Moldings #

Skill Level: Beginner (Level 1) #

Estimated Time: 20-30 Minutes Per Door #

Introduction #

Knowing how to install exterior door casing is essential — it’s the trim that frames the door opening, covering the gap between the door jamb and the surrounding wall material. It serves both a functional and aesthetic purpose — it conceals the rough framing, insulation, and shims behind the jamb while providing a clean, finished look that complements the rest of the home’s trim package. Whether you’re working on a production tract home or a custom build, when you install exterior door casing properly, it’s one of the details that separates professional-quality work from amateur hour.

In new construction, exterior doors are typically cased on one side only — the interior side. The reason is straightforward: the exterior side of the door is already finished. Most pre-hung exterior doors come from the manufacturer with brickmold or exterior trim already attached, or the exterior trim was applied during the framing and siding stages. Your job as a finish carpenter is to install exterior door casing on the interior side, which means wrapping 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 — that’s hung in the frame during the rough construction phases to protect the opening and allow the framers, drywallers, and painters to work without damaging the real finish slab. When you show up to do trim, the bang door may or may not have been swapped out for the finish slab. Either way, you need to verify that the jamb is plumb, the reveal is consistent, and the wall surface is flush with or slightly proud of the jamb before you start nailing casing.

Learning to install exterior door casing on one side is one of the most fundamental trim tasks you’ll perform, and it’s an excellent place to build your skills. The techniques you learn here — setting a consistent reveal, cutting tight miters, and nailing in the right sequence — carry directly over to window casing, two-side door casing, and more complex molding assemblies. Master this task and you’ve built a solid foundation for everything else in finish carpentry.

Before You Begin #

Prerequisites #

• Before you install exterior door casing, the exterior door must be properly hung, shimmed, and secured in the rough opening
• Drywall must be completed, taped, and finished around the door opening
• The finish slab should be installed, or the bang door must be properly adjusted so the jamb is in its final position
• The jamb should be flush with the finished wall surface — check both sides and the header
• Painting or priming of the jamb and wall should be completed if the spec calls for pre-painted trim
• Flooring should be installed or you need to know the finished floor height to set the correct casing leg length
• Baseboard thickness and style should be determined so the casing plinth or bottom termination is planned

What You Need To Know #

• When you install exterior door casing, 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 specify a different reveal, so always ask before you start marking
• Standard 3-piece casing uses 45-degree miter joints at the top corners per This Old House door casing guide
• Always dry-fit all three pieces before nailing anything — this lets you verify the miters close tight and the reveals are consistent
• Casing is nailed with two different nail sizes: a thinner nail (such as a 15-gauge or 18-gauge) near the inside edge into the jamb, and a heavier nail near the outside edge into the framing
• Exterior door jambs are typically wider than interior door jambs (4-9/16″ or 6-9/16″ vs. 4-9/16″) to accommodate thicker wall assemblies
• If the jamb is not flush with the wall, you’ll need to address it before casing — plane down proud drywall or add jamb extensions for a recessed jamb

Tools Required #

Power Tools #

• Miter saw (10″ or 12″ compound miter saw) — for cutting accurate 45-degree and 90-degree cuts
• Finish nailer (15-gauge or 16-gauge) — for nailing the outside edge of the casing into the framing
• Brad nailer (18-gauge) — for nailing the inside edge of the casing into the jamb
• Air compressor with hose — to power pneumatic nailers (if not using cordless battery nailers)
• Random orbital sander (optional) — for smoothing filler and prepping for paint

Hand Tools #

• Tape measure — 16′ or 25′ tape for measuring casing lengths
• Combination square — essential for marking a consistent reveal on the jamb
• Pencil — sharp carpenter’s pencil for marking reveal lines and cut marks
• Utility knife — for scoring caulk lines and trimming shims
• Block plane or No. 5 jack plane — for trimming casing or shaving proud drywall
• Nail set — for driving nail heads below the surface of the casing
• 4-foot level or 2-foot level — for checking plumb on side casings and level on the header
• Speed square — for quick reference and marking 90-degree cuts
• Hammer — for setting nails and making minor adjustments
• Coping saw or oscillating multi-tool (optional) — for back-cutting miters if needed

Supplies #

• 15-gauge or 16-gauge finish nails, 2″ to 2-1/2″ — for securing the outer edge of casing to wall framing
• 18-gauge brad nails, 1-1/4″ to 1-1/2″ — for securing the inner edge of casing to the door jamb
• Wood glue (Titebond II or III for exterior applications) — for gluing miter joints before nailing
• Wood filler or lightweight spackling — for filling nail holes and minor gaps
• Sandpaper (120 and 150 grit) — for smoothing filled nail holes and joint surfaces
• Painter’s caulk — for sealing the joint between the casing and the wall surface
• Primer (if trim will be painted) — for priming raw wood or MDF before finish paint

Materials #

• MDF (Medium Density Fiberboard) Casing — The most common choice when you install exterior door casing on production homes. Comes pre-primed, machines cleanly, and provides a smooth, consistent surface for paint. MDF is cost-effective and won’t warp or split like solid wood, but it cannot get wet — keep it away from any area where moisture could be an issue. Standard profiles include Colonial, Ranch, and Craftsman styles.
• Finger-Jointed Pine Casing — Made from short pieces of pine glued end-to-end and milled to profile. More affordable than solid pine, comes pre-primed, and is a good choice for paint-grade work. Slightly more moisture-resistant than MDF but still a paint-grade material.
• Poplar Casing — A popular hardwood for paint-grade trim in mid-range to custom homes. Poplar is easy to work with, takes paint beautifully, and is more durable than MDF or finger-jointed pine. It’s a good upgrade when the budget allows.
• Alder Casing — A softwood that’s commonly used for stain-grade trim in the western United States. Alder has a warm, even grain pattern that takes stain and clear-coat finishes well. It’s softer than oak or maple, so it machines easily and is forgiving for beginners learning miter work.
• Oak Casing — A traditional hardwood choice for stain-grade trim. Red oak and white oak both provide a strong, durable casing with prominent grain character. Oak is harder to cut and nail than softer species, so use sharp blades and pre-drill if hand nailing. White oak is preferred for areas with any moisture exposure.
• Maple Casing — A dense, hard, closed-grain wood used in high-end stain-grade applications. Maple produces extremely smooth, refined joints and takes clear finishes beautifully. However, it’s the hardest to work with — blades dull faster, and it’s prone to burn marks on a miter saw if you cut too slowly.

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

Step 1: Inspect the Door and Set Reveal Lines #

Before you install exterior door casing, you need to inspect the door opening and establish your reveal lines. This step sets the foundation for the entire installation — if your reveals are off, every joint and alignment downstream will suffer. 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 surface on all three sides (both legs and the header). Place a straightedge or the flat of your combination square across the joint — the blade should sit flat on both surfaces simultaneously.
• If the drywall is proud of the jamb (sticks out past it), you’ll need to plane or rasp the drywall down until it’s flush. A Surform rasp works well for this. Don’t skip this step — proud drywall causes the casing to rock and creates a visible gap at the jamb edge.
• If the jamb is proud of the wall (sticks out past the drywall), you have two options: plane the jamb flush if it only protrudes slightly (1/16″ or less), or apply a jamb extension strip if the gap is larger.
• Open and close the door to verify it operates freely. If the door binds or rubs, the jamb position may need adjustment before you case it — don’t case a door that doesn’t work properly.
• Check the hinge-side and strike-side jamb legs with a level. They should be plumb or very close. Minor out-of-plumb conditions (1/8″ or less over the height of the door) can be absorbed by the casing.

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 this 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 your 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 continuous line — just enough marks to guide your eye when positioning the casing.
• Pay special attention to the corners where the side jambs meet the header. Mark the reveal clearly at these intersections — this is where your miter joints will land, and the reveal must be consistent here for the miters to look right.

Pro Tip: Some experienced carpenters skip the combination square and use a reveal gauge — 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 up the process significantly when you install exterior door casing on 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 to install exterior door casing 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 used) up to the header. Accuracy here is critical because the side casings establish the miter angle at the top — if the sides are the wrong length, your miters won’t close properly.

Measure the Side Casings #

• Measure from the finished floor (or the top of the flooring, if flooring is installed) up to the reveal mark at the corner where the side jamb meets the header jamb.
• If flooring is not yet installed, ask the GC or project lead for the finished floor height and measure from that point. Getting this wrong means you’ll either have a gap at the bottom or need to re-cut.
• Add 1/16″ to your measurement. This extra length ensures the miter joint closes tight — the casing will “spring” slightly when you nail it and the pressure keeps the miter joint pulled together.
• When you install exterior door casing, measure both sides independently. Door openings are rarely perfectly square or equal height on both sides, so never assume the left and right casings are the same length.

Cut the Side Casings #

• Cut the bottom end of each side casing at a straight 90-degree cut (square cut). This end sits on the floor or on top of the plinth block.
• Cut the top end at a 45-degree miter. For the left side casing, the long point of the miter faces the wall side (back); for the right side casing, the long point also faces the wall side. The short point of both miters faces the door (the jamb side).
• Always cut with the casing face-up on the miter saw to prevent chip-out on the visible face. If you’re getting chip-out, apply a strip of painter’s tape across the cut line before cutting.
• After cutting, hold each piece up to the door in its installed position. The bottom should sit on the floor, the inside edge should align with your reveal marks, and the miter at the top should land right at the corner reveal mark.

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

Step 3: Install Side Casings #

With both side casings cut and dry-fitted, you’re ready to install exterior door casing side pieces. The nailing pattern and sequence matter — done correctly, the casing pulls tight to the jamb and wall, and the top miter stays positioned 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 aligned to your reveal marks and the bottom sitting flat on the floor.
• Tack the casing in place with one brad nail near the middle of the casing, on the jamb side (inside edge). Use an 18-gauge brad, 1-1/4″ to 1-1/2″ long. This tack holds the piece in position while you verify alignment.
• Check the reveal along the entire length of the casing. Adjust as needed before adding more nails. The casing should follow your reveal marks consistently from bottom to top.

Nail the Side Casings #

• Once alignment is confirmed, nail the inside edge (jamb side) with 18-gauge brads spaced every 12″ to 16″. Start at the middle and work toward the ends — this lets the casing settle gradually and prevents bowing.
• Nail the outside edge (wall side) with 15-gauge or 16-gauge finish nails spaced every 16″. These nails are longer (2″ to 2-1/2″) and penetrate through the casing and drywall into the wall framing (jack stud or king stud).
• Keep nails 3/4″ to 1″ from the edges to prevent splitting. On the miter end, stop nailing about 2″ from the miter — you’ll secure this area when you cross-nail the miter joint after the header is installed.
• Repeat for the strike side casing. Double-check that both side casings produce a consistent reveal and that the top miters are at the correct height for the header.

Pro Tip: When you install exterior door casing, start nailing at the middle and work toward the ends. This is a fundamental technique that prevents the casing from bowing away from the wall or jamb. If you start nailing at one end and work to the other, any slight curve in the material gets locked in and compounds as you go. Middle-out nailing lets you push or pull the casing into alignment gradually.

Step 4: Measure and Cut the Header #

The next step to install exterior door casing is the header — the horizontal piece that spans across the top of the door, connecting the two side casings with miter joints at each end. This is the piece where precision matters most — the miters on the header must 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 is especially important 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 cutting. A common mistake is cutting the miter the wrong way on one end, which wastes material. Visualize how the piece sits above the door before you cut.
• Dry-fit the header by holding it up against both side casing miters simultaneously. 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 require just 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 to install exterior door casing. Positioning the header locks the miter joints and completes the casing assembly. Take your time with the miter joints — a little extra care here produces results that look professional and last for years.

Glue and Set the Header #

• Apply a thin, even layer of wood glue to both miter faces on each joint — that means glue on the side casing miter AND the header miter. Spread it with your finger to cover the entire face. Don’t overdo it; a thin film is all you need. Excess glue squeezing out means more cleanup.
• Press the header up into position, seating both miters simultaneously. You may need a helper to hold one end while you position the other, especially on wider doors.
• Align the header with the reveal marks on the header jamb. The inside edge of the header casing should be consistent with the reveal you set on the sides.
• Tack the header in place with one brad nail at center, then check both miter joints. Adjust if needed before nailing fully.

Nail the Header and Lock the Miters #

• Nail the header using the same dual-nail pattern: 18-gauge brads on the inside edge into the header jamb, 15-gauge or 16-gauge finish nails on the outside edge into the framing (header framing above the door).
• Space nails every 12″ to 16″, starting from the middle and working toward the ends.
• Cross-nail the miter joints: drive one 18-gauge brad nail through the top of the header, angled down through the miter joint into the top of the side casing. Then drive one nail from the side casing, angled horizontally through the miter into the end of the header. This “cross-nailing” locks the joint tight and prevents it from opening over time.
• Wipe off any glue squeeze-out immediately with a damp rag. Dried glue is much harder to remove and can show through paint.

Fill, Sand, and Prime #

• Set all nail heads below the surface using a nail set. Most finish nailers have an adjustable depth setting, but some nails inevitably sit a little proud — tap them down with the nail set and hammer.
• Fill all nail holes with wood filler or lightweight spackling. For paint-grade trim, spackling works well and sands easily. For stain-grade trim, use a color-matched wood filler.
• Fill any minor gaps in the miter joints with wood filler. If the gap is larger than 1/32″, you may need to re-cut — filler can’t hide a bad joint.
• Let filler dry completely per manufacturer’s instructions, then sand smooth with 150-grit sandpaper. Sand with the grain direction on stain-grade work.
• Apply caulk along the outside edge of the casing where it meets the wall. Use painter’s caulk and tool it with a wet finger for a smooth, clean line.
• Prime any raw wood or MDF with a quality primer. This seals the surface and provides a base for the finish paint coat.

Pro Tip: When you install exterior door casing, cross-nailing the miter is the single most important technique for keeping joints tight long-term. Wood moves with humidity — it expands in summer and contracts in winter. A miter that’s only held by glue will eventually open. The cross-nails mechanically lock the two pieces together so even as the wood moves, the joint stays closed. Drive the nails at opposing angles to create a pinching effect through the joint.

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 and cross-nailed
• ✓ Casing sits flat against both the jamb and the wall — no rocking or gaps behind the trim
• ✓ Side casings are plumb (or follow the jamb line if the jamb is slightly out of plumb)
• ✓ Header is level (or follows the head jamb reveal consistently)
• ✓ All nail heads are set below the surface
• ✓ All nail holes are filled and sanded smooth
• ✓ No glue squeeze-out visible at the miter joints
• ✓ Caulk applied between the casing and wall — clean, consistent line
• ✓ 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 issue is likely that the wall or jamb isn’t perfectly square. The fix is to back-cut the miter: use a block plane or utility knife to slightly hollow out the back of the miter face. This allows the front edges to meet first and close tight. Only remove a tiny amount of material — you’re adjusting by thousandths, not sixteenths.

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

When you install exterior door casing and it 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, you can shave it down with a Surform rasp or a drywall sanding screen. If the wall has a dip or depression, you can shim behind the casing at the low spot — use a thin wood shim between the casing and the wall, then nail through both. The shim fills the void 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 that you need to address before you install exterior door casing. If the jamb is recessed (wall is proud), your best option depends on the severity. For gaps of 1/8″ or less, you can sometimes flex the casing to bridge the difference, but this is not ideal. For larger gaps, you need a jamb extension — a thin strip of wood ripped to match the gap width and glued/nailed to the jamb edge to bring it flush with the wall. If the jamb is proud of the wall (sticks out past the drywall), plane the jamb edge down to flush using a block plane. Test frequently with a straightedge as you plane.

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 must nail closer to the end (for example, to lock a miter joint), pre-drill with a bit slightly smaller than the nail shank. You can also blunt the tip of the nail with a hammer tap — a blunted nail point crushes the wood fibers instead of wedging them apart, which significantly reduces splitting.

Miter Opens Over Time After Installation #

If miters open weeks or months after you install exterior door casing, the issue is wood movement from humidity changes. This is normal and expected — the question is whether you took steps to minimize it during installation. First, make sure you’re using glue on every miter joint. Second, cross-nail every miter as described in Step 5. Third, if you’re in a climate with extreme humidity swings, consider using wood species that are more dimensionally stable (like poplar or MDF) rather than species that move a lot (like maple or red oak). If the joint has already opened, you can fill it with caulk for paint-grade work, or use color-matched wood filler for stain-grade, then refinish.

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