Moldings Skill Level: Beginner (Level 1) · Estimated Time: 20-30 Minutes Per Door

Introduction #

Exterior door casing is the trim that frames a door opening. It covers the gap between the door jamb (the frame the door hangs in) and the wall around it. Casing does two jobs. It hides the rough framing, insulation, and shims (thin wedges used to plumb and lock the jamb in place) behind the jamb. And it gives the doorway a clean, finished look that matches the rest of the room's trim. Whether you're on a production tract home or a custom build, tight, even casing is one of the first things people notice about a finish carpenter's work.

In new construction, exterior doors usually get casing on one side only: the interior side. That's because the exterior side is already finished. Most pre-hung exterior doors arrive with brickmold (the exterior trim that frames the door from outside) already attached. Or the siding crew added exterior trim during framing and siding. Your job as the finish carpenter is the interior side only: wrap that jamb with trim that matches the rest of the interior molding package.

You'll often hear the term "bang door" on job sites. A bang door is a temporary slab, usually a beat-up old door, hung in the frame during rough construction. It protects the opening so framers, drywallers, and painters don't damage the real finish slab (the actual door that stays). When you show up to trim, the bang door may or may not have been swapped for the finish slab yet. Either way, before you nail any casing, check three things: the jamb is plumb, the wall is flush with the jamb or sits slightly proud of it, and you have room to set a consistent reveal.

Casing one side of an exterior door is one of the most basic trim tasks you'll do, and the skills carry straight over to other jobs. A consistent reveal, tight miters, and the right nailing sequence are the same techniques you'll use for window casing, two-side door casing, and bigger molding assemblies.

Before You Begin #

Prerequisites #

• Before you install exterior door casing, the exterior door must be hung, shimmed, and secured in the rough opening (the framed opening in the wall before any trim goes on)
• Drywall around the door opening must be hung, taped, and finished
• The finish slab should be installed. If it isn't yet, the bang door needs to be adjusted so the jamb sits in its final position
• The jamb should be flush with the finished wall surface. Check both side jambs and the header (the horizontal piece across the top)
• If the job specs call for pre-painted trim, the jamb and wall should already be painted or primed
• Flooring should be installed. If it isn't, get the finished floor height so you can set the correct length for the casing legs (the vertical side pieces)
• Know the baseboard thickness and style ahead of time. That decides how the casing ends at the bottom, whether it lands on a plinth block (a small foot block the casing rests on) or runs straight to the floor

What You Need To Know #

• The standard reveal (the small offset between the edge of the jamb and the casing) is 3/16" to 1/4". Your builder or project lead may want a different reveal, so ask before you start marking
• Standard 3-piece casing uses 45-degree miter joints (angled cuts that meet to form a corner) at the top corners, per the This Old House door casing guide
• Casing must follow the customer's approved style; verify the profile and header layout against the job spec before you start. This guide covers the core style, 45-degree miter joints at the top corners. Other header styles are covered in separate Variant guides off this core process.
• Always dry-fit all three pieces (hold them in place without nailing) before you nail anything. This lets you check that the miters close tight and the reveal stays consistent
• Casing gets nailed with one nail size: 18-gauge brads, 1-3/4" long, into the outside edge, the wall framing side. Casing brads never go into the jamb
• Exterior door jambs are usually wider than interior door jambs (4-9/16" or 6-9/16" versus 4-9/16") because exterior walls are thicker
• If the jamb is not flush with the wall, fix it before you case the door. If drywall is proud (sticking out past the jamb), pound it back flush with a hammer first, then cut back with a multi-tool or rasp if needed, never a block plane; or add jamb extensions (thin strips that widen the jamb) if the jamb sits back from the wall

Tools Required #

Power Tools #

• Miter saw (10" or 12" compound miter saw) - for cutting accurate 45-degree and 90-degree cuts
• Brad nailer (18-gauge) - for every fastener in this guide
• Air compressor with hose - to power pneumatic (air-driven) nailers, if you're not using cordless battery nailers

Hand Tools #

• Tape measure - 16' or 25' tape for measuring casing lengths
• Combination square - for marking a consistent reveal on the jamb
• Pencil - sharp carpenter's pencil for marking reveal lines and cut marks
• Utility knife - for scoring marks and back-cutting miters
• Nail set - for driving nail heads below the surface of the casing
• Speed square - for quick reference and marking 90-degree cuts
• Hammer - for setting nails and making minor adjustments
• Oscillating multi-tool (optional) - for cutting back proud drywall if a hammer and rasp aren't enough

Supplies #

• 18-gauge brad nails, 1-3/4" - for every fastener in this guide
• Wood glue (Titebond II or III for exterior applications) - for gluing miter joints before nailing

Materials #

• Casing material. MDF, finger-jointed pine, and poplar are the common choices for paint-grade casing. For stain-grade casing, alder, oak, and maple are the usual options. Species isn't critical for paint-grade work, but on stain-grade casing, get the miter joint tighter than usual: sand the joint smooth and use only a little filler if needed, don't rely on filler to close a gap.

Install Exterior Door Casing: Step-by-Step Process #

Step 1: Inspect the Door and Set Reveal Lines #

Before you install exterior door casing, inspect the door opening and mark your reveal lines. This step sets up everything that follows. If your reveals are off, every joint and every alignment after this will suffer for it. Take your time here. A few extra minutes of setup saves you from fighting the trim later.

Inspect the Opening #

• Check that the door jamb is flush with the finished wall on all three sides: both legs (the vertical side jambs) and the header. Lay a straightedge or the flat of your combination square across the joint. The blade should sit flat on both surfaces at the same time.
• If the drywall is proud of the jamb (it sticks out past it), pound it back flush with a hammer first. If that's not enough, cut it back with a multi-tool or rasp it down until it's flush. A Surform rasp (a coarse hand tool for shaving down material fast) works well for this. Don't skip this step: casing is kerfed on the back so the outside edge and the jamb-side edge can both seat cleanly, and proud drywall in the middle rocks the casing like a teeter-totter if you don't deal with it first.
• If the jamb is proud of the wall (it sticks out past the drywall), that's a jamb adjustment issue, not a casing task. Stop and get it adjusted before you case the door: see How to Adjust an Exterior Door.
• Open and close the door to check that it swings freely. If it binds or rubs, the jamb may need adjusting before you case it. Don't case a door that doesn't work right.
• Check the hinge-side jamb leg (the side the hinges are on) and the strike-side jamb leg (the side with the latch) with a level. They should be plumb, or very close to it. A small out-of-plumb condition, 1/8" or less over the height of the door, is fine. The casing can flex enough to cover it.

Mark the Reveal #

• Set your combination square to the specified reveal, typically 3/16" for most production work, though some builders prefer 1/4". Lock the blade at that measurement.
• Hold the head of the combination square against the face of the jamb. The end of the blade marks your reveal line, the edge where the casing will sit.
• Make small pencil marks every 12" to 16" along both side jambs and across the header jamb. You don't need a solid line, just enough marks to guide your eye when you position the casing.
• Pay extra attention to the corners where the side jambs meet the header. Mark the reveal clearly at these points. This is where your miter joints will land, and the reveal has to be consistent here for the miters to look right.

Pro Tip: Some experienced carpenters skip the combination square and use a reveal gauge instead: a small block of wood or plastic cut exactly to the reveal dimension. You hold it against the jamb edge and butt the casing against it. This speeds things up a lot when you're casing dozens of openings on a production job. You can make your own from a scrap piece of 3/16" plywood or hardboard.

Step 2: Measure and Cut Side Casings #

With your reveal lines marked, the next step is measuring and cutting the two side casings. These are the vertical pieces that run from the floor, or the top of the plinth block if you're using one, up to the header. Accuracy matters here because the side casings set the miter angle at the top. If the sides are the wrong length, your miters won't close right.

Measure the Side Casings #

• Measure from the finished floor (or the top of the flooring, if it's installed) up to the reveal mark at the corner where the side jamb meets the header jamb.
• If flooring isn't installed yet, ask the GC (general contractor) or project lead for the finished floor height, and measure from that point. Get this wrong and you'll either have a gap at the bottom or need to re-cut.
• Add 1/16" to your measurement. That extra length makes the miter joint close tight: the casing "springs" slightly (flexes under a little pressure) when you nail it, and that pressure keeps the miter joint pulled together.
• Measure both sides independently. Door openings are rarely perfectly square, and the two sides are rarely the exact same height, so don't assume the left and right casings match.

Cut the Side Casings #

• Cut the bottom end of each side casing square (a straight 90-degree cut). This end sits on the floor or on top of the plinth block.
• Cut the top end at a 45-degree miter. On a mitered end, the long point is the tip of the angled cut that sticks out further, and the short point is the tip that's cut back shorter. For the left side casing, the long point faces the wall side (the back). For the right side casing, the long point also faces the wall side. The short point on both faces the door (the jamb side).
• Always cut with the casing face-up on the miter saw. This keeps the blade from chipping out (splintering) the visible face. If you're still getting chip-out, put a strip of painter's tape across the cut line before you cut.
• After cutting, hold each piece up to the door in its installed position. The bottom should sit on the floor. The inside edge should line up with your reveal marks. The miter at the top should land right at the corner reveal mark.

Pro Tip: When you cut the 45-degree miter, let the blade come to full speed before you lower it into the material, then feed it through slowly and steadily. Rushing a miter cut leaves a rough, splintered face that won't close cleanly. A sharp, fine-tooth blade (80 teeth on a 10" saw) makes a big difference in miter quality.

Step 3: Install Side Casings #

With both side casings cut and dry-fitted, you're ready to install the side pieces. The nailing pattern and sequence matter. Done right, the casing pulls tight to the jamb and wall, and the top miter stays in position for a clean joint with the header.

Position and Tack #

• Start with the hinge side of the door. Hold the side casing in position with the inside edge lined up on your reveal marks and the bottom flat on the floor.
• Drive one 18-gauge brad, 1-3/4" long, all the way through near the middle of the casing, into the outside edge (the wall framing side), not the jamb. This is a full nail, not a partial tack, but it still lets the piece shift slightly if the reveal needs adjusting.
• Check the reveal along the full length of the casing. Adjust it if needed before you add more nails. The casing should follow your reveal marks all the way from bottom to top.

Nail the Side Casings #

• Nail the top of the side casing first. Make sure the corner is going to line up with the header before it's committed.
• Then nail the very bottom, confirming the length is right and the reveal looks good down there.
• Finish by nailing off the middle. Use 18-gauge brads, 1-3/4" long, spaced every 12" to 16", all into the outside edge (wall framing side), never into the jamb. Keep nails 3/4" to 1" from the edges to keep the wood from splitting, and stop about 2" short of the mitered end, since that's the thinnest, most split-prone part of the piece.
• Repeat for the strike side casing. Double-check that both side casings hold a consistent reveal and that the top miters sit at the right height for the header.

Pro Tip: Nail the top first so the miter corner locks into position, then the bottom so the length and reveal are confirmed there too, and finish with the middle. Nailing the ends first instead of the middle keeps both reference points, the miter at top and the floor line at bottom, locked in before you commit the rest of the piece.

Step 4: Measure and Cut the Header #

The next step is the header: the horizontal piece that spans the top of the door and connects the two side casings with miter joints at each end. This is the piece where precision matters most. The miters on the header have to match the miters on the side casings exactly for a tight, gap-free joint.

Measure the Header #

• With both side casings nailed in place, measure from the outside edge of the left side casing to the outside edge of the right side casing. This is your "long point to long point" measurement for the header.
• Alternatively, you can measure from the reveal mark on the left side of the header jamb to the reveal mark on the right side, then add the width of the casing twice (once for each side). Both methods should give you the same number.
• Add 1/16" to your measurement. Just like the side casings, cutting the header slightly long creates spring pressure that keeps the miter joints tight. This matters even more on the header because gravity works against you: the weight of the piece wants to pull the miters open.

Cut the Header #

• Cut both ends of the header at 45-degree miters. The long point is on the bottom edge (the side facing the door), and the short point is on the top edge (the side facing the ceiling). This is the opposite orientation from the side casings.
• Double-check the miter direction before you cut. A common mistake is cutting the miter the wrong way on one end, which wastes material. Picture how the piece sits above the door before you cut.
• Dry-fit the header by holding it up against both side casing miters at the same time. Both joints should close tight with light pressure. If one joint is open on the face and tight at the back, your miter angle is slightly off. Adjust the saw and re-cut.

Pro Tip: Cut the header 1/16" long so the miters close under spring tension. When you push the header into position, it should take a little bit of force to seat. That spring pressure is what keeps the miter joints pulled together over time, as the wood expands and contracts with seasonal humidity changes. If the header drops right in with no resistance, it's too short, and the miters will eventually open up.

Step 5: Install the Header and Finish #

This is the final step. Setting the header locks the miter joints and completes the casing. Take your time with the miter joints. A little extra care here gives you results that look clean and hold up for years.

Glue and Set the Header #

• Apply a thin, even layer of wood glue to both miter faces on each joint: glue on the side casing miter and on the header miter. Spread it with your finger to cover the whole face. Don't overdo it. A thin film is all you need. More glue just means more squeeze-out to clean up.
• Press the header up into position, seating both miters at the same time. You may need a helper to hold one end while you position the other, especially on wider doors.
• The header doesn't always go in exactly as cut. You may need to back-bevel the ends slightly or adjust the miter angle a touch to get a tight fit against both side casings, especially if the opening isn't perfectly square.
• Line up the header with the reveal marks on the header jamb. The inside edge of the header casing should match the reveal you set on the sides.
• Drive one 18-gauge brad, 1-3/4" long, all the way through at the center of the header, into the framing above the door, not the header jamb, then check both miter joints. Adjust if needed before you finish nailing.

Nail the Header and Lock the Miters #

• Nail the header with 18-gauge brads, 1-3/4" long, into the framing above the door, never into the header jamb.
• Space nails every 12" to 16", starting from the middle and working toward the ends.
• Glue does the holding at the miter, not cross-nailing. Apply a liberal bead of glue to both miter faces and pull the joint tight; skip the angled cross-nail brads through the joint. This matters most on MDF, since nails don't hold well in MDF's end grain and a cross-nail there doesn't add the strength it would in solid wood. Once the glue is dry, putty and sand the joint smooth, on every miter, no exceptions.
• Wipe off any glue squeeze-out immediately with a damp rag. Dried glue is much harder to remove and can show through paint.

Set the Nails and Close Out #

• Set all nail heads below the surface using a nail set. Most finish nailers have an adjustable depth setting, but some nails still sit a little proud. Tap them down with the nail set and hammer. Run your fingers along both edges of the casing to catch anything your eye misses.
• Check both miter joints one more time. If a gap is bigger than 1/32", re-cut and reset the piece instead of planning to fill it. A miter that only looks closed because it's filled will open back up as the house moves.
• Confirm the reveal is still consistent along both sides and the header before you move on.
• Sweep up your cutoffs and clear the floor around the opening.

Filling the fastener holes and sealing the seam where the casing meets the wall belong to the painter. He fills and coats the whole room in one pass with one set of materials, so spot-filling ahead of him flashes differently under the finish coat and gets reworked anyway. Miter joints are different: glue, putty, and sand those yourself, since a wood-to-wood joint needs to be flush and clean before the painter ever gets to it. What he needs from us is tight, glued miters that are puttied and sanded smooth, a consistent reveal, casing flat to the wall and jamb, fasteners set below the surface, and a swept floor.

Pro Tip: Glue is what keeps a miter tight over the long run, not cross-nailing. Wood moves with humidity: it expands in summer and shrinks in winter. A liberal bead of glue on both miter faces, pulled tight, holds better than a cross-nail, especially in MDF, where nails don't grip the end grain well. Putty and sand every miter once the glue is dry.

Quality Check #

Run through this checklist after you install exterior door casing on every opening. Catching issues now is far easier than fixing them after paint.

• After you install exterior door casing, the reveal is consistent (3/16" or as specified) along both sides and header
• Both miter joints are tight with no visible gaps on the face
• Miter joints are glued, tight, and puttied and sanded smooth
• Casing sits flat against both the jamb and the wall - no rocking or gaps behind the trim
• All nail heads are set below the surface
• No glue squeeze-out visible at the miter joints
• The door still opens, closes, and latches properly after casing is installed
• Bottom of side casings sits tight on the floor or plinth block with no visible gap

Install Exterior Door Casing Troubleshooting #

Miter Joint Is Open on the Face but Tight at the Back #

This is the most common miter problem, and it means your saw angle is slightly more than 45 degrees. First, check your miter saw. Is it actually set to exactly 45.0 degrees? Even half a degree off shows at the joint. If the saw is accurate, the wall or jamb probably isn't perfectly square. The fix is to back-cut the miter: use a utility knife to hollow out the back of the miter face slightly. That lets the front edges meet first and close tight. Only remove a tiny amount of material. You're adjusting by thousandths of an inch, not sixteenths.

Casing Rocks and Won't Sit Flat Against the Wall #

If the casing rocks, it almost always means the wall surface isn't flat or the jamb isn't flush. Run a straightedge across the area where the casing will sit. If you see a hump in the drywall, shave it down with a Surform rasp or a drywall sanding screen. If the wall has a dip, shim behind the casing at the low spot: slide a thin wood shim between the casing and the wall, then nail through both. The shim fills the gap and the nail locks it in place. After nailing, trim the shim flush with the casing edge.

Jamb Is Not Flush with the Finished Wall #

This is a framing or drywall issue you need to fix before you install exterior door casing. If the jamb is recessed (the wall is proud of it), your best option depends on how bad the gap is. For gaps of 1/8" or less, you can sometimes flex the casing to bridge the difference, but that's not ideal. For bigger gaps, you need a jamb extension: a thin strip of wood ripped (cut to width, lengthwise along the grain) to match the gap, then glued and nailed to the jamb edge to bring it flush with the wall. If the jamb is proud of the wall instead (it sticks out past the drywall), that's the same jamb adjustment issue covered in Step 1: stop and get it adjusted before you case the door.

Casing Splits When Nailing Near the End #

MDF rarely splits when you install exterior door casing, but solid wood, especially hardwoods like oak and maple, can split when you nail too close to the end or edge. The fix is to keep nails at least 3/4" from any edge and at least 1" from the end of the piece. If you have to nail closer to the end (to lock a miter joint, for example), pre-drill with a bit slightly smaller than the nail shank (the body of the nail). You can also blunt the tip of the nail with a hammer tap. A blunted point crushes the wood fibers instead of wedging them apart, which cuts down on splitting a lot.

Miter Opens Over Time After Installation #

If miters open weeks or months after you install exterior door casing, the cause is wood movement from humidity changes. That's normal, and the real question is whether you took steps to prevent it during installation. First, make sure you glued every miter joint. Second, make sure the joint got puttied and sanded once the glue dried, the way Step 5 describes. Third, if you're in a climate with big humidity swings, consider a wood that's more dimensionally stable (it swells and shrinks less), like poplar or MDF, instead of a species that moves a lot, like maple or red oak. If the joint opens after the door is already installed and painted, that's a finish callback (a follow-up fix handled at the paint stage), not a reinstall. A hairline gap that shows up months later from seasonal movement is exactly what the painter seals during a touch-up. Driving fresh nails through cured, painted trim won't reliably pull it back closed. Prevent it up front instead, the way Step 5 describes: glue the miter and pull it tight while it's fresh, then putty and sand it smooth.

Related Guides #

• How to Install Baseboard -- Learn the fundamentals of baseboard installation, which shares many techniques with door casing including miter cutting and nail patterns
• How to Case a Window -- Window casing uses the same reveal and miter techniques as door casing but with four mitered corners instead of two
• How to Install a Pre-Hung Door -- Understanding how the door was hung helps you troubleshoot jamb alignment issues before casing
• How to Trim a House -- The complete overview of finish carpentry workflow, showing where door casing fits into the overall trim sequence