Trim Out a Window: Build the Jamb Box, Shim It Square, Then Case It

The Problem

Trimming a window has two failure points: jamb extensions installed piece by piece that end up out of plane with each other, and casing run before the jamb is actually square — after which every miter is a negotiation. The fix is sequencing: build, then set, then case.

The Technique

Build the jamb as one box:

1. Measure the window's width, height, and jamb depth — the distance from window frame to wall face. Cut the jamb boards to length and rip them to the measured depth (7⅛" in this house).
2. Glue and nail the four jamb boards together into a complete box on the bench.
3. Mark each corner a set distance in — a quarter inch — as the registration marks for where the casing will sit later.

Set the box:

4. Place the box in the opening and shim the two bottom corners first, keeping the margin to the window frame even on both sides.
5. Nail off the shimmed corners with a 15-gauge trim gun, then shim the middle of each run, checking with a level so nothing bows.
6. Do the top corners, then shim and nail everything else off. Trim the shims flush with an oscillating multi-tool.

Case it:

7. Cut the casing to length with 45° miters. Install the bottom piece first — brads on the inside edge into the jamb, 16-gauge nails on the outside into framing.
8. Glue the miter faces, install the two side pieces, and finish with the header.