13.3 Interior Finishes: Gypsum, Stucco, Flooring & Paint
Key Takeaways
- Gypsum board levels of finish run 0 through 5, and the specified level must match the lighting and the final coating — level 4 under critical lighting or gloss paint will telegraph joints
- Type X gypsum board provides fire resistance only as part of a tested assembly; substituting board or fastener spacing invalidates the rating
- Three-coat portland cement plaster over metal lath requires a weather-resistive barrier and a weep screed at the foundation line
- Concrete slabs must be tested for moisture before resilient flooring is installed, using relative humidity probes or calcium chloride testing to the manufacturer's limits
- IBC interior finish classes A, B, and C are set by flame spread index, and the required class varies by occupancy and location within the means of egress
The Work Everyone Sees
Quick Answer: Finishes are 6 of 100 items on the AZ ROC commercial outline and 9 of 100 on the residential outline. The tested content is less about technique than about specification compliance: the right gypsum level of finish, the right plaster assembly, the right substrate moisture condition, and the right interior finish flame-spread class.
Finishes are also where AZ ROC complaints concentrate, because they are what the owner can see. A structurally perfect building with ridged drywall joints and a blotchy paint job produces a complaint; the reverse rarely does.
Gypsum Board
| Type | Use |
|---|---|
| Regular | General interior |
| Type X | 5/8 in., fire-resistance-rated assemblies |
| Type C | Enhanced fire performance for specific tested assemblies |
| Water-resistant / moisture-resistant | Damp areas — not for direct tile-backing in wet areas |
| Glass-mat / cement board | Tile backer in showers and wet areas |
| Abuse- and impact-resistant | Corridors, schools, healthcare |
Type X and the tested assembly. Type X board does not carry a rating by itself. The assembly is rated — a specific board type and thickness, on specified framing at a specified spacing, with a specified fastener type and spacing, with joints treated. Substituting 1/2-inch board, widening fastener spacing, or omitting joint treatment invalidates the listing even though the wall looks identical.
Levels of Finish (GA-214)
| Level | Description | Typical use |
|---|---|---|
| 0 | No taping or finishing | Temporary construction |
| 1 | Joints taped only | Concealed plenum spaces |
| 2 | Tape plus one coat over joints and fasteners | Tile substrate, garages, warehouses |
| 3 | Tape plus two coats | Heavy texture, or where a heavy wall covering follows |
| 4 | Tape plus three coats, smooth | Flat paint, light texture, light wall coverings |
| 5 | Level 4 plus a skim coat over the entire surface | Gloss or semi-gloss paint, critical lighting, dark colors |
Level 4 is the residential default and the source of most callbacks. Under raking window light or beneath a semi-gloss dark paint, level 4 joints telegraph — and the fix is a full skim coat of the whole wall, not a spot repair. If the design has floor-to-ceiling glass or wall-washer lighting, level 5 belongs in the specification, and if it is missing, ask before drywall starts rather than after paint.
Portland Cement Plaster (Stucco) and EIFS
Stucco is Arizona's dominant exterior finish, and it is a finish over a drainage assembly, not a waterproof skin.
Three-coat over metal lath, the traditional assembly:
| Coat | Nominal thickness | Notes |
|---|---|---|
| Scratch | 3/8 in. | Keyed into the lath, scratched horizontally |
| Brown | 3/8 in. | Rodded straight and true; must cure before finish |
| Finish | 1/8 in. | Color and texture |
Behind it: a weather-resistive barrier over the sheathing (two layers where required), then metal lath fastened through to the framing. At the base: a weep screed at or below the foundation plate line, at least 4 inches above grade or 2 inches above paving, with the WRB lapped over its attachment flange so water exits rather than getting behind it. That single detail is responsible for a large fraction of stucco moisture complaints.
Control joints relieve shrinkage and thermal movement. Without them, stucco cracks on its own schedule, and AZ ROC's workmanship standards treat cracking beyond tolerance as a defect.
EIFS — exterior insulation and finish system — is a different assembly: insulation board, base coat with reinforcing mesh, and a synthetic finish. Modern EIFS is specified as drainable EIFS with a drainage plane behind the insulation, precisely because early barrier EIFS trapped water. Do not treat the two as interchangeable.
Flooring and Substrate Moisture
The most common flooring failure has nothing to do with the flooring. Adhesives and resilient products fail when moisture in the concrete slab exceeds the manufacturer's limit.
| Test | What it measures | Common limit |
|---|---|---|
| In-situ relative humidity probe (ASTM F2170) | RH deep within the slab | Commonly 75–85% RH, product-specific |
| Calcium chloride (ASTM F1869) | Moisture vapor emission rate | Commonly 3–5 lb/1000 ft²/24 hr |
| pH test | Surface alkalinity | Product-specific; high pH destroys some adhesives |
Rules that follow:
- Test before installing, and test at the number of locations the standard requires — not one probe in the middle of a 20,000 sq ft slab.
- A slab on grade needs a vapor retarder beneath it; omitting it makes the moisture problem permanent.
- Substrate flatness governs the finish: tile, luxury vinyl plank, and wood each have flatness tolerances, and grinding or self-leveling underlayment is the remedy.
- Acclimate wood and many resilient products in the conditioned space before installation.
Arizona's dry air cuts both directions: slabs dry relatively quickly, but wood flooring installed at ambient humidity and then exposed to a humidifier or evaporative cooler will move.
Paint and Interior Finish Classification
Painting systems are specified as a sequence — surface preparation, primer, and topcoats — and preparation is where the money is. New gypsum needs a drywall primer/sealer to equalize porosity between paper and joint compound, or the joints flash through the topcoat. Bare masonry needs a block filler. Ferrous metal needs a corrosion-inhibiting primer, and it needs it before the monsoon, not after the flash rust.
Interior finish classification is the code side and a reliable exam item. IBC Chapter 8 classifies interior wall and ceiling finishes by flame spread index measured under ASTM E84:
| Class | Flame spread index | Smoke developed |
|---|---|---|
| A | 0–25 | 0–450 |
| B | 26–75 | 0–450 |
| C | 76–200 | 0–450 |
The required class depends on occupancy and on where in the means of egress the finish sits. The general pattern is that the most restrictive class is required in exit enclosures and exit passageways, a less restrictive class in corridors, and the least restrictive in rooms and enclosed spaces — and sprinklered buildings are permitted a reduction in many occupancies.
Exam tip: questions here usually give a location and ask which class applies, or give a wall covering's flame spread index and ask for its class. Memorize the three numeric bands — 0–25, 26–75, 76–200 — and remember that egress enclosures are always the strictest location.
A specification calls for level 4 gypsum finish, but the design has floor-to-ceiling glass on the north wall and dark semi-gloss paint. What should the contractor do?
A one-hour rated wall assembly specifies 5/8-inch Type X board with fasteners at a stated spacing. The crew installs 5/8-inch Type X but doubles the fastener spacing to save time. What is the effect?
Where must a weep screed be located in a three-coat stucco assembly?
A wall covering tests at a flame spread index of 60 under ASTM E84. What interior finish class is it?