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How to Install Window Trim

How to Install Window Trim

Hi everyone, I’m Cristina from Remodelando la Casa, and I’m beyond excited to join the team of Pretty Handy gals and guys!  I’m still pinching myself at such a wonderful opportunity.

Today I’m going to show you a fairly easy way to transform your builder grade windows from plain and boring to beautiful with loads of character and with a more finished appearance.

trimmed left window lg

Right now I’m working on updating my bedroom, where I have a couple of these windows.

small window sills lg

 They look  like they are wearing skirts, but forgot to put on the tops! 🙂  Yeap, naked!

plain builder's windows lg

Let’s change that by dressing up those windows!

This is only a decorative treatment.  All the wooden material (Poplar or Pine), can be found at your local home center.

(Affiliate links are included for your convenience. Brittany earns a small percentage from a purchase using these links. There is no additional cost to you. You can read more about affiliate links here.)


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

parts of window - trim
moldings for window trim


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

Cutting the caulking lg

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

pray bar to take away sill lg

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

taking away window sill lg

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

window sill removed lg

Don’t throw away the old window stool, you can use it to trace the pattern onto your new stool.   Measure the width of your window, add the width of both side casings: 7 inches, add 1/2″ for reveals (1/4″ on each side) and add 2 more inches for the “horns” (The horns are those extra parts on the window stool that go beyond the apron. Allow one inch on each side of the stool.)

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

tracing new stool with old one lg

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.

two window sills ready to be cut lg

Use your jig saw to cut away those marked ends.

Cutting window stool with jig saw lg

Place it on the window and make sure the new stool is level.  Nail it in place.

nailing stool to a window

The apron can be installed by centering it underneath the stool.  The dimensions are the same as for the stool, but without the horns (2″).  As shown below, the horns protrude an inch on each side beyond the apron.

attaching apron to a window lg

The side casings can be installed now.  To figure the length of the side casings, measure the distance from the stool to the top of the opening of the window, add 1/4″ for reveal line. Secure the side casings with nails.

attaching sides of window lg

After the side casings are installed, measure the distance from the outer edges of the side casings. That will give you the head casing length.

Head Casing Length

Nail the head casing in place.

attaching top part on window lg

 Cut the decorative molding, crown and half round.

crown molding and half round to trim window lg

When cutting the decorative molding, miter the ends (cut at a 45 degree angle as shown below.)  Remember to cut the molding upside down.

cutting crown for window lg

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

nailing crown molding to window lg

The returns (side pieces of the decorative molding) are glued in place and held overnight with tape.

attaching returns of crown to window lg

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

sanding window before painting lg

 Paint the trim with one coat of primer and two coats of paint. Then you’ll have good looking windows in your room!

new trimmed window1 lg

Add new paint in a contrasting color on the walls and the room is almost ready!

new window trim-crown lg

 I had some leftover paint that I mixed to achieve this hue.

trimmed left window lg

window stool lg

These twin windows really add a lot of character to the room.

finished two window trims lg

 The trim makes a big difference, wouldn’t you agree?

new trimmed window2 lg

 Thank you so much for having me here at Pretty Handy Girl! I can’t wait to share more with you!

~ Learn more about Cristina~

Don’t miss Brittany’s great tutorial for trimming a casement window: How to Install Trim and Casing Moulding on a Casement Window by PrettyHandyGirl

112 replies
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  1. Kimberley
    Kimberley says:

    Thank you for the precise instructions. I have two sets of 3 side by side windows, I cased all three as if they were one. They turned out beautifully!

  2. Sarah
    Sarah says:

    I’m confused… so is the side casing trim 4inches or 3.5? Because if they were 4″ as per your materials diagram, adding the side casings together would be 8″, not 7.5.

    I would like to use your exact measurements 🙂

  3. vijay
    vijay says:

    I am installing the window trims and the crown on the top. We have our present windows exactly like what you had before installing the new trim. I just want to know if we need to install any wood to the inside dry wall of the window (just inside and next to the trim). I am not sure if we need to paint white inside the window or if that goes with a wood panel.
    Appreciate your help!

  4. kitkat
    kitkat says:

    Hi this tutorial is excellently dictated. I have seen other types and have been left without complete instructions. This was clearly shown with pictures of the items needed, the tools used and descriptions of the window trim pieces. I plan on doing this to my home office window as a trial before tackling the others. I will have to rent a nail gun with the pneumatic. Thank you so much for the details!! awesome.

  5. Karen
    Karen says:

    I don’t have any wood around my windows, just the wall material. Where would you suggest I start? Should I add wood around the window first then start off at the first step of your tutorial?

  6. Kate
    Kate says:

    Hello! I’m so glad I found this post. We recently bought our first home and overlooked the fact that every window is dry walked in without trim… I hate it!! Our problem is that one window is set very deep and the dry wall is textured. Also, each window has a about 2″ piece of trim over the drywall next to the frame of the window. How do I go about this? I can send pictures if you need clarification! I would appreciate any insight you might have!! Thank you!!


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