DIY Window Casing

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A step-by-step tutorial on how to case a window, update your homes interior windows by using this simple DIY window casing tutorial.

Do you want to update the look of your windows? We did, and it’s official our windows are entirely done, well almost, only the one above the kitchen sink is left. But we are going to finish that one when we repaint our kitchen cabinets. But here are the steps we took to create this simple DIY window casing.

install window trim for interior windows

Let’s start at the beginning. When we purchased our home, we found out very quickly that our windows were not very good at keeping the cold winter air out, which made us replace them within a couple of months. 

red brick home in snow

After we replaced all the old outside windows, we still needed to remove the inside. That’s right; our home had two windows (an interior and an exterior).

When we removed the exterior window, there were lots of holes that needed to either re-plastered or framed out by wood. We decide to install window jambs around the inside of the windows to solve this problem.

But the problems did not stop there. We also needed to remove the old tile window sill, which was a lot harder than we anticipated and was the longest part of this whole project. Here’s what our windows looked like after removing the tile window sill.

remove tiles from window sill

SIDE NOTE: We did have to patch the plaster walls around the window jambs because they got a little beat up.

After all these projects, we were ready to install interior window casing.

What does it cost to install a window casing?

Our cost for seven windows was about $260 for the supplies for the casing. So plan on about $38 per window, but your price could be more or less depending on the size of the casing you will be installing plus the size of the window.

How to case a window

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  • 1″x4″ board (primed is best if you will be painting)
  • 2″ nails
  • liquid nails (optional)
  • wood filler or spackling paste
  • #220 grit sandpaper
  • white caulk
  • primer
  • paint



SIDE NOTE: I do share a more detailed tutorial on how to install interior window trim here.

Because you will be installing an interior window casing, I would recommend using a 16 or 15 gauge nail gun to make sure the board will stay in place.

TIP: If you want to save time later, sand the boards with #220 grit sandpaper. Using an orbital sander to do this will also save time. Also, apply primer and paint to the wood before cutting. This way, you will only have to touch up the window casing later after its installed.

STEP 1: Cutting the boards

SIDE NOTE: For this style of window casing there will be an overhang of 1″ on both sides of the top and bottom casing.

To determine the amount of wood needed, you will need to measure the opening of the window (top, bottom, and sides).

TIP: Make sure to measure each individually because the sizes can be different. Also, write your measurements on a piece of paper.

For the top and bottom casing remember there will be an overhang of 1″ on both sides. So you will need to add the width of the sideboards + 1″, in our case the 4″ board’s actual size is 3.5″ plus the 1″, with a total of 4.5″. But this amount needs to be added for both ends of the casing, so an extra 9″ will need to be added to both the top and bottom casings in our case.

TIP: I would recommend cutting only the top and bottom casing for each window then after installing the casing, remeasure the side casing and cut the board to give you an exact fit.

Or if you want to cut everything now. Add .25″ to both side casing measurements. For example, if your length of the window opening is 36″, your cut measurement would be 36.25″ for both sides.

STEP 2: Install the casing to the window

Start by adding the casing to the bottom of the window. Remember you will have an overhang of 1″+ the width of the board on each side of the window opening.

SIDE NOTE: I would recommend dropping the casing 1/8″ from the window opening. This way if the window jambs or drywall around the window are uneven, you hopefully can still have a level casing.

Align the bottom casing, level it, then secure the boards to the wall using a 2″ nail. You will want at least four nails (for each end of the casing) but place a nail about every 8″-12″.

install base of craftsman window trim

TIP: It helps to have a scrap board cut to the extra overhang amount, 4.5″ in this example, then use this board to mark the casing for where the ends of the window will be on each end of the casing.

SIDE NOTE: Our house has plaster then brick for our exterior walls, so to secure the casing to the wall we did use Liquid Nails before nailing the casing into place.

nailing interior window trim

After installing the bottom, its time to install the sides. Remember I recommended remeasuring the sides to give you an exact cut (add 1/4″ for the 1/8″ gap). Once the board fits into the opening, install the side casing the same way as the other two.

Install the window casing for the top of the window the same way as the bottom casing

STEP 3: Prepping the window for paint

Once everything has been trimmed out its time to prep the window casing for paint by caulking and filling in the nail holes.

SIDE NOTE: If you want to have a wood finish, you do not have to fill in the nail holes but if you do make sure to apply a stainable wood filler. Also, do not apply caulk to the casing.

Start by filling in all the nail holes with wood filler (spackling paste works too). Let the filler dry completely.

filling in nail holes on window casing

Once the filler has dried, smooth the area with #220 sandpaper. If the wood is not painted already an orbital sander works great for this.

After sanding wipe the wood with a cloth to remove any dust then apply caulk to all the gaps where the wood hits wood, window, or wall.

apply sealant onto window casing

TIP: To have a smoother caulk line, place a paper towel in a bowl with a little bit of water. After applying a bead of caulk, wet your finger then smooth the bead with your finger. Use a dry paper towel to remove excess from your finger.

Let the caulk dry according to the tube.

STEP 4: Painting the window casing

Now its finally time to paint your new window casing. Start by priming the casing with primer then let it dry completely.

paint window casing

SIDE NOTE: If you are painting pine boards with knots, use an oil-based primer so the knots will not bleed through and learn from my mistakes…

To achieve the best finish, apply two coats of primer.

Once the primer has dried, paint the casing. Apply as many coats needed to give you the finish you want (at least two is recommended).

using a roller to paint window casing

After you have finished painting the casing make sure to touch up the walls too. And you will now have a simple craftsman window casing in your home! Plus you installed the window casing yourself, way to go!!

diy window casing
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  1. Looks great! Amazing what a difference molding can make. I just posted about a DIY Molding we did in our little girls room. We will probably do something similar in our Master very soon.

    @Seeking Lavender Lane

    1. Thanks Deb! I really enjoyed looking around your blog, your home is looking amazing!!

  2. Great post! I would love it if you joined and contribute your awesome posts at my link party at City of Creative Dreams, starts on Fridays :D Hope to see you there at City of Creative Dreams Link Party.

  3. Wow, what a difference. I would have never thought to DIY the jams but they look amazing. Thanks for linking up with us at The DIY’ers!

    1. Thanks Ashley! And thank you for having a great link up…

  4. What a great job!

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