so the wall board supports the ceiling edges. Run board perpendicular to framing where possible and stagger end joints.
• Fastening: Screws are typically spaced about 12 inches on ceilings and 16 inches on walls when framing is 16 inches on center (closer at 24-inch framing and in rated assemblies). Drive heads just below the surface without breaking the face paper. Fire-rated assemblies must follow the tested design's fastener type and spacing exactly.
• Joints at openings: Avoid joints that line up with door and window jambs. Joints there crack, so place them over the header.
• Control joints: Long runs of wall or ceiling need control joints at the spacing in the manufacturer's literature (commonly about 30 feet for walls). Put them at door frames and wherever the substrate changes.
• Rated assemblies: Fire-taping (at least Level 1) is required in concealed spaces of rated assemblies. Every penetration needs a listed firestop system that matches the wall rating and the item penetrating it.
• Moisture: Keep board dry and off the floor. Do not hang drywall until the building is dried in. Mold-damaged board is removed, not painted over.

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Ceilings, Flooring, Tile & Paint

1. Suspended Acoustical Ceilings

• Hangers: ASTM C636 calls for 12-gauge hanger wires spaced no more than 4 feet on center, plus extra wires at light fixtures and other concentrated loads.
• Seismic: In higher seismic design categories, ASTM E580 adds wider perimeter closure angles, lateral bracing wires, and compression posts on larger ceilings. Light fixtures need independent support wires. Much of Nevada is assigned SDC D, so check the structural notes.

2. Resilient Flooring over Concrete

• Moisture testing: Test new slabs before installing vinyl, VCT, wood, or other moisture-sensitive floors. Use in-situ relative humidity probes (ASTM F2170) or calcium chloride tests (ASTM F1869), and compare the results with the flooring manufacturer's limit. Slab moisture is a leading cause of flooring failures.
• Surface preparation: Smooth and level the slab and remove curing compounds that would block adhesion. Patch with cementitious underlayment.

3. Ceramic Tile (ANSI A108/A118 and the TCNA Handbook)

• Mortar coverage: At least 80% contact in dry interior areas and 95% in wet areas and exteriors (ANSI A108.5). Back-butter large tiles to reach this coverage.
• Substrate flatness: Tile with any edge of 15 inches or longer needs a flatter substrate. ANSI A108.02 allows no more than 1/8 inch of variation in 10 feet for these tiles.
• Movement joints: Install them at perimeters, at changes of plane, and over structural joints, following TCNA detail EJ171. Never grout over a slab control joint.
• Wet areas: Use a waterproof membrane or backer system, and slope shower pans to the drain.

4. Painting & Coatings

• Surface preparation drives the life of most coatings. Concrete and masonry must be cured and dry. Steel needs the surface preparation the coating requires. Drywall needs the specified finish level and a primer.
• Film thickness: Specifications state the dry film thickness in mils. Inspectors check wet film with a wet-film gauge as the paint goes on.
• Lead paint (EPA RRP rule, 40 CFR 745): Renovations that disturb painted surfaces in pre-1978 housing or child-occupied facilities need an EPA-certified firm and certified renovator when they disturb more than 6 square feet inside per room or more than 20 square feet outside. Lead-safe work practices and the Renovate Right pamphlet are required.