Interior Finishes

Interior finishes transform a house from a bare frame and rough utilities into a livable, attractive home. While finishes may seem cosmetic, their proper installation affects durability, indoor air quality, fire resistance, sound control, and long-term maintenance costs. This lesson covers the major interior finish systems: drywall, trim carpentry, flooring, and painting.

Drywall (also called gypsum board, wallboard, or Sheetrock — a brand name) is the universal interior wall and ceiling finish in residential construction. It is a panel made of gypsum plite (calcium sulfate dihydrate) sandwiched between two sheets of paper. Gypsum is naturally fire-resistant (the water in its crystal structure absorbs heat during a fire).

Drywall Types

Drywall Installation

Hanging (installing panels):

• Panels are installed horizontally on walls (long dimension horizontal) to minimize visible joints and to land joints on studs
• On ceilings, panels are installed perpendicular to joists
• Attached with drywall screws (1-1/4" for 1/2" board into wood framing) at 12" o.c. on ceilings and 16" o.c. on walls
• Screws must be set slightly below the surface (creating a dimple for compound) without breaking the paper face
• Joints between panels should be staggered so they do not align with the joints on the opposite side of the wall

Taping and finishing: The process of concealing joints, screw heads, and corners with tape and joint compound (mud):

1. First coat (taping): Apply a bed of joint compound, embed paper tape or fiberglass mesh tape, and cover with a thin layer of compound. Fill all screw dimples.
2. Second coat (blocking/filling): Apply a wider coat of compound over the tape, feathering the edges. Use a wider knife (8–10 inch).
3. Third coat (skim/finish): Apply the widest coat (12-inch knife or wider), feathering smoothly into the surrounding surface. The finished joint should be smooth with no visible edge or ridge.
4. Sanding: After drying, sand lightly to remove any ridges or imperfections.

Drywall Finish Levels

The Gypsum Association defines five finish levels, specified by the project requirements:

Most residential work is finished to Level 4.

Trim (also called millwork or finish carpentry) includes all the decorative and functional wood (or composite) elements that conceal joints, transitions, and edges:

• Baseboard: Trim at the base of walls, covering the gap between the drywall and the floor. Heights range from 3.25" to 7.25". Attached with a nail gun (15 or 18 gauge finish nails).
• Casing: Trim around windows and doors, covering the gap between the jamb and the drywall. Installed with finish nails. Many styles: colonial, craftsman, modern flat stock.
• Crown molding: Decorative trim at the junction of walls and ceilings. Ranges from simple cove to elaborate multi-piece assemblies. Installation requires compound miter cuts (angled in two planes simultaneously).
• Chair rail and wainscoting: Horizontal trim at mid-wall height (chair rail) and paneling below (wainscoting). Primarily decorative.
• Door and window sills/stools: Horizontal trim at the bottom of window openings (the interior sill).
• Closet shelving: Wire or wood shelving systems, typically 12" deep with a clothes rod.

Trim installation tips:

• Cope inside corners (cut one piece to fit the profile of the other) rather than miter — cope joints remain tight even when walls are not perfectly square or when wood shrinks.
• Pre-prime or pre-finish trim before installation for better paint coverage.
• Use a nail gun with appropriate gauge nails (18-gauge brad nails for small trim, 15-gauge finish nails for baseboard and casing).
• Fill nail holes with wood filler, sand, and touch up with paint.

Hardwood Flooring

Solid hardwood (3/4" thick tongue-and-groove planks) is nailed or stapled to the subfloor. Common species: oak (most popular), maple, hickory, walnut. Finished on-site (sanded and coated with polyurethane) or pre-finished (factory-applied finish). Hardwood is sensitive to moisture — it expands in humid conditions and contracts in dry conditions. A 1/2-inch expansion gap is left at walls (hidden by baseboard).

Engineered hardwood has a thin hardwood veneer over a plywood or HDF core. More dimensionally stable than solid hardwood, suitable for installation over concrete slabs and with radiant heat.

Luxury Vinyl Plank (LVP) / Luxury Vinyl Tile (LVT)

Rigid or flexible vinyl planks or tiles that click-lock together (floating installation — not attached to the subfloor). Waterproof, durable, and available in realistic wood and stone appearances. The fastest-growing flooring category due to its practicality, water resistance, and moderate cost. Suitable for all rooms including kitchens and bathrooms.

Ceramic and Porcelain Tile

Tiles set in thin-set morite adhesive on a suitable substrate (cement board, concrete, or uncoupling membrane). Grout fills the joints between tiles. Tile is waterproof (when properly installed with waterproofing membrane in wet areas), extremely durable, and available in unlimited styles. Common in bathrooms, kitchens, and entries.

Substrate requirements: The substrate must be rigid — tile cracks over flexible surfaces. The deflection of the floor structure under load must not exceed L/360 for ceramic tile (more stringent than the standard L/240 for other finishes). This requirement often drives the selection of deeper joists or closer spacing in tile areas.

Carpet

Carpet and pad are stretched over tack strips at the perimeter of the room. Carpet provides comfort, sound absorption, and warmth. Common in bedrooms and living areas. Not suitable for wet areas.

Interior painting is typically the final finish operation. The standard system for residential interiors:

1. Primer: A preparatory coat that seals the drywall, provides adhesion for the topcoat, and blocks stains. PVA (polyvinyl acetate) primer is standard for new drywall. Stain-blocking primer (shellac or latex stain-blocker) is used over knots, water stains, and smoke damage.
2. Topcoat (finish paint): Two coats of interior latex (water-based) paint. Sheens range from flat (no sheen, hides imperfections, standard for ceilings) to eggshell/satin (low sheen, washable, standard for walls) to semi-gloss (moderate sheen, water-resistant, standard for bathrooms, kitchens, and trim) to high-gloss (maximum durability, for trim and doors in high-use areas).

Paint application:

• Cutting in (brushing edges) before rolling the field
• Roll in a W-pattern, then smooth with parallel strokes
• Maintain a "wet edge" to avoid lap marks
• Allow full drying between coats (typically 2–4 hours for latex)