7 steps to installing flawless window trim

Cut one end of the first piece of window trim. To miter corners, cut a 45-degree angle using a handsaw and miter box or a power miter saw.

Installing flawless window trim efficiently and correctly, requires a variety of carpentry skills from precise measuring to proficient cutting.

Basic window trim consists of one style of casing on all four sides, known as a “wraparound.” In some cases, a different style of trim – head casing – is used at the top of the window for added style. Similarly, the bottoms of some windows feature a sill created using “stool” and “apron” molding, which require special handling.

Making the process even more demanding is the request by many homeowners to use stained, rather than painted, trim. If you have to stain the trim, you won’t be able to use caulk or putty to fill gaps caused by poor measurements or inattentive cutting.

Window trim comes in a variety of materials and profiles. While the most common is a simple casing using mitered corners, blocked corners that feature corner blocks or rosettes offer homeowners a nice alternative, and in a home with matching trim can be a defining design element.

Steps for installing flawless window trim

1. Mark a reveal line ¼ inch from the window frame around the perimeter of the window using a pencil and combination square. This is where the inside edge and ends of the casing will align.

2. Measure each side of the window opening from reveal line to reveal line to determine the length of each casing; add several inches to the length of each piece.

3. Cut one end of the first piece of trim. For wraparound casing, start with the head casing.

• To install a sill, cut and fit the stool first and then proceed to the head and side casings; cut the apron last.
• To miter corners, cut a 45-degree angle using a handsaw and miter box or a power miter saw.
• For butted or blocked corners, make a straight cut.

4. To determine the location of the second cut, hold the inside edge of the trim along the reveal line and mark the inside length of the second cut with a pencil.

• For mitered corners, the shortest end of the 45-degree angle is at the end of the reveal line.
• For butted corners, the head casing usually extends beyond the side casing, so add twice the width of the trim to the length of the head casing.
• For blocked corners, cut the casing to fit flat along the edges of the corner blocks.
• For a sill, the stool should extend about ½ to ¾ inches beyond the width of the casing. The apron, fitted just underneath the stool, should be the same width as the widest part of the window casing.

Once the measurement is confirmed, make the second cut at the other end of the casing. Continue cutting and fitting each piece, making sure it is cut correctly before moving on to the next one. Avoid leaving gaps or uneven spaces at the corners.

5. Check that each piece is level and attach trim to drywall using wood glue or construction adhesive and two or three finish nails. Nails should be secure, but not tight in case they have to be removed to reposition the trim.

When all pieces are fit correctly, drive finish nails into each piece every 8 to 12 inches. You may want to drill starter holes to avoid splitting the trim when you drive the nails.

6. When all pieces are fit correctly, drive finish nails into each piece every 8 to 12 inches. You may want to drill starter holes to avoid splitting the trim when you drive the nails. Cross-nail corners from the side to draw the trim pieces together and secure them in place.

7. Countersink nails with a nail set. Fill nail holes with wood putty or wax filler. Caulk any small gaps between trim and drywall.

Because it requires few specialized tools or skills, trimming an interior window is an easy way to add a custom look to factory or standard windows.

Cutting the stool

You can buy stool precut with a groove milled into the bottom to rest on the inside edge of the sill. Measure side to side inside the window frame, add ½ inch to account for the ¼-inch reveal on each side. Because the stool extends at least to the edge of the side casing, add the width of the casing twice (one for each side).

Finally, add the horn—the distance you want the stool to overlap the casing on each side, usually ½ to ¾ inch. Add it twice. Cut the stool to length.

To allow the stool to slide around the casing and window frame, it has to be notched. Measure between the window frames and mark that distance on the stool. Then measure the depth the notch should be and mark it on the stool.

Cut the notch. Secure the stool to the sill with finish nails.