Installing Window Trim

Using a power miter saw, cut a length of window stool (the flat piece upon which the window shuts down) that's 1-1/2" longer than the outside dimension of your installed casing. Hold a piece of casing in place and lightly mark the outside edge on the wall. Repeat on the other side and measure between the marks. Mark the center of the angled sill and the stool. Then align the two marks for the next step.

To determine the notch width, center the stool on the window and mark the edge of the stool at the inside of each side-jamb. If necessary, rip or plane the window-edge of the stool so it's parallel to the sash. To determine notch depth, measure from the stool to the sash at both jambs, subtract 1/16" and connect the two points. Mark and cut the notches. Then bevel or round the ends of the horns and tack the stool to the rough sill with 8d finishing nails 10" on center.

Helpful Tip

Because surfaces are rarely perfectly flat at wall openings, use a scribe (compass) to mark any necessary cuts on the back edge of the stool and at the horns. This will assure the stool fits nicely against the sash and the horns against the wall.

You'll have to extend your window jambs if they aren't flush to the interior wall surface. Hold a straight edge on the wall surface and measure the distance between the jamb and the straight edge to determine how thick the extension must be. Check three points on both sides and the top, then choose the largest dimension.

Helpful Tip

If there is more than a 1/8" difference between thickness measurements, you can split the difference by ripping the extension 1/16" thinner than your measurement. Or you can rip a taper and plane or sand it once it's in place.

It isn't as common, but sometimes a jamb is wider than the wall. In this case, use a block plane to plane the jamb flush with the interior wall surface.

Rip strips of 3/4" stock to the thickness you need. Cut a full-length head-jamb extension. Glue and nail it so that 1/16" to 1/8" of the head jamb will be left exposed—this is called a reveal. Then cut the side-jamb extensions to fit snugly between the stool and head-jamb extension.

Mark a 3/16" reveal on both the side- and head-jambs. Measure between the side-jamb marks and cut a 45-degree miter on both ends of the head casing so that the short dimension equals that distance. Then position the casing on your marks and tack it to the jamb with 4d finishing nails and to the framing with 8d or 10d casing or finishing nails.

Helpful Tip

You should always tack your casing in place initially. Wait until mating pieces are in place before driving the nails home. This way, if you must remove a piece to fine-tune a cut, you can do so without damaging the trim.

Measure from the stool to the top of the head casing at each side of the window and miter-cut each side casing so that its long point will equal that measurement. Apply glue to the miters and lock the joints with a finish nail from the top and side. Nail side casings to the jamb and wall as done with the head casing.

Helpful Tip

To avoid possible measuring errors, cut a miter on one end of the casing. Stand it upside down on the stool with the top touching the head casing and mark the length at that intersection.

Ideally, the end of the apron should have the same profile as the face. If you have nicely detailed trim that will be stained and want to do a first-class job, make a mitered return on the ends of the apron. Cut an outside miter on the ends of the apron and on two short molding scraps.

Glue and nail the scraps to the apron to form a 90-degree corner at each end. When the glue dries, cut off any excess. Nail through the apron into the wall framing, then nail through the stool into the casing with 6d finishing nails.

Helpful Tip

If you cut in the other direction — pushing the saw into the face of the molding — you could use the back of the casing to guide your cut. You're less likely to splinter the face of the trim when you do this.

For less demanding applications, you can cut the ends of the apron to match the molding's profile. Cut an inside miter (long point on backside of trim), and cope the resulting profile straight back. Use a sanding block to smooth the cut.

Helpful Tip

If you plan to stain the end grain, apply a wood conditioner before staining so the end won't soak up too much stain.

Use a nail set and hammer to drive all nails slightly below the surface. If you plan to paint, fill the holes with wood putty and sand smooth. If you plan to stain and varnish, do that first and fill nail holes using putty sticks.

Congratulations! By trimming your windows, you've added architectural detail and aesthetic appeal to the interior of your home.