Trimming Basement Windows

by | Trims,Windows&Doors

Basement windows bring much-needed sunlight into dark areas, but even in finished basements they often get ignored on the trim front. This is partly because most basement foundation walls are at least 8″ thick, and often a lot thicker.

Add a furred-out wall and the window starts to look more like a tunnel with a pane of glass at the end. But with some well designed and well-executed trim carpentry, you can turn the depth disadvantage into a positive.

today I will explain and simplify the way to trim your basement windows, so, keep reading and focus.

A basement window opening may be finished with wallboard, but the easiest way to trim one is by making extra-wide custom jambs that extend from the inside face of the window frame to the interior wall surface.

Because of the extra width, plywood stock is a good choice for the custom jambs. The project shown here is created with veneer-core plywood with an oak veneer surface. The jamb members are fastened together into a nice square frame using rabbet joints at the corner.

The frame is scribed and installed as a single unit and then trimmed out with oak casing. The casing is applied flush with the inside edges of the frame opening. If you prefer to have a reveal edge around the interior edge of the casing, you will need to add a solid hardwood strip to the edge of the frame so the plies of the plywood are not visible.

Tools&Materials pencil ■ tape measure, table saw ■ drill with bits ■ 2-ft level ■ framing square ■ utility knife ■ straightedge ■ finish-grade 3⁄4″ oak plywood ■ spray-foam insulation ■ composite or cedarwood shims ■ 11⁄4,2″ finish nails ■ 15⁄8″ drywall screws ■ carpenter’s glue

How to Trim a Basement Window (standard)

  • Check to make sure the window frame and surrounding area are dry and free of rot, mold or damage. At all four corners of the basement window, measure from the inside edges of the window frame to the wall surface. Add 1″ to the longest of these measurements.
  • Set your table saw to make a rip cut to the width arrived at in step 1. If you don’t have a table saw, set up a circular saw and straightedge cutting guide to cut strips to this length. With a fine-tooth panel-cutting blade, rip enough plywood strips to make the four jamb frame components.

  • Crosscut the plywood strips to correct lengths. In our case, we designed the jamb frame to be the exact same outside dimensions as the window frame, since there was some space between the jamb frame and the rough opening.
  • Cut 3⁄8″-deep × × 3⁄4″-wide rabbets at each end of the head jamb and the sill jamb. A router table is the best tool for this job, but you may use a table saw or hand saws and chisels. Inspect the jambs first and cut the rabbets in whichever face is in better condition. To ensure uniformity, we ganged the two jambs together (they’re the same length). It’s also a good idea to include backer boards to prevent tear-out.

  • Glue and clamp the frame parts together, making sure to clamp near each end from both directions. Set a carpenter’s square inside the frame and check it to make sure it’s square.
  • Before the glue sets, carefully drill three perpendicular pilot holes, countersunk, through the rabbeted workpieces and into the side jambs at each corner. Space the pilot holes evenly, keeping the end ones at least 3⁄4″ in from the end. Drive a 15⁄8″ drywall screw into each pilot hole, taking care, not to overdrive. Double-check each corner for square as you work, adjusting the clamps if needed.

  • Let the glue dry for at least one hour (overnight is better), then remove the clamps and set the frame in the window opening. Adjust the frame so it is centered and level in the opening and the exterior-side edges fit flush against the window frame.
  • Taking care not to disturb the frame’s position (rest a heavy tool on the sill to hold it in place if you wish), press a steel rule against the wall surface and mark trimming points at the point where the rule meets the jambs at each side of all four frame corners, using a sharp pencil.

  • Remove the frame and clamp it on a flat work surface. Use a straightedge to connect the scribe marks at the ends of each jamb frame side. Set the cutting depth of your circular saw to just a small fraction over 3⁄4”.Clamp a straightedge guide to the frame so the saw blade will follow the cutting line and trim each frame side in succession. (The advantage to using a circular saw here is that any tear-out from the blade will be on the nonvisible faces of the frame).

  • Replace the frame in the window opening in the same orientation as when you scribed it and install shims until it is level and centered in the opening. Drive a few finish nails (hand or pneumatic) through the side jambs into the rough frame. Also, drive a few nails through the sill jamb. Most trim carpenters do not drive nails into the head jamb.

  • Insulate between the jamb frame and the rough frame with spray-in polyurethane foam. Look for minimal-expanding foam labeled “window and door” and don’t spray in too much. Let the foam dry for a half hour or so and then trim off the excess with a utility knife. Tip: Protect the wood surfaces near the edges with wide strips of masking tape.
  • Remove the masking tape and clean up the mess from the foam (there is always some). Install case molding. We used picture-frame techniques to install a fairly simple oak casing.


While casing for a window is much the same as for a door, the horizontal elements below the window are different. In most cases, you need to install a stool (often called a sill), jambs, and an apron as well. If the window is set deep in the wall, you will end up with a stool that is also a deep shelf, which may be made of hardwood plywood, solid-surface countertop, or another material.

The sequence here shows a window set about 5 in. back from the drywall. Start with the stool. The board you use should be wide enough to extend in front of the drywall by at least 3⁄4 in., 1 in. or 11⁄2 in. is common.

First, figure the total length of the stool, this does not have to be precise, since it will run past the casing on each side by an inch or so. Position a scrap piece of the casing you will install where it will end up: Take into account the thickness of the jamb as well as the reveals on the window frame and the jamb.


  • Look at the steps ahead to help visualize this. Then make a mark about 11⁄2-in. past the casing. Do the same on the other side, and measure between the outside marks to find the stool’s total length.
  • Next, mark for cutting notches on each side of the stool. Use a square to mark for the width of the notch; allow yourself a gap of 1⁄4 in. or so, where it will be covered by the jamb later.

  • To determine the depth of the notch, use a compass, as shown. If the depth is too deep, measure and mark with a tape measure and square. Find the depth and transfer that measurement to the stool.

  • Do the same on the other side and cut with a miter saw and/or a jigsaw.

  • Place the stool in the opening, shim as needed to keep it level and even with the window frame, and drive finish nails.
  • Cut a length of casing or other trim material for the apron and install it against the wall under the stool, checking the stool for level.
  • Cut side jambs to run from the stool to the reveal mark on the window frame and install with shims to keep them square. Install the head jamb as well.

  • To install mitered casing, mark a reveal line on the jamb. Cut two scraps of the casing at 45° miters and hold them in position against the jamb to test for a tight miter fit.
  • If the joint is wider at one side, slightly adjust your saw and test again until you achieve a great-looking joint. Miter-cut the side casing pieces so the short side of the cut reaches the reveal line, and partially hand-drive a few nails to temporarily hold them in place.
  • Measure across the top and cut the head casing. Position it; you may need to remove the temporary nails in the side casing to adjust for tight miter fits.
  • Finish attaching the casing with 2-in. power-driven nails, or with wire brads driven into the door frame and 6d nails driven into the wall framing.

More Casing Possibilities

Here are some more possibilities for casing windows and doors. All use trim pieces commonly available at lumberyards and home centers, and none call for making miter cuts. And for all, you can choose stained wood or painted trim.

To install this “traditional” style of the casing, attach plain casing (also called sanitary casing) to the sides. Top them with a “fillet” of 11⁄8-in. casing stop or a similar small piece. Then add a head casing of 1x4, and top that with another fillet, this time one that is a bit thicker.

A distinctive “classical” look is achieved with fluted casing attached to the sides. A fillet of lattice is placed on top of that, and then a head casing that is about 5⁄8 in. thick and 31⁄2 in. wide. Place a wider and thicker fillet on top of the head casing, then attach bed molding below the top fillet.

This one is simplicity itself and can look either rustic or modern, depending on the materials you use. On the sides, install plain (“sanitary”) casing, then install a head casing of 1x4 (which is 3⁄4 in. thick). Install the head casing so it overhangs the side pieces by 3⁄4 in. on each side.


Brick molding, as shown here, is the most popular choice for outside window and door trim. Here, I use PVC vinyl brick molding, which will never need painting, lasts forever, and is easy to work with.

  • Measure the opening. If there are any masonry or old caulk protrusions, chip them away or plan for a trim size that takes them into account. (You can always install a thick bead of caulk around the molding, so there’s no need to try for a tight fit.) Measure the diagonals.
  • If they are equal, the window is square; if not, remove the fasteners and use shims to bring the window square. You can draw a reveal line, or just plan on making the inside of the trim 1⁄2 in. wider and taller than the inside of the window frame, for a 1⁄4-in. reveal all around. Also, a measure for the width of the molding all around. In this case, the bottom trim will need to be narrowed with a rip cut.
  • Miter-cut trims pieces, with the short side of the miters running from revealing to reveal. You can’t hook your tape measure to a miter-cut end, so “burn an inch”: Hold the tape at the 1-in. mark and make the cut mark at a point that shows 1 in. longer than the desired length.

  • The bottom piece rests on a sill that is sloped down away from the house. So the piece will follow that line, rip-cut with a table saw or a circular saw set to a slight angle.
  • Working on a flat surface, assemble all four pieces of trim with four finish nails driven into each corner.
  • Where the bottom piece was rip-cut, you will need to cut off a little nub.
  • Set the trim assembly in place and nail or screw in place, taking care not to drive fasteners where they will interfere with the operation of the window.
  • Paint the trim. Finish with a bead of high-quality caulk. If a gap is wider than 1⁄4 in., first push in some foam backer rod.