Easy DIY Modern Window Casing - Pretty Passive

I did it, guys! I finally cased out one of our windows with a DIY modern window trim. It was so much cheaper and easier than I expected, and I expected it to be really simple all around. This small weekend upgrade DIY took only 4 pieces of wood & has made the biggest impact on the look of our home.

Alright I’ll stop gushing, but this project looks and feels so good that even my husband Tano is drooling over it.

Table of Contents

• The easiest way to replace old tile window sills and avoid demo
• Choosing a Design for your window trim
• DIY Modern Window Trim Supply list
• Window Trim Cut list
• Step-by-step diy window trim and sill replacement
  • 1. Measure the depth of the sill and decided on a style.
  • 2. Buy the wood/trim and any other supplies.
  • 3. Cut the windowsill to width and draw out the edge boxes
  • 4. Do a dry fit and trim the boxes as needed for it to sit comfy on top of that old sill
  • 5. Once it fits, apply the adhesive caulk to the naked tile sill and sit your fresh one right on top of it.
  • 7. Cut the remaining pieces (2 sides and the top) and install the sides with multiple nails.
  • 8. Fill the nail holes and wood joints with wood filler
  • 9. Start caulking!
  • 10. Paint and you’re DONE!
• The final product
• Mistakes I made

Replacing our windowsills and casing the windows has been a must-do item for the last 5 years, but we put it off because they are currently a blank slate with the most disgusting yellow tile sills. V old Florida. And not in the old school cool pastel art deco and rattan Golden Girls way.

We have done enough big DIY projects to know that there’s always a lot more mess than you anticipate, and the clean up and repair job on these windows was… undesirable, at best. So I came up with a lazy-girl’s alternative to ripping out those tile sills and it looks damn good.

Here is the before from the nursery makeover. Excuse the mess. We live here, but you can see how dang plain it is. If the window sills werent bad enough, the whole thing is screaming for TLC

The easiest way to replace old tile window sills and avoid demo

The simple secret to replacing the dingy tile was so literally cover them up. I know that sounds really gross and totally half-assed, but in addition to dreading the giant mess mess of knocking them off one by one I honestly believe they maintain some of the structural integrity of those windows and in turn, the house.

That sounds stupid, but in the case of this house I am amazed this place is still standing. I would hate to accidentally chip away from of the wood frame casing out the structural frame for those windows. And I swear it’s like the house was built AROUND those windows. It’s weird, man.

So many things are like that here, but I digress.

Also, we have a bunch of windows to handle so I needed to find the easiest way. Otherwise it simply wouldnt happen. Just being honest over here.

The windows are newer and not due for replacement any time soon, but when they were installed we were left with a solid 2-2.5 inches of window frame above the tile, so I knew I could use the standard 1″ boards from Home Depot or Lowes (they are actually 3/4 inch thick).

The next part was the trickiest piece of the whole equation.

Choosing a Design for your window trim

Decisions are everywhere in DIY. Finding the “right” design for your windows is very personal, and if you’re like me then you will forever imagine all of the different ways it could have been.

Daydreamers, unite.

But for the sake of buying very little wood, I decided on a modern square frame with the extended sill. I’m sure there’s a better way to describe this, but here we are.

It is pretty standard to have the window sill extend horizontally to the edge of the trim casing, and I think it would look weird if it didn’t. Unless you went with a shallow sill (like the original tiles). The tiles are only about 3 inches deep with a bullnose edge. Practically worthless.

Ultimately this is the design I settled on:

I chose to not miter the top corners simply because the size of that window allowed me to cut one standard 8 foot board in half for the two sides. But before I get into too many details, let me cover the supplies.

DIY Modern Window Trim Supply list

• Miter Saw
• Jigsaw
• Nail Gun
• Wood filler
• 240 grit sandpaper sheet
• Adhesive Caulk
• Paintable acrylic latex caulk with silicone)
• caulk gun
• Preprimed lumber

The lumber I used for this one 52″x50″ window was all preprimed pine:

• 1 1x6x8
• 3 1x4x8

And that is all lol

Window Trim Cut list

1 1×6 at 59″ (the window width + width of the 2 edge trim pieces [or more, if you’re into that sort of thing])

2 1×4 at 59″ (the windowsill width for top and bottom trim)

2 1×4 at 48″ (I cut one board equally in half, otherwise my window size would’ve required an additional board and NOPE. Not for 1 more inch on each side)

Step-by-step diy window trim and sill replacement

1. Measure the depth of the sill and decided on a style.

I went with a plain wood box framing the entire window with the windowsill extending horizontally to the end of the box frame. The design I chose required the least amount of wood and it looks modern. Win-win!

2. Buy the wood/trim and any other supplies.

3. Cut the windowsill to width and draw out the edge boxes

that let it sit flush along the wall. Use a jigsaw to cut them out.

*We can’t find our jigsaw, so the alternative I opted for was to screw a ton of tiny holes (pumpkin carving style) and ask Tano to use the little hand saw we have. The cuts were a little rough but it was all going to get covered with wood/caulked anyway.

4. Do a dry fit and trim the boxes as needed for it to sit comfy on top of that old sill

as well as adjusting the width to eventually be flush with the side pieces.

5. Once it fits, apply the adhesive caulk to the naked tile sill and sit your fresh one right on top of it.

6. Cut the bottom of the frame (remeasuring width as needed) and place it flush underneath the new windowsill. Nail it into the wall and varying angles for a more secure fit, and nail the horizontal overhang of the sill into the bottom frame (this will be covered by the sides of the frame).

Since there is a tile windowsill underneath, we couldn’t nail directly into that, but I very carefully got one nail through the middle of the sill into the bottom frame without completely missing it and having an exposed nail.

7. Cut the remaining pieces (2 sides and the top) and install the sides with multiple nails.

Once those are up, you can easily place the top over them and nail that one where it sits, flush on both sides.

8. Fill the nail holes and wood joints with wood filler

and lightly sand it with 240 grit sandpaper after it dries (30 minutes – 1 hour).

9. Start caulking!

The rule of thumb is to caulk everything that meets at a 90 degree angle. Any flat joints are best served with wood filler.

Pro tip: use baby wipes for caulk clean up and thank me later. 😉

10. Paint and you’re DONE!

Since it’s preprimed, the only “issues” are the edges, but they’re really not issues in my book. Slap some paint over them.

Pro tip: use frogtape painters tape for *crisp* lines and a wooster angled brush to all things edging.

I captured most of this process and more mistakes on Instagram. Theyre all rolled into a highlight for you, too 😉

The final product

There she is. My favorite frugal home improvement project to date. This one window cost about $35 in lumber but she looks like a million bucks. I already had all of the other tools, which just goes to show how handy a lot of these things are. Many of them (including the wood filler and adhesive caulk) have been sitting around the house for several months (years??).

Stalk me on instagram to catch the latest project.

Mistakes I made

This project was easier than I expected, but naturally some mistakes were made. I am by no means a perfectionist which is probably why I love DIY so much! Its almost impossible to do everything right the first time around.

Luckily I have a ton of other windows to get it right with. Ive already included the correct method in the steps above, but this is what I did wrong to catch my mistakes for you:

cutting the bottom frame before fitting the sill – we cut everything first, even before fitting the sill. This might be more efficient going forward, but my measurements were not perfect this time around. Surprise, surprise!

Because of this and my strong desire to have the side frame pieces lay flush with the edge of the sill the width of the bottom board, a *little bit of the window frame is “uncovered” by the trim I added. I’m cool with this. It’s a testament to the live and learn DIY life I live!

not sanding the unprimed edges – they are raw wood, so painting them looks different from the rest of the trim. If I wanted it totally uniform then I would sand AND prime them before painting.

That is about it. Everything else went remarkably smooth and quick!) .I got all of this done in a total of about 3 hours.

Thank you for reading. Pin for later & happy building!

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