How to Install Window Trim

install classical window trim with crown molding in two windows in bedroom - teal walls

My bedroom makeover is finally at that point where pretty things are starting to arrive. Yippee!

I've been working in this room for a very long time and I haven't seen much progress. Well, that's it 'til today.

Installing window trim was the project that turned this room from a working zone into a "Hello, this is a pretty bedroom!"

Adding architectural interest to a room is SO important! I call it the structural details that help add weight, interest, and scale to a room. I already added beams on the slanted ceiling and I'm almost finished with a built-in unit. But, all of a sudden I looked at those windows on each side of the built-in, and they looked naked! 😳

Look at that. BORING.

No trim. Well, a little skirted sill was all their dressing. Ha, it's about time to change that!

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This is only a decorative treatment. All the materials can be found at your closest Home Depot.

The tools you'll need:

Miter box and saw (A Miter Saw will be even better)
Jig Saw
Sander
Nailer
Pry bar
Utility knife
Wood glue

How much material you need depends on the size of your window, but this is what I used:

detailed list of moldings used to update a window

Directions

1. DO THE MATH

Measure your windows. Write down those measurements. They're going to be helpful to calculate the amount of material you need to buy and you can also use those measurements to plan your window trim design.

If you need to trim many windows, go and measure each and every one of them. Sometimes they look similar, but the numbers might be different.

2. REMOVE THE OLD WINDOW TRIM IF ANY

In my case, I had to get rid of the old window stool and apron.

Using a utility knife, I went all around those two pieces, cutting the caulking. This will prevent rips on the drywall.

using a utility knife to score the caulking around the window

Use the pry bar to lift the apron. A wood shim will give you the needed support and also it will prevent damage to the wall.

Once the apron is off, you can begin lifting the stool, again, using the pry bar.

I found lots of wood shims underneath the sill. They were used to level the stool. I also left them in place.
Then, I went ahead and scraped and removed all the old caulking from the window and wall.

Don't throw away the old window stool just yet. You can use it to trace its silhouette onto your new material.

3. CUT & INSTALL THE WINDOW SILL OR STOOL

Once again, you need to do some measuring to come up with the number to cut that 1 x 5 to size.

This is how it worked for my windows:

width of your window: ( ), in my case it was 32"
add the width of both side casings: ( ), mine was 7" (width of the 1 x 4 side casings)
add 1/4" on each side for reveals: ( 1/2" )
add 1" on each side for the "horns" ( 2" ).

The total length of the window stool I used was 41½"

Mark the center on the old and new stool boards, align those centers, and trace the new pattern.

tracing the old window sill into the new material

If you are working on more than one window, it is a good idea to mark each board, even though they look pretty much the same, their measurements can vary.

Use the jigsaw to notch off those marked ends.

You can now install the new stool on the window. Make sure is level. If not, level it up with wood shims placed underneath. Using a brad nailer with 2 1/2" nails, set it in place.

4. CUT & INSTALL THE APRON

The apron dimensions are the same as for the stool, BUT without the horns (2") and made out of 1 X 4 material. The horns are those extra parts on the window stool that go beyond the apron in this next picture.

Center the apron below the stool and nail it in place.

5. CUT & INSTALL THE SIDE CASINGS

You can find the length of the side casings by measuring the distance from the stool to the top of the window opening. Add 1/4" (top reveal line) to that measurement.

Set the side casings in place with brad nails.

6. INSTALL THE HEAD CASING

Once both side casings are installed, you need to measure the distance from the outer, top points of both side casings. However, a better way to determine the length of the head casing is by simply placing the 1 X 6 board on top of the side casings. Align it to one side and mark the ending point on the other side, as shown in the picture below.

Cut and install.

using a brad nailer to install window trim

7. EMBELLISH THE HEAD CASING WITH CROWN MOLDING AND HALF-ROUND

The decorative molding, crown, and half-round need to be cut (short point) at the same length as the head casing. For help on how to cut crown corners click HERE.
You also need to cut the 3/4" returns.

Mitering the ends.

The crown is installed at the very top of the head casing and the half-round is aligned to its very bottom.

The returns are glued in place and held overnight with tape.

head casing with crown molding and half round

8. FILL NAIL HOLES AND GAPS WITH WOOD FILLER & SAND

After filling the nail holes and gaps with caulking, good sanding is a necessity. Don't forget to sand those sharp edges too.

9. PRIME AND PAINT OR STAIN AS DESIRED

I went with one coat of primer and two coats of paint and I got a set of brand new, good-looking windows in my room!

Window in bedroom with built-in and teal color wall

No naked windows anymore! Love the classy element the crown molding imparts.

Paint remnants came in handy for achieving this hue.

And here, I updated this post with a wide shot of the two windows on each side of the newly built bookcase. Waiting for the electric fireplace to arrive