Window casing — the trim frame that surrounds a window on the interior — is one of the highest-impact finish carpentry upgrades you can make in a GTA home. A bare window opening with no trim looks unfinished; crisp casing immediately signals quality. Better still, installing window trim is a manageable DIY project once you understand the key skill: marking the reveal. Get that right and everything else follows.
This guide covers the full process for a typical GTA home window — from marking the reveal on the jamb through nailing, filling, caulking, and painting. For related finish work, see our guide to installing baseboards in a GTA home and our subfloor installation guide and our room painting guide for how to prep and paint once the trim is up.
Materials and 2026 GTA Pricing
Window casing comes in two main materials: MDF (medium-density fibreboard) and solid wood. Pre-primed MDF is the practical choice for most GTA renovations — it arrives ready to paint, costs less, and won't expand or contract with humidity the way solid wood does. Solid wood (pine, poplar) is preferred if you're staining the trim or matching existing wood millwork.
🛒 Materials Needed
- MDF casing trim (pre-primed): $1.50–$3.00/lf
- Solid wood casing (pine/poplar): $3.00–$6.00/lf
- 1 window needs approx. 12–14 lf of casing
- Finish nails (1-1/4" and 2"): ~$8/box
- Wood filler / spackle: ~$10
- Paintable caulk: $6–$8/tube
- Primer (if using solid wood): $20–$30/can
- Paint (trim paint, semi-gloss): $30–$50/can
🔧 Tools Needed
- Miter saw (or miter box for hand cuts)
- Pneumatic finish nailer (rental: $40/day) or hammer
- Combination square (set to 3/16" for reveal)
- Tape measure
- Pencil
- Caulk gun
- Putty knife
- Sandpaper (120 and 220 grit)
| Item | Unit | 2026 GTA Cost (CAD) |
|---|---|---|
| MDF casing (pre-primed) | Per linear foot | $1.50–$3.00 |
| Solid wood casing (pine/poplar) | Per linear foot | $3.00–$6.00 |
| Finish nails (box of 1,000) | Per box | ~$8 |
| Wood filler | Per tub | ~$10 |
| Paintable caulk | Per tube | $6–$8 |
| Nail gun rental | Per day | ~$40 |
| 10-window home — total materials | Approx. | $200–$450 |
💡 Pre-Primed MDF Saves Significant Time
Pre-primed MDF casing from Home Depot or RONA arrives ready for a topcoat — you can apply your trim paint directly without sanding or priming the face. This alone saves 1–2 hours per room compared to unprimed wood. Seal the back of the MDF pieces with a quick brush coat of primer before installing to prevent moisture absorption from wall drywall, especially in bathrooms or kitchens.
The Key Skill: Marking the Reveal
The reveal is the small setback between the inside edge of the casing and the interior face of the window jamb. The standard reveal is 3/16" (about 5 mm). This deliberate offset serves two purposes: it looks better than flush (the shadow line reads as intentional detail), and it hides minor imperfections in jamb plumb and squareness.
Mark the reveal on all four sides of the jamb using a combination square set to 3/16". Run a pencil line along the full height of both side jambs and the full width of the head jamb. These lines are your alignment guide when nailing the casing in place.
⚠️ Don't Skip the Dry Fit
Always test-fit all three casing pieces dry — without nails — before you nail anything. Check that the 45-degree mitre joints close tightly at both top corners. Gaps at the mitre joint are visible and hard to fix after nailing. If there's a gap, adjust your cut before committing. A 1-degree correction on the miter saw closes most mitre gaps. Do not rely on caulk to fill a poor mitre joint — it will look exactly like what it is.
Step-by-Step: Installing Window Casing
Mark the Reveal on All Jamb Edges
Set your combination square to 3/16" (the distance from the end of the ruler to the stop). Run it along the interior face of the head jamb and both side jambs, drawing a pencil line. This line is where the inside edge of your casing will sit. Be consistent — if your reveal wanders between 1/8" and 1/4" around the window, the trim will look off even if the cuts are perfect. Take 5 minutes to mark all four sides carefully.
Measure and Cut the Head Casing
The head casing (horizontal top piece) is the first piece to install. Measure the distance between the reveal lines on the two side jambs. Add the width of one casing piece on each side — the head casing extends past the reveal marks to overlap where the side casings will meet it at 45-degree angles. Cut the head casing with 45-degree mitre cuts on each end, pointing outward (like a picture frame). Make the piece slightly long on the first cut, then trim to fit precisely. Mark which way the mitre goes before cutting to avoid a reversed cut.
Cut the Two Side Casings
Each side casing gets a square (90-degree) cut at the bottom — this sits on the floor, sill, or apron depending on the window type — and a 45-degree mitre cut at the top to meet the head casing. Hold the side casing up against the jamb with its inside edge on the reveal line, and mark the top cut length where it meets the head casing mitre. Cut at 45 degrees. The mitre on the side casing and the mitre on the head casing should form a tight closed joint when held together. Test this at both corners before cutting the second side casing.
Dry-Fit All Three Pieces
Hold all three pieces in position simultaneously — head casing at top, two side casings below — and check: (a) inside edges align with reveal lines, (b) mitre joints close tightly at both top corners, (c) bottom of side casings sit flat. Have a helper hold pieces if working alone or use painter's tape to temporarily hold pieces in place. Adjust any cuts before nailing. This is the most important step — a minute of checking saves an hour of repair.
Nail to Jamb, Then to Wall Framing
Start with the head casing. Use 1-1/4" finish nails to tack the inside edge of the casing to the jamb — one nail every 16". Then use 2" finish nails to nail the outside edge through the drywall into the wall framing (king stud or header). Nail the side casings the same way: inside edge into jamb with 1-1/4" nails, outside edge into framing with 2" nails. Drive nails every 16" along the length. At the top corners, drive a nail through the face of the mitre joint to close it — a 1-1/4" nail through the head casing into the end grain of the side casing pulls the joint closed.
Fill Nail Holes and Caulk the Gaps
Fill all nail holes with wood filler or spackle. Let dry completely (30 minutes for most products), then sand flush with 120-grit sandpaper. Apply paintable caulk to the joint between the outside edge of the casing and the drywall wall surface — this gap is always visible and must be caulked for a professional finish. Also caulk the mitre joints if there are any small gaps. Smooth caulk with a wet finger. Do not caulk the inside edge (casing-to-jamb joint) — leave that joint visible, as it is the reveal.
Prime and Paint
If using pre-primed MDF, apply two coats of trim paint (semi-gloss or satin) directly — no additional primer needed on the face. If using unprimed solid wood, prime all surfaces first with a quality trim primer, let dry, sand lightly with 220-grit, then apply two finish coats. Use a 2" angled brush for trim work. Paint the casing before painting the adjacent wall — it's easier to cut a clean line at the wall-casing boundary with wall paint than to paint the wall first and cut in at the casing. See our room painting guide for the full sequence.
Profile Choices: Colonial vs. Craftsman vs. Flat
GTA homes span a wide range of architectural styles, and trim profile should match the home's era and style. Colonial (ogee) profile is the traditional Ontario standard with its curved S-shape detail — found in most pre-2000 GTA homes. Craftsman (flat with slight easing) is the dominant profile in new construction and renovations since the mid-2010s — it reads clean and modern. Flat/square-edge casing is the most contemporary option, used in minimalist and high-design interiors. All three profiles are widely available at Home Depot and RONA in MDF and solid wood. Match the new window trim profile to whatever is on your baseboards for a consistent look.
Frequently Asked Questions: Window Trim Installation GTA
❓ What is the reveal on window trim?
The reveal is the 3/16" setback between the inside edge of the casing and the edge of the window jamb. You mark it with a combination square. It creates a small shadow line, hides imperfect jambs, and makes the trim look intentional rather than crammed flush. Standard reveal is 3/16" — some carpenters use 1/4" in older homes with rougher framing.
❓ Inside vs. outside corner trim — are they different?
Window casing uses mitre joints at the top two corners — these are outside corners of the jamb frame. Coped joints are for baseboard and crown at inside wall corners, not for window trim. For a standard GTA window, cut 45-degree mitres at the top two corners where head casing meets side casings. That's it.
❓ What size nails for window trim?
Two sizes: 1-1/4" finish nails for the inside edge into the jamb, and 2" finish nails for the outside edge into wall framing. Using smaller nails at the jamb prevents splitting the jamb material. Using longer nails at the wall edge ensures the casing is anchored to framing and won't work loose over time.
❓ Should I paint trim before or after installing?
For pre-primed MDF: apply a first coat before installation, then touch up cut ends and nail holes after. For stained solid wood: install first then finish. Pre-painting MDF is significantly faster than cutting in after installation and is the approach used by most GTA finish carpenters on painted trim jobs.
❓ Cope vs. mitre at corners — which should I use?
For window casing corners: always mitre. Cope joints are for baseboard and crown at inside wall corners. Window casing corners are outside corners of the jamb frame, where a mitre is the correct joint. Don't overcomplicate it — cut 45-degree mitres at the two top corners and you're done.