Door trim (casing) frames the doorway and covers the gap between door jamb and wall. The job is similar to window trim but with one less piece (no stool/sill at bottom). Two main styles: traditional with butt-joined head casing on top, and picture-frame with mitred corners on all four sides.
This guide covers both styles.
- Skill level
- Intermediate
- Time
- 1–2 hr per door
- Steps
- 7
- Tools
- 5, listed below
- Materials
- 5, listed below
Before you start
Three things to settle:
1. Style: traditional vs picture-frame. Traditional: head casing sits on top of the side casings, butt-joined. Common in older Canadian homes. Picture-frame: all three sides (top, both sides) meet at 45-degree mitres. More modern look.
2. Reveal. Reveal is the small visible strip of jamb before the casing starts — typically 1/4 inch all around. Maintain consistent reveal across all three sides.
3. Match existing trim. Door trim should match the rest of the house's trim style. Mismatched styles read as unintentional mix.
What you'll need
Tools
- Mitre saw
- Tape measure
- Pencil and combination square
- Brad nailer
- Caulk gun
Materials
- Door casing (matching existing or new style)
- 18-gauge brad nails
- Wood filler
- Painter's caulk
- Primer and paint
Maintain 1/4 inch reveal consistently
The reveal must be uniform around the door. Use a combination square set to 1/4 inch as a gauge.
Inconsistent reveal is immediately visible. Take time to set reveal correctly before nailing.
Step-by-step instructions
Mark the reveal lines
Set a combination square to 1/4 inch. Hold against each edge of the door jamb (top, both sides).
Pencil-mark the reveal line at multiple points. These are where the casings' inside edges will sit.
Cut side casings
For picture-frame: 45-degree mitres at top end of each side piece. Length = jamb height + 1/4 inch (for top reveal).
For traditional: square cut at top end. Length = jamb height + 1/4 inch.
Install side casings
Position along the reveal line on the jamb. Top mitre (or square cut) aligned with where head casing will land.
Nail to rough framing through outer edge with brad nails. 3 nails per side casing.
Verify reveal is uniform along full length.
Cut head casing
Picture-frame: 45-degree mitres on both ends. Length = top jamb width + 1/2 inch (1/4 reveal each end).
Traditional: square cut on both ends. Length = top jamb width + 2× side casing width (extends past side casings).
Install head casing
Position aligned with reveal line on top jamb.
Picture-frame: mitres meet side casings cleanly.
Traditional: head casing sits on top of side casings, extends past their outer edges.
Nail to framing.
Fill nail holes and caulk
Fill brad-nail holes with wood filler. Sand smooth.
Caulk along inner edge of casing (where it meets jamb) and outer edge (where it meets wall).
Prime and paint
Pre-primed casing: only topcoat needed.
Two coats of trim-grade paint, sanding lightly between.
Common mistakes that cost you
- Inconsistent reveal. Visible variation in reveal width. Use a square as gauge throughout.
- Mitres don't meet cleanly. Saw angle off. Test cut on scrap; adjust if needed.
- Nailing only into drywall. Pulls free over years. Nail through casing into rough framing behind.
- Skipping caulk. Slight gaps visible. Always caulk both edges.
Frequently asked questions
Picture-frame or traditional?
Match existing trim style. If updating throughout, picture-frame is more modern; traditional is more classic.
Picture-frame's mitred corners are more demanding to fit cleanly.
Should I install trim before or after door?
After. The door must be properly hung and operating before trim, since trim covers the door frame's outer edge.
What reveal should door casing have?
Door casing usually sits back from the jamb edge with a 1/4 inch reveal, the small strip of jamb left visible before the casing starts. The reveal has to be the same on the top and both sides, because any variation is immediately visible.
Set a combination square to 1/4 inch, hold it against each jamb edge, and pencil the reveal line at several points. Those marks show where the inside edges of the casings sit.
How long should door casing pieces be cut?
Side casings are cut to the jamb height plus 1/4 inch for the top reveal, with a 45-degree mitre at the top for picture-frame style or a square cut for traditional. For picture-frame, the head casing is the top jamb width plus 1/2 inch, mitred on both ends.
For traditional, the head casing is square cut and equals the top jamb width plus twice the side casing width, so it extends past the side casings.
How do you nail and finish door trim?
Door trim is nailed through the casing into the rough framing behind, using 18-gauge brad nails, three per side casing. Nailing only into drywall lets the trim pull free over the years. Once the casings are up, fill the nail holes with wood filler and sand smooth, then caulk the inner edge where the casing meets the jamb and the outer edge where it meets the wall.
Pre-primed casing needs only two coats of trim paint, sanded lightly between.
When to call us instead
Trim throughout a renovation should match across all doors and windows. We handle trim consistently as part of larger work.
Book a free 60-minute site visit →Dream Touch Renovations · 33 Roydon Pl, Suite 210, Ottawa, ON K2E 1A3 · (613) 862-4555
Renovating homes across Ottawa, Kanata, Nepean, Barrhaven, Orléans and Stittsville.
Related Ottawa services
If you're updating trim throughout, our residential renovation team handles trim consistently as fixed-price work. Book a free 60-minute consultation.
Official references
- Hand Tools: General Hand Tool Operation, Canadian Centre for Occupational Health and Safety