What You Need to Do Before You Start
Door casing — the trim that frames the sides and top of a doorway — can be replaced without removing the door itself. You'll need a pry bar, a utility knife, a miter saw or hand saw, a nail gun or hammer, finishing nails, wood filler, sandpaper, and paint or stain that matches your existing trim. The job takes two to four hours for a single doorway, depending on how firmly the old casing is attached and whether you're painting or staining.
Before you begin, decide whether you're replacing the casing on one side of the door (the side facing the room) or both sides. Interior doors usually have casing on both sides; exterior doors may have casing on the inside only. Measure the height and width of the existing casing so you can buy the right profile and width at a home center. Bring a photo or a sample piece if you want to match the style exactly.
Key Takeaways
- Remove old casing by cutting the paint seal with a utility knife, then prying it away from the door frame with a flat bar, working from the top down.
- Pull out all nails left behind using a pry bar or nail puller, and sand the frame smooth before installing new casing.
- Cut new casing pieces at 45-degree angles where they meet (called a miter joint), using a miter saw for the cleanest cuts.
- Nail the new casing to the frame with finishing nails spaced every 12 to 16 inches, then fill nail holes and sand before painting or staining.
Removing the Old Casing Without Damaging the Frame
Start at the top of the doorway. Run a utility knife along the seam between the casing and the wall to cut through any paint or caulk that's holding it in place. This step prevents the drywall from tearing when you pry the casing away.
Insert a flat pry bar behind the casing near the top corner and gently lever it away from the frame. Work your way down one side, then repeat on the other side. The casing should come away in one or two pieces per side if the nails aren't too tight. If a piece splinters or breaks, you can still use the frame underneath — that's what matters.
Once the casing is off, use a pry bar or nail puller to remove every nail left in the frame. Tap the nail from the back side if you can reach it, or pull it straight out from the front. Sand the frame lightly with 120-grit sandpaper to smooth any rough spots or old paint drips.
Measuring and Cutting New Casing Pieces
Measure the height of the door opening from the floor to the underside of the top casing (called the head casing). Measure the width of the opening as well. You'll cut two side pieces (called legs) and one top piece (the head).
The legs run from the floor to the point where they meet the head casing. The head casing sits on top of the legs at a 45-degree angle on each end — this angled joint is called a miter. To cut a miter, set your miter saw to 45 degrees and cut the end of each leg piece at that angle. The head casing gets mitered on both ends as well.
A common mistake is cutting the legs too long. Measure twice: the leg should reach from the floor to a point about 1/16 inch below where the head casing will sit. This small gap prevents the pieces from binding when you nail them together. Cut the head casing to span the width of the opening plus the thickness of the two mitered leg ends.
Installing the New Casing
Start with one leg. Position it against the frame so the outer edge is flush with the edge of the frame and the mitered end points upward at 45 degrees. Drive finishing nails through the casing into the frame, spacing them every 12 to 16 inches. Use nails long enough to penetrate the frame by at least 1.5 inches — typically 2-inch finishing nails work well.
Install the second leg the same way on the opposite side. Then fit the head casing on top, pressing the mitered ends firmly into the mitered ends of the legs. The joint should be tight with no visible gap. Nail the head casing to the frame above the door, again spacing nails every 12 to 16 inches.
If you're casing both sides of the door, repeat the process on the other side. The casing on the opposite side does not have to match the first side exactly — many homes have different styles on the interior and exterior of a door.
Filling Nail Holes and Finishing
Once all the casing is nailed in place, use a nail set (a small metal punch) to drive the nail heads slightly below the surface of the wood. This creates a small hole that you'll fill later. Fill each hole with paintable wood filler, overfilling slightly so the filler stands proud of the surface. Let the filler dry according to the product instructions — usually 30 minutes to an hour.
Sand the filled holes smooth with 120-grit sandpaper, then wipe away the dust with a damp cloth. If the casing will be painted, explore primer and paint in the same color as your existing trim. If it will be stained, sand with 150-grit sandpaper before staining to match the rest of your woodwork.
Common Mistakes and How to Avoid Them
The most common error is cutting the miters at the wrong angle or length, which leaves visible gaps at the corners. Double-check your miter angle before cutting — 45 degrees is standard for interior casing. If a miter joint is loose after nailing, you can drive a small finishing nail through the joint itself (called a "toe nail") to pull the pieces together.
Another frequent problem is nailing the casing too close to the edge of the frame, which can split the wood. Keep nails at least 1 inch from the edge. If you're using a nail gun, set the pressure so it drives the nail without blowing through the wood.
Uneven casing — where one side is thicker or wider than the other — happens when the frame itself is out of square. Check the frame with a level before you start. If it's not plumb (perfectly vertical), you may need to shim behind the casing to make it sit flush against the frame.
When to Call a Professional
If the door frame itself is damaged, warped, or rotting, replacing the casing alone won't fix the problem. A professional can assess whether the frame needs repair or replacement before new casing goes on. Similarly, if the opening is severely out of square or the walls are uneven, a carpenter can adjust the frame to make the casing sit properly.
For exterior doors, especially those exposed to weather, consider having a professional install new casing if you're not confident in your ability to seal the joints against water. Poorly sealed exterior casing can lead to water damage inside the wall.
Frequently Asked Questions
Can I reuse the old casing if it's still in good condition?
Yes, if the casing comes off in one piece and isn't warped or split. Sand it smooth, repaint or restain it, and reinstall it using new nails. This works especially well if you're just refreshing the paint color.
What's the difference between casing and baseboard?
Casing frames doorways and windows; baseboard runs along the bottom of walls where they meet the floor. They're often the same profile and color, but baseboard is thicker and wider. Door casing is installed first, then baseboard butts up against it.
Do I have to use a miter saw, or can I use a hand saw?
A hand saw works, but a miter saw gives you cleaner, more accurate cuts with less effort. If you're cutting only a few pieces, a hand saw is fine — just mark the 45-degree angle clearly with a pencil and cut slowly to stay on the line.
How do I match the existing casing profile if I can't find the exact same trim?
Bring a photo or a sample piece to your home center and ask the staff to help you find the closest match. Many profiles are standard — ranch, colonial, and contemporary styles are widely available. If the original casing is custom or very old, a woodworker can mill a matching profile, but that costs more.
What if the mitered corners don't fit tightly?
Small gaps can be filled with paintable caulk after the casing is installed. For gaps larger than 1/8 inch, the miter angle or length is probably wrong — remove the casing and recut. Caulk is a patch, not a solution to a bad cut.