Introduction #
Casing a window is a basic skill in trim carpentry. Window casing is the trim that frames a window opening. It covers the gap between the window jamb (the wood frame built into the window opening) and the drywall around it. Casing does two jobs: it hides the rough framing and shims behind the wall, and it gives the window a clean, finished look.
On new construction, we case windows after the drywall is hung, taped, and has at least a primer coat of paint. Before you start, the jamb needs to sit flush with the finished wall. If the jamb sits back behind the drywall, install jamb extensions first (thin strips added to the edge of the jamb to bring it out flush with the wall). Don’t skip this step. Casing over a jamb that isn’t flush will leave gaps, make the casing rock, and keep the joints from staying tight.
Window casing uses many of the same skills as door casing: marking an even gap along the jamb, cutting clean mitered corners (corners where two pieces meet at an angle, usually 45 degrees, instead of butting square), and following the right nailing pattern. If you’ve already cased a door, you already have most of what you need. The main difference: most windows get picture-frame casing, with mitered corners on all four sides. Most doors only get three pieces of casing (two legs and a header), with the bottom meeting the floor. Once you can case a window cleanly on all four sides, casing a door is the easier version of the same skill.
This guide covers the standard picture-frame method. Stool-and-apron windows are different: a protruding sill (called a stool) sits at the bottom, with a flat piece nailed beneath it. That technique is covered in a separate guide. Picture-frame casing is the method you’ll use on most windows.
How to Case a Window Step by Step #
Step 1: Inspect the Window and Mark Reveal Lines #
Start by inspecting the opening. Run your hand along each jamb edge to check it’s flush with the drywall on both sides. Open, close, and lock the window to make sure it works smoothly. Fix any problems now, before the casing goes on.
Set your combination square to 3/16″ and use it to mark the reveal line along all four jamb edges. The reveal is the strip of jamb you leave showing between the edge of the jamb and the casing, so the casing doesn’t sit flush against the window frame. Hold the body of the square against the face of the jamb and draw a light pencil line along the blade. Mark the reveal on the top jamb, both side jambs, and the bottom jamb. These lines are what you line up every piece of casing against.
Keep your pencil marks light, just dark enough to see while you position the casing. Heavy pencil lines can show through paint, and they’re hard to remove once the casing is on.
Pro Tip: Cut a small piece of cardboard exactly 3/16″ wide. Use it as a spacer between the jamb edge and the casing. It’s faster than marking reveal lines on every jamb, and it gives you a consistent reveal with no pencil marks to clean up later.
Step 2: Measure and Cut the Header (Top Piece) #
With picture-frame trim, cut the header (the top piece) first. Measure the distance between the reveal marks on the top jamb. This is your short point measurement, the length along the casing’s inside edge, where each miter cut comes to a point closest to the window. To get the full length of the header, add twice the width of your casing stock to that number. For example: if the reveal-to-reveal distance is 36″ and your casing is 2-1/4″ wide, the header is 36″ + 2-1/4″ + 2-1/4″ = 40-1/2″ measured at the long points, the outside edge of the casing, where each miter cut reaches its widest point.
Set your miter saw to 45 degrees and cut a miter on each end of the header. Both miters point outward: the long point of each miter is on the face of the casing, and the short point is on the back. When you hold the header in position, the mitered ends should angle down toward the side jambs.
Cut the header about 1/16″ longer than your measurement. That little bit of extra length makes sure the miters close tight when the side casings push into them. A piece cut exactly to length, or worse, a little short, leaves open miters that are hard to fix.
Pro Tip: Always cut the left end first. Then measure from that cut to mark and cut the right end. This stops small errors from stacking up. If you measure the full length and cut both miters from a rough piece, any error in the first cut throws off the second cut too.
Step 3: Install the Header #
The header needs to sit exactly right. Hold it in position with the back edge lined up on your reveal marks on the top jamb. Check that the mitered ends line up with the reveal marks on the side jambs. If the piece is a little long, the miters will stick out just past the side reveal marks. That’s what you want. It will compress into a tight joint once the side casings go on.
Nail the header with 18-gauge brads. Use two rows of nails: one row into the jamb edge, about 3/8″ from the inside edge of the casing, and one row into the wall framing, about 3/8″ from the outside edge. Start in the center and work outward toward the ends. Space nails every 12″ to 16″ along each row.
Don’t nail within 2″ of the mitered ends yet. Leave the ends free so you can adjust them when you install the side casings. Nail the ends too early and you lock them in place, and you won’t be able to fine-tune the miter joints.
Step 4: Measure and Cut the Side Casings #
Measure from the point of the header’s miter (where the short point of the header meets the reveal line) straight down to the bottom reveal mark on the side jamb. That measurement is the short-point length of your side casing. On a picture-frame installation, it’s a short-point-to-short-point measurement: both ends get miters.
Cut a 45-degree miter at the top of the side casing to match the header’s angle. The miter should angle inward, so the long point sits on the wall side and the short point sits on the jamb side. For picture-frame style, cut a matching 45-degree miter at the bottom, angled inward toward the bottom jamb. If you’re installing a sill or stool at the bottom instead, make a square cut there.
Cut both side pieces at the same time and compare them. They should match in length. If the window opening is slightly out of square, you may need to adjust one side. Measure each side on its own instead of assuming they match.
Pro Tip: Hold each side piece in position before nailing, and check the miter fit at the top. If it doesn’t close perfectly, adjust your saw angle by half a degree and re-cut. It’s better to sneak up on the right angle with a few test cuts than to install a bad joint and hope it disappears later.
Step 5: Install the Side Casings #
Glue strengthens every miter joint. Apply a thin bead of wood glue to the mitered end of the side casing before you set it in place. Press the miter tight against the header’s miter, and line up the back edge of the casing with the reveal marks on the side jamb. The glue helps keep the joint from opening up as the wood moves with the seasons.
Nail the side casing using the same two-row pattern: one row into the jamb edge, one row into the wall framing. Start at the top and work down, keeping the casing lined up with the reveal marks as you go. Space nails every 12″ to 16″.
Once the side casing is nailed along its length, cross-nail the miter joint at the top corner. Cross-nailing means driving brads through the edges of both pieces at an angle, so they lock into each other across the joint. Drive one brad horizontally through the edge of the side casing into the end of the header, and one brad vertically through the edge of the header into the top of the side casing. This locks the miter shut and keeps it from opening. Now nail the ends of the header you left free in Step 3.
Repeat for the other side casing.
Step 6: Measure and Cut the Bottom Casing (Sill Piece) #
For picture-frame casing, measure and cut the bottom piece exactly like the header. Measure the short-point distance between the bottom reveal marks on the side jambs, add twice the casing width, and cut 45-degree miters on both ends, pointing outward. Cut the piece about 1/16″ long again, so the miter joints close tight.
Apply wood glue to both mitered ends before you set the bottom piece in place. Press the miters tight against the bottom ends of the side casings, and line up the back edge with the reveal marks on the bottom jamb. Nail it using the same two-row pattern, starting in the center and working outward. Cross-nail both bottom miter joints, just like you did at the top corners.
If you’re installing a stool-and-apron bottom instead of picture-frame casing, the process is different. The stool extends past the casing on both sides (the extensions are called “horns”), and the apron is a flat piece nailed beneath the stool. That method is covered in a separate guide.
Step 7: Tighten, Clean, and Check #
With all four pieces on, go back around the opening and close it out. At this stage you’re checking joints and fasteners, that’s our work. The finish coat is the painter’s job, not ours.
- Tighten any loose miter: Work a small amount of glue into a miter that isn’t fully closed, press it shut, wipe off the excess with a damp cloth, and pin it with a brad driven across the joint. Do this while the glue is still wet.
- Set every brad below the surface: Run a finger over the casing face and both miters. If anything sticks up above the wood (proud), tap it down with a nail set, a small hand tool made for driving a brad head just below the surface. A brad left sitting at the surface will show through the finish.
- Check the reveal one last time: It should be consistent all the way around, with the casing tight to the jamb on the inside edge and no gap.
- Test the window: Open, close, and lock it before you walk away. If the casing is pinched against the sash (the part of the window that slides or swings), it’s much easier to pull and reset now than after the painter has been through.
- Sweep up: Collect the cutoffs and leave the sill and floor clear.
Filling the fastener holes and sealing the outside edge where the casing meets the wall are the painter's steps, not ours. He does the whole room in one pass with one set of materials. Anything we spot-fill ahead of him tends to flash differently under the finish coat. What he needs from us: tight miters, a consistent reveal, casing flat to the wall, and fasteners set below the surface.
A gap on the inside edge is the one thing the painter can never hide. If the casing isn't sitting tight to the jamb, it wasn't installed right. The fix is to pull it and reset it.
Quality Check After You Case a Window #
Before you move to the next window, check your work against this list:
- Reveal is consistent at 3/16″ on all four sides of the window
- All four miter joints are tight with no visible gaps
- Casing sits flat against the wall - no rocking or bowing
- Casing sits tight against the jamb edge - no gaps on the inside edge
- No gaps between casing pieces at any of the corners
- Every brad is set below the surface, nothing proud to the touch
- Window still operates properly - opens, closes, and locks without binding against the casing
- Casing profile matches the door casing used in the rest of the house
Troubleshooting #
Even careful work can run into small problems. Here are the most common ones and how to fix them.
Miter Won’t Close Tight #
This is the most common problem with picture-frame casing. If the miter joint has a gap on the face but is tight on the back, your saw angle is a little less than 45 degrees. If it’s tight on the face but open on the back, the angle is a little more than 45 degrees. Adjust your saw by half a degree and re-cut. If the joint is tight on one end but open on the other, the wall or jamb isn’t perfectly square. Back-cut the miter, shave a little extra angle off the back edge with a utility knife or block plane, so the face edges can close.
Casing Rocks Against the Wall #
This usually means the wall isn’t flat at the window opening, or there’s a bump of drywall compound near the jamb edge. Hold a straight edge against the wall around the opening to find the high spot. Scrape or sand down any drywall compound bumps. If the wall is bowed, you may need to shim behind the casing to bring it flat. A shim is a thin, tapered strip of wood, cedar works well, that you nail in behind the casing to fill the low spots.
Reveal Not Consistent #
If the reveal varies from one side of the window to the other, either the jamb isn’t straight or the casing wasn’t aligned carefully. Double-check alignment at each nail location before you nail. If the jamb has a slight bow, split the difference: let the reveal vary slightly so the casing stays straight, rather than following a bowed jamb and ending up with a wavy edge.
Miter Corner Opens After Nailing #
This happens when nailing pulls the casing away from the miter joint. The fix is cross-nailing: drive a brad through the edge of one piece into the end grain (the cut end of the wood) of the piece next to it. Use two cross-nails at each corner, one from each direction. Apply wood glue to the joint first, then cross-nail and wipe away any squeeze-out, the glue that oozes out when you press the joint shut. On future installs, cross-nail the corners before you nail the ends of the adjoining pieces, so this doesn’t happen.
Window Won’t Open After Casing Is Installed #
If you case a window too tight, the casing or the nails can interfere with the sash, the moving part of the window. Check that no nails went through the jamb into the window frame or sash. Make sure the casing isn’t overlapping the window stop (the strip that holds the sash in its track) or reaching past the reveal into the window channel (the track the sash slides in). If the casing is too tight against the sash, carefully pry it away from the jamb, reposition it with a slightly wider reveal, and re-nail. Always test the window before you leave the opening, while the casing is still easy to pull and reset.
Related Guides #
Once you can case a window with consistent reveals and tight miters, the same layout and nailing skills carry over to doors and cased openings. For more on window casing techniques and material selection, see this guide from Family Handyman on window trim installation.
- How to Case an Exterior Door
- How to Install Baseboard
- How to Trim a House
- How to Install a Door Jamb Extension
Reference Photos #
The following photos show examples of this type of work completed in the field. These serve as visual references for quality standards, proper installation techniques, and finished results.
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