When we moved into our house, all of the windows were finished with sheetrock returns. They were really boring, so one of the first things we decided to do was get rid of the sheetrock and add white painted trim. I’d never done this before, so I had to learn how to trim a window. I started in our daughter’s nursery to get my process down and then moved on to the rest of the house. It was a lot of work, but we are really happy with how they turned out!

How to Trim a Window

Contents

Tools:

Materials:

1 x – Lumber (width and amount vary based on number of windows and their opening size)
Window Casing
Pocket Hole Screws
Brad Nails (at least 1-5/8” long)
Wood Glue
Shims
Wood Filler
Paintable Caulking

Tips:

Work safely, use proper protective equipment, and don’t rush.
Read the entire plan before getting started.
Use glue on all joints between wood pieces.
Predrill holes before using screws to avoid splitting.

How to Trim Out a Window

Step 1

First, measure the rough opening around your window. Measure the height and width as well as the depth (from the finished wall to the window frame). It’s a good idea to get these dimensions at various locations around the window opening, as framing is rarely square. Record the smallest width, height, and depth measurements – these will be used later to construct the window sill and jambs.

Step 2

Choose material for the window jambs and sill based on the width measurement from Step 1. If you are lucky, the width will correspond to a dimensional lumber size and you can use that for the jambs. If not, you will have to rip lumber down on a table saw to the correct width. In our case the jamb depth was 5.5”, the exact same width as a 1×6 board. The sill should be approximately 2” wider than the jambs, so I chose 1×8 lumber for those.

Jamb and Sill Width

Step 3

Determine the length of the top jamb by subtracting ½” from the width measurement you recorded in Step 1. Cut the top jamb to this length. Then cut the sill 5” longer than the top jamb.

Top Jamb and Sill Length

Step 4

Subtract 2” from the height recorded in Step 1. Cut the left and right jambs to this length. Drill pocket hole screws in each end of the side jambs.

Side Jamb Length

Step 5

If you want to use a router on the edge of the sill, do so before you notch it to fit in the opening. Then notch the sill with a jig saw. The notch should be 2-1/2” wide, and the same depth as the jambs (5-1/2” in our case).

Route and Notch Sill

Step 6

Assemble the jambs and sill using pocket hole screws and glue.

06-assemble-jambs-and-sill

Step 7

Install the jamb and sill assembly into the window. Start by leveling the bottom with shims, then add shims to the sides as appropriate to square everything in the opening. Nail through the shims with a finish nailer to secure the assembly to the framing.

Step 8

Next, measure and cut the casing for the sides and top. I like to leave a 3/16” reveal (the casing is set back from the edge of the jamb by 3/16”). This is more forgiving than trying to line up the trim with the edge of the jamb and adds more dimension to the trim. (Hint: Rather than trying to measure the jamb and cut the casing accordingly, place marks on the edges of the jamb 3/16” from the edge. Then hold the casing in place and transfer the mark to the casing. You are far less likely to cut a piece too short this way!) (Bonus Hint: When you are holding the casing in place to mark the cut, make a mark on the trim to indicate the direction of the miter. This makes it easy to remember which way to set the miter saw.)

07-mark-and-cut-casing

Step 9

Install the casing around the top and sides with a brad nailer. Add glue at the miters to keep them from separating. If drywall sticks out past the jamb, “coax it” (read: beat it back) with a hammer until its flush. It’s OK, you’re going to cover it with trim anyway (just don’t get carried away!).

08-glue-and-nail-casing-around-window

Step 10

Cut the casing and returns for underneath the sill. This piece should be as wide as the top piece of casing. Glue the returns onto the end of the large piece of casing. Nail the casing into place with a brad nailer.

09-cut-a-casing-return

Step 11

Caulk all seams and fill nail holes with wood filler/putty/spackle, then paint. (Optional: I like to paint the jamb assemblies before they are installed in the window opening, and the casing before it is cut. That way, when everything is installed, I just have to touch up the nail holes and seams.)