Interior Casing and Trim: Layout, Fitting, and Fastening

Interior Casing and Trim: Layout, Fitting, and Fastening

Interior casing is the molding that frames a door or window opening on both the room side and, where applicable, the opposite side. It covers the gap between the jamb edge and the finished wall surface and gives the opening a clean, finished appearance. Doing it well requires accurate layout, clean miter cuts, and a fastening pattern that pulls the casing tight without splitting the wood.

Setting the Reveal

Before cutting a single piece of casing, you must mark the reveal — a small setback from the face edge of the jamb toward the door stop. The reveal is almost always 3/16 inch for interior doors. It serves two purposes: it creates a slight shadow line that is visually pleasing, and it gives the casing a solid flat surface to nail against rather than the fragile edge of the jamb.

Use a combination square set to 3/16 inch. Hook it over the face of the jamb and drag a pencil along the blade to scribe a continuous line on both side jambs and the head jamb. Work around all three sides of the opening without lifting the square, keeping the pencil pressure steady. The casing's inside edge will align with this line — not the jamb edge.

Sequencing the Cuts: Head Casing First

Always install the head (top) casing first, then fit the two side casings to it. This sequence lets the miter joints at the top corners close tightly from below.

Measuring the head casing: The inside length of the head casing equals the distance between the two reveal lines on the side jambs. Add the width of one casing thickness on each end to account for the miter return, then mark and cut 45-degree outside miters on both ends of the head piece — the top face of each miter faces up and the point of each miter faces outward.

Dry-fit the head casing on the reveal line. Once it seats correctly, hold it in place (or tack it with one brad) and mark the side casings directly from the mitered end of the head casing. This direct-transfer method is more accurate than arithmetic alone.

Cutting side casings: The top of each side casing is a 45-degree miter that completes the corner with the head casing. The bottom of each side casing is a square (90-degree) cut that sits on the floor or on a plinth block if one is used. Cut the miter first, then measure down to your floor or plinth height and make the square cut.

Miter Saw Safety

Before operating the miter saw:

• Wear safety glasses and hearing protection.
• Keep fingers at least 6 inches from the blade path.
• Hold the workpiece firmly against the fence and table; never freehand small cutoffs.
• Allow the blade to come to a complete stop before raising the head or removing the workpiece.
• Never reach across the blade path.

Gluing and Nailing the Miter Joints

Once the head casing is positioned on the reveal line, apply a thin, even bead of wood glue to both faces of each miter joint. Glue significantly strengthens the joint and helps prevent the gap that opens as wood moves seasonally.

With the head casing held in position, use an 18-gauge brad nailer loaded with 1-1/2-inch brads to drive a pin nail through the face of the miter — straight through the point of one casing piece and into the adjoining piece. One or two pins per corner are enough to clamp the miter while the glue sets. Do not overdrive; an 18-gauge brad is slender and will not split the miter point.

Face-Nailing Pattern

Casing is fastened with two rows of nails:

1. Jamb edge row — Use 1-1/2-inch finish nails (or 1-1/2-inch brads from the finish nailer) angled slightly toward the jamb. Space them approximately every 12–16 inches and at every miter. These nails pull the casing tight to the jamb face.

2. Wall-framing row — Use 2-inch finish nails spaced approximately every 16 inches, aligned with the rough framing behind the drywall. The rough king studs and header are your targets. These nails carry the structural load and keep the outer edge of the casing flush to the wall surface.

Adjust compressor pressure so nail heads sit just below the surface without crushing the wood fiber. Finish the holes with a nail set if using a hammer, or rely on the nailer's depth-of-drive adjustment.

Inspection and Finishing

After all casing is nailed, perform a systematic quality check:

• Reveal consistency: Measure from the jamb edge to the inside edge of the casing at three points on each side jamb and at the center of the head jamb. All measurements should be within 1/16 inch of 3/16 inch.
• Miter gaps: Use a feeler gauge to probe each corner. A gap greater than 1/16 inch is a failing condition and must be corrected by re-cutting or carefully planing the miter face.
• Flush to wall: Sight along the casing face. Any proud edge can trap paint and shadow, and typically means the framing or drywall requires shimming before reinstalling the casing.

Fill nail holes with a wood filler appropriate to the finish (paintable or stainable). Allow to dry fully, then sand lightly with fine-grit sandpaper, following the grain. Wipe away dust before priming or finishing.

General Safety Reminders

• Secure air hoses so they do not create a trip hazard.
• Never point a nailer at yourself or others; treat it as you would any tool capable of causing injury.
• Disconnect the nailer from the air supply before clearing a jam or changing tips.