How to Install Window Trim for a Finished Look

Last Edited: 4/12/24

window trim on stone wall.
How to Install Window Trim

Installing window trim is a great way to change the interior of your home from basic to refined. It takes patience and a little finesse, and we’re here to help you through it. If you’re altering your current windows or looking for tips for new window trim installation, use the guide below to achieve a weathertight seal and professional finish. 

Installing Window Trim

Gather all your tools and materials before you begin. You may be able to rent a mitre saw and Brad Nailer from your local hardware store. Buy more window trim than you need in case you make a mistake.

Before trimming a window, fill in any gaps or cracks between the window jamb and the rough opening with loose insulation or non-expanding foam to ensure a weathertight seal. Allow this to dry and cut away any excess foam.

Tools and Materials for Installing Window Trim

  • Mitre saw.
  • Pencil
  • Brad Nailer
  • 1-inch and 2 1/4-inch Brad nails
  • Air compressor
  • Rafter square
  • Wood glue
  • Window trim

These tools will make quick work of any window trim installation project. However, if you’re just trimming one window, you may only need a mitre box, mitre hand saw, finish hammer, nail set, and finish nails. Warning: This takes skill and a steady hand! We recommend practicing with these tools before installing your window trim.

Before You Begin

Put that tape measure away – you won’t need it! The hard part about trimming a window is measuring the length of the trim to cut and cut the right angle. It may look like a 45-degree angle at each corner, but most rough openings for windows are – well – rough. For a precise miter joint, you’ll need to make small adjustments to your mitre saw for each cut.

Always start with the biggest window; that way you can use trim from miscuts on smaller windows. Since the trim is sold in long sections, it may help to trim it to rough lengths first so it’s easier to handle.

1. Install the Top Piece of Trim

First, use a rafter square to make two pencil marks that form an “L” around each corner of the jamb. This “L” should follow the right angle of the jamb and be 3/16 of an inch from the perimeter of the window frame. This will be your “reveal,” or the space between the jamb and the trim. These marks will also help you quickly measure the length of each piece of trim.

Use your mitre saw to cut one end of the window trim to a 45-degree angle. Always put the thickest side of the trim against the miter saw fence to get the cleanest cut and avoid a tear-out.

Place this piece of trim at the top of the window, and line up the bottom corner of your 45-degree cut with the corner of your penciled “L” in the upper left-hand side of the window. Holding the trim firmly in place, mark the other (uncut) end of the trim where the bottom edge lines up with the right-hand pencilled corner. Use your mitre saw to cut the trim at a 45-degree angle where you make this mark. Remember to flip the trim over before you cut!

Use your brad Nailer to tack the trim to the top of the jamb. The nails should be spaced roughly 6 inches apart. Use 1-inch brad nails to secure the inside edge of the trim to the jamb and use 2 1/4-inch nails to secure the outside edge of the trim to the drywall and frame. To prevent the trim from splitting, don’t nail closer than 2 inches from the end of the trim.

2. Install the Side Trim Pieces

Now that the top trim piece is installed, you can install the window trim for each side. Cut a new piece of window trim at a 45-degree angle with your saw. Match it up to the mitre joint on the window trim you installed earlier. It probably won’t match perfectly. Adjust your miter saw and trim the angle until it matches up.

You can’t take back what you already cut. Don’t aim for the pencil mark with one quick slice of the saw; instead, make several cuts as you “sneak up” on the pencil mark with the mitre saw blade, checking the fit as you go.  

Once you’re happy with that joint, mark the uncut side of the trim where it lines up with the pencil marks you made earlier, and cut your 45-degree angle. Spread some wood glue in the upper mitre joint before you nail the trim into place. Put several nails in the face of the trim and one nail in the outside where the corners meet. Repeat this process on the other side.

3. Install the Bottom Piece of Trim

The last piece of trim is the hardest to install since you must cut two mitre joints perfectly. The idea here is to trim the angle for each mitre joint before cutting the trim to length. To do this, cut one mitre joint on one end and adjust the saw until you have a tight fit for your first mitre joint. Next, cut the mitre at the other end of the trim, but don’t cut it to length. Check the fit of that mitre joint, and adjust your saw as necessary, again “sneaking up” to a good fit. Once those joint fits well, cut it to length with the same saw angle setting you just used.

4. Apply Finishing Touches

If you’re not painting the trim, use a wood putty that matches the wood finish to cover the nail heads. Otherwise, use wood putty to cover the nail holes before you prime and paint the wood trim. You can also use painter’s caulk (paintable caulk) on the edges where the trim meets the wall or window frame. If there are small gaps between the trim and drywall (unevenness due to finishing allows for gaps), then apply a very fine bead of the painter’s caulk.

Professional Window Trim Installation

Since this installation job requires a compressor, mitre saw, and brad Nailer, you may be better off hiring a professional for window trim installation. For only a few windows, the cost of tools will significantly outweigh the cost of professional installation, especially after your account for your time. Just give Mr. Handyman a call or request service online. We will trim your windows and can take care of any other repairs while we’re there!