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:
- Utility knife
- Combination square
- Tape measure
- Table saw
- Miter saw or miter box
- Jigsaw or hand saw
- Router (optional)
- 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.