A Guide to Installing Window Sill Trim

Upgrading the trim around doors and windows is a popular home improvement projects that can help to increase the value of a home. Installing your window trim does take some time, patience, and attention to detail, however. One stumbling block that many DIYers or contractors might come up against is cutting and installing window sill trim. This guide to installing window sill trim will help.

What is window sill trim?

While a window sill is only the flat, horizontal piece at the interior bottom of the window, window sill trim refers to all of the trim components inside the window. These components include the sill, the apron underneath, the side and head jambs inside the window opening, and the side and head casing around the window.

Window sill trim exists to give the rough opening around the window a finished look. Rather than ragged drywall and framing lumber, the window sill trim is smooth and clean (if done correctly).

Tools and materials you’ll need for installing window sill trim

Installing window trim requires quite a few different tools:

• Prybar
• Hammer
• Utility knife
• Pencil
• Combination square
• Tape measure
• Level
• Table saw 
• Miter saw or miter box
• Jigsaw or hand saw
• Router (optional)
• Sandpaper
• Finish nailer (2-inch)
• Finishing nails (8D and 10D, optional)
• Wood shims

It’s certainly possible to hang window sill trim with nothing more than a hand saw, hammer, and nails. However, the accurate cuts required for installing window sill trim are easier to achieve with power tools, and avoiding errant blows with a hammer is a skill that requires time to develop.

How to install window sill trim

Installing window sill trim might take several steps (and tools), but the time is well worth the project’s impact. 

1. Remove the existing trim

If you’re replacing existing trim, start by removing it. Use the utility knife to score along the edges of the window casing, sill, jambs, and apron to cut the paint or caulk. Slide the pry bar behind each piece of trim and gently pry it loose. Be sure to place a piece of scrap between the wall and pry bar to prevent damage whenever extra leverage is necessary.

2. Cut the jamb material to width

Use the combination square to measure the width of the new jambs. Transfer these measurements to the jamb material and cut them to width (not length yet) on the table saw. Cut a short piece off of one of these jambs to use as a reference on the window. 

3. Cut the window sill to length

Mock up the window jamb and casing using the short piece of jamb and a small piece of the new casing. With the jamb in place, slide the casing toward the wall slightly to create a ¼-inch reveal on the jamb. Use a pencil to mark the drywall where the casing meets the wall. Repeat this on the other side of the window as well. 

The distance between these two marks represents the width of the sill. For aesthetics, you can add 1 ½-inches to this length to allow for a ¾-inch overhang on each side. Transfer this measurement to the sill material and cut it to length on the miter saw or miter box. 

4. Cut the sill to shape

Use the tape measure to find the center of the window sill and the window, marking both for reference. Holding the window sill in place so the marks are aligned, use the combination square to mark where the inside of the window framing will meet the sill. Next, set the combination square to match the distance between the surface of the drywall and the window, and transfer this measurement to the backside of the sill. 

The marks for where the sill meets the wall and the depth marks will create small boxes on each end of the sill that need to be removed. Use a jigsaw or hand saw to remove these boxes. 

5. Install the window sill

Use some sandpaper to smooth the cuts, or a router to create a rounded profile along the front of the sill. The sill should fit snugly into the window opening at this point, but you can adjust it slightly with sandpaper if necessary.

With the sill in place, lay the level on top to ensure that it’s close to level. If not, adjust with shims. Use a pneumatic nailer or hammer to nail the sill into the framing below.

6. Cut and install the jambs

With the jambs already cut to width, installing them is a matter of cutting them to length and nailing them in place. 

Starting with the head jamb, hold it in place between the studs in the window opening and mark it to length. Cut and nail this jamb in the opening, using shims to get it as level as possible. Next, repeat the process with the side jambs: hold them in place, mark them to length, and nail them into the framing.

It can be nearly impossible to hold a full-length jamb in place for final marking, so use the tape measure to get an overall length and add a few inches before cutting. This will provide a piece of jamb material at a workable length.

7. Cut and install the side casing

Set the combination square to ¼-inch and mark along the inside of the window jambs to create a reveal line. Hold a piece of casing along this line on the head jamb, and use a pencil to mark the wall along the top edge of the casing at both top corners. Measure from this line to the sill and add an inch or two. Transfer these measurements to two pieces of casing.

Cut each piece of casing to length on the miter saw or saw box. Next, set the miter saw or saw box to 45 degrees and cut opposing miters on the ends of both pieces of casing. This should result in two perfectly mitered pieces of casing that are longer than necessary. 

Hold the right-side casing in place, but flip it upside down so the point of the miter is resting on the sill. Then, use the pencil to mark the casing where it meets the line drawn on the wall earlier in this step. Reset the saw to 0 degrees and cut the casing to length. Repeat on the left side and nail both pieces in place, using the ¼-inch reveal for reference.

8. Cut and install the head casing

Cut a 45-degree miter on the end of the head casing. Next, flip the casing upside down and rest the point of this miter on the point of the miter on the left or right casing. At the other side of the window, use the pencil to mark where the point of the other miter meets the head casing. Cut the final miter at this line and install it in place with a hammer and nails or pneumatic nailer. 

9. Cut and install the apron

The final step is to install the apron underneath the sill. The apron should be the same length as the distance between the outside edges of the casing, so use them for reference. Hold the apron piece against the casing, and make a mark at the outside edge of each piece of casing.

For a truly finished look, install returns (45-degree miters on the ends of the apron). Mark the apron as noted above. Set the saw to 45 degrees and cut the apron at those marks and nail in place. Next, cut two small mitered pieces and glue them on the end of the apron, giving the apron a custom look.

MT Copeland offers video-based online classes that give you a foundation in construction fundamentals with real-world applications, like drywall finishing. Classes include professionally produced videos taught by practicing craftspeople, and supplementary downloads like quizzes, blueprints, and other materials to help you master the skills.