How to Install Window Trim

This post was first shared at Pretty Handy Girl right HERE
How to install window trim

I'm right now working on updating my bedroom, it's a good size room that's so plain it feels cold. So, I've been adding some architectural interest like beams, custom bookcases and now trim in order to make it feel cozy, interesting.

Just the same as all around the house I got these kind of windows:

Boring I know! No trim and only a little skirted sill. Well, it's about time to change that!

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

The tools you'll need

  • Miter box and saw  (miter saw will be even better)
  • Jig saw
  • Sander
  • Pneumatic nail gun
  • Pray bar
  • Utility knife

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
I only found the 1x5 pine for the stool at Home Depot
moldings used to update a window

The first thing to do is getting rid of the old window stool and apron.
Using the utility knife, go all around cutting the caulking, very important to prevent rips on the drywall.

Using the pray bar begin lifting the apron, use a wood shim as support, again, to prevent damage on the wall.

Once the apron is off, you can begin lifting the stool.

I found lots of wood shims underneath, that were used to level the stool.  They were left in place.  Scrape the old caulking from the window and wall.

Don't throw away the old window stool just yet.  You can use it to trace the pattern onto your new stool.   
Time to measure:
width of your window:                   (          ),  in my case it was 32"
add the width of both side casings: (          ),  mine were 7" 
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.

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.

Now it's time to use your jig saw to cut away those marked ends.

You can now place it on the window, and making sure is level, nail it in place.

The apron can be installed next.  The dimensions are the same as for the stool, but without the horns (2").  The horns are those extra parts on the window stool that go beyond the apron in this next picture.

The side casings can be installed now.  For their length, measure the distance from the stool to the top of the opening of the window, add 1/4" for reveal line.

Once both side casings are installed, measure the distance from the beginning of the first casing to the end of the second, that's going to be your head casing length. However, a better way to determine the length of the head casing (as I did in my dining room windows), by simply placing the board above the side casings, aligning it to one side and marking the ending point on the other side, as shown in the picture below.

Once cut, secure it to the wall with nails.

The decorative molding, crown and half round, needs to be cut at the same length of the head casing. For help on how to cut crown corners click HERE.

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.

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

One coat of primer and two coats of paint and you have brand new, good looking windows in your room!

A new color on the walls and the room is almost ready!

Remnants of paint came handy for achieving this hue.

A wide shot of the two windows on each side of the newly built bookcase:
bookcase in between two trimmed windows

The trim makes a big difference, don't you agree! 
moldings used to update a window

For more DIY - Home ideas in this room check these out:

how to build a set of bookcases with fireplace

DIY wooden beams how to

These are all the Master bedroom projects so far:
Install faux wooden beams
DIY Bookcases with Fireplace
Install trim around door niche
Repurpose a Wooden Divider as Headboard
Christmas in the Bedroom


  1. You are probably the handiest girl I've come across so that's no surprise! I love that window trim. That's the same style we've been using in our home. I can't wait to see your easy tips!

  2. I went a read the whole thing!! You did great! I'm so excited for you.

  3. Felicidades Cristina por tu nuevo esquilo. y muy buen tutorial

  4. You are a perfect candidate for empowered! Good for you- xo Diana

  5. That post at Pretty Handy Girl is where I found out about you!


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