Window trim (casing) frames the window opening and covers the gap between window and wall. The job is similar to door casing but with one extra component: the stool (interior sill that the window rests on) for traditional bottom-trim styles. Modern picture-frame casing skips the stool and treats all four sides identically.

This guide covers two common styles: traditional with stool and apron (bottom trim) on the underside of the stool, and picture-frame (four matching sides). Both styles are within Intermediate DIY range.

Before you start

Three things to settle:

1. Traditional vs picture-frame. Traditional: the bottom of the window has a horizontal 'stool' (a flat piece extending into the room), with an 'apron' below it. The two side casings sit on top of the stool. Top casing spans the width above. This is the classic look in most older Canadian homes. Picture-frame: four pieces of casing form a uniform rectangle, mitred at the corners. Modern look, simpler installation. Pick based on existing trim style in your home.

2. Reveal. The 'reveal' is a small intentional offset between the window jamb and the casing — typically 1/4 inch all around. Without a reveal, the trim sits flush with the jamb, which looks unfinished. Mark and maintain reveal during install.

3. Mitre angles for picture-frame. All four corners are 45-degree mitres meeting at 90 degrees. Test cuts on scrap first — mitre saws drift slightly out of true and need adjustment for clean joints.

What you'll need

Tools

Mitre saw
Tape measure
Pencil and combination square
Brad nailer or finish nailer (18-gauge)
Stud finder
Caulk gun
Sander or sanding block

Materials

Window trim/casing (matching existing or new style)
Stool material (for traditional style — typically thicker than casing)
Apron material (for traditional style)
18-gauge brad nails (1.5-2 inches)
Wood filler
Painter's caulk
Primer and paint

Maintain a uniform 1/4 inch reveal

The reveal — the small visible strip of jamb before the casing starts — must be uniform around the entire window. Use a combination square set to 1/4 inch as a gauge: hold against the jamb, mark casing position, repeat at multiple points.

Inconsistent reveal (1/4 inch on one side, 1/8 on another) is immediately visible from across the room. Take time to set the reveal correctly.

Step-by-step instructions

Mark the reveal lines

Set a combination square or speed square to 1/4 inch. Hold against each edge of the window jamb (top, both sides, bottom for traditional or all four for picture-frame).

Pencil-mark the reveal line at multiple points — these are where the casing's inside edge will sit.

Traditional: install stool first

Cut the stool (a thick horizontal piece, typically 1x4 or 1x6) to length. The stool typically extends 3/4 inch beyond each side casing's outer edge.

Notch the stool's back edge so it fits around the window's jamb. The stool sits in front of the jamb, projecting into the room.

Nail the stool to the rough framing below the window with finish nails.

Cut the side casings

For picture-frame: cut a 45-degree mitre on each end of each side piece. Length = window jamb height (inside corner of jamb to inside corner of jamb) + 1/2 inch (1/4 reveal at each end).

For traditional: side casings sit on top of the stool, with a square cut at the bottom and a 45-degree mitre at the top.

Install side casings

Position the side casing along the reveal line on the jamb. The mitred top end (or square top end for picture-frame is 90 degrees) should align with where the head casing will land.

Nail the casing to the rough framing through the casing's outer edge with brad nails. 3 nails per side casing minimum.

Verify the reveal is uniform along the casing's full length.

Cut the head casing

For picture-frame: cut 45-degree mitres on both ends. Length = top jamb width + 1/2 inch.

For traditional: square cut on both ends, length = window width + 2× side casing width.

Test fit. Mitres should meet cleanly with no gaps.

Install head casing

Position the head casing aligned with the reveal line on the top jamb. For picture-frame, mitres meet the side casings' mitres at the corners.

Nail to rough framing.

Verify reveal is uniform across the top.

For picture-frame: install bottom casing

Picture-frame uses a fourth piece on the bottom, mitred at both ends to match the side casings.

Install with same nailing pattern as the other pieces.

Traditional: install the apron

The apron is the trim piece below the stool, on the wall surface.

Cut to a length matching the stool's length (or slightly shorter). Nail to the rough framing through the wall.

The apron typically has a profile cut at each end (decorative ends) to break the visual line.

Fill nail holes and caulk seams

Fill brad holes with paintable wood filler. Sand smooth.

Caulk along the inner edge of the casings (where they meet the jamb) and the outer edge (where they meet the wall). Caulk hides slight irregularities.

Prime and paint

Pre-primed casing needs only topcoat. Raw wood needs primer first.

Two coats of trim-grade paint. Sand lightly between coats.

Common mistakes that cost you

Inconsistent reveal. Even small variation is visible. Use a square as gauge; verify at multiple points.
Mitres that don't meet cleanly. Saw drift, mismeasured cuts, or wall not square. Test cuts on scrap; adjust saw angle if needed.
Nailing into drywall only. Casing pulls free over years. Nail through casing's outer edge into the rough framing.
Skipping caulk. Slight gaps between casing, jamb, and wall are visible without caulk. Always caulk.
Wrong style for the home. Traditional trim in modern home (or vice versa) reads as misstyle. Match existing trim style throughout.

Frequently asked questions

What's the difference between casing and the window itself?

The window itself is the glass and frame in the wall opening. The casing is the trim around the inside that frames the window aesthetically and covers the gap between window jamb and wall.

The window installation is structural; casing is decorative.

Can I install window trim without a mitre saw?

Picture-frame style requires precise mitres — difficult without a mitre saw. Traditional style with butt joints can be done with hand-saw cuts but the joints aren't as clean.

Mitre saws are inexpensive to rent by the day. For a multi-window project, renting or borrowing is the right call.

What about windows with narrow walls (no room for casing)?

Some modern installations use returns — a small piece of trim that wraps from the casing back to the window jamb instead of running across the wall.

Or skip casing entirely and use 'drywall return' — the drywall extends to the window jamb with no casing trim. Modern minimalist look.

When to call us instead

Window trim throughout a renovation is part of the trim package. We do trim work as part of full renovations, with consistent style across all windows and doors.

Book a free 60-minute site visit →

Related Ottawa services

If you're updating windows and trim throughout, our residential renovation team handles windows, trim, and finishing as fixed-price scopes. Book a free 60-minute consultation.