Easy DIY Window Casings (No Miter Cuts!) - Addicted 2 Decorating®

So many people hate installing window and door casings because of the dreaded miter cuts. Well, I’ve been trimming out my windows and doors in a way that I think is really beautiful (much more so than just standard, plain casings), and guess what! These easy DIY window casings require no miter cuts! This style of window casing is so easy for a DIYer, and yet it has a very substantial and custom look.

Isn’t that so much nicer than the standard narrow pieces that are mitered at the upper corners? I sure think so! So let me show you how I installed this super easy DIY window casing.

I’ll show you step-by-step how I did this, but please note that I’m installing casings on windows that have never had trim installed on them before, so that means that I’m starting from the very beginning. If your windows are already trimmed, and you’re just wanting to beef up the trim and change the style, that means you probably already have jambs installed, so you can skip ahead a few steps.

This is what my window looked like before I got started…

So let me show you how to get from that completely untrimmed window to a finished window with beautiful casing.

How to install DIY window casings (window trim)

Step 1 – Trim away any excess insulation and drywall.

If you’re starting with newly installed windows like I am, the first thing you’ll want to do is make sure that the drywall is cut flush with the framing around the window, and also make sure that there’s no excess insulation that will get in the way.

If you have excess insulation like I did, or if you need to trim away any drywall that didn’t get cut flush with the window framing, just use a utility knife or serrated kitchen knife to cut away the excess.

Step 2 – Cut and install the window sill (stool).

The first piece that will be installed is the window sill (technically called the stool). To determine the size of lumber you’ll need for this, measure the distance from the window frame to the front edge of the drywall…

The measurement for my window was 2.5 inches. To that measurement, add 1.5 inches, which gave me a width of 4 inches for my window sill. That meant that for my window sill, I would need to use a 1″ x 6″ piece of lumber. (Remember that a 1″ x 4″ is actually only 3.5 inches wide, and a 1″ x 6″ is actually 5.5 inches wide.)

To determine the length of the piece I’d need for my window sill, I took the inside measurement from one side of the rough window opening to the other…

The measurement for my window was 48 7/8 inches. To that measurement, I added 8.5 inches, which would allow the window sill to overlap the wall past the rough window opening 4.25 inches on each side. That gave me a measurement of 57 3/8 inches, which I used to cut my 1″ x 6″ lumber for the window sill.

After cutting the length of the board with my miter saw, I used the table saw to rip the board to the 4-inch width that I needed for my window sill.

Before I could install the window sill, I needed to cut a notch out of each end for the part that extends on the wall past the rough window opening. Using a speed square, I marked the 2.5-inch measurement that I had taken, as well as the 4.25-inch measurement for the extra width that I added to the window sill board on each end.

If you have to rip a board for the window sill, be sure that the cut edge is the back edge that will go against the window, and keep the factory-cut edge on the front edge that will show.

I used a jigsaw to cut the notch out of each end.

Then I put the window sill in place to check for fit…

And I want to show y’all this, because while I am a perfectionist, I’m not perfect. 🙂 I didn’t do a great job cutting this end, but things like this don’t stress me out at this stage…

A lot of that gap will be covered up by the side casing piece, and the part that still shows can be filled with caulk before painting. Those of us who aren’t excellent finish carpenters have to rely on wood filler and caulk, and that’s something I do with just about every project I do. And I’m okay with it. 🙂 However, if you’re using wood that will be stained for your window trim, you’ll want to take extra care to cut it as perfectly as possible since caulking isn’t an option.

I did a little better on the other end, but I’ll still need to use caulk to fill in a gap there…

Before nailing the window sill in place, I checked for level using a long bubble level.

It wasn’t level (not a surprise), so I had to shim one end. You don’t have to have actual shims for this. Any scrap piece of wood can be cut/ripped to create a shim. I actually used pieces of a paint stir stick to shim the sill…

The paint stick shims did the trick, so I nailed the sill in place. I used 16-gauge 1.5-inch nails to install all of these pieces.

Step 3 – Cut and install the top jamb.

Next, I measured, cut and installed the top jamb. Unlike the sill, the edges of the top and side jambs need to be flush with the face of the drywall. This piece also needed to be shimmed to get it level, and then I nailed it into place.

Step 4 – Cut and install the side jambs.

Next, I installed the two side jambs. These need to be ripped to the same width as the top jamb so that the edges are flush with the face of the drywall. In a stroke of luck, mine were 2.5 inches wide, which happens to be the exact width of a 1″ x 3″. Using a bubble level, I made sure the side jambs were plumb before nailing them into place. Both of mine had to be shimmed.

As a side note, the other two windows in this room happened to be the super easy windows to trim. The windows were level, and the trim went on so easily and quickly. This window, however, is so unbelievably unlevel, like nothing I’ve ever seen on a newly-installed window. So if you’re having new windows installed, I highly recommend double checking the installer’s work yourself with your own level before any siding and drywall goes up.

Step 5 – Cut and install the side casings.

With all the jambs installed and square/level/plumb, the two side casings can be installed. For this, I use 1″ x 4″ lumber. For this piece, I measured from the window sill to the bottom of the top jamb, and then added 1/4 inch to that measurement for the length of the side casings. Always measure each side separately, and don’t just assume that they’ll be exactly the same.

When I install the side casings, I don’t line up the inside edge of the casing so that it’s flush with the face of the side jamb. Instead, I move it over so that there’s about a 1/4-inch reveal on the edge of the side jamb. Also, when installing these, I make sure that I put at least three nails through the side casings and into the edge of the side jambs, so that the two pieces are connected to each other. I also nail the casing piece to the wall in at least three places (top, middle, bottom).

And of course, always use your level before attaching the pieces.

Step 6 – Build and attach the top decorative header casing.

To create the top header piece of the casing, I measure from the outside of the left side casing to the outside of the right side casing.

Then I take that measurement and cut a piece of 1″ x 6″ lumber to that exact length.

Next, I add one inch to that measurement, and cut two pieces of 1″ x 2″ lumber to that longer length. These three pieces create the top header.

Using the same nails plus wood glue, I attach these three pieces so that the 1″ x 6″ is sandwiched between the two 1″ x 2″ pieces. Before nailing them together, I measure to make sure that the 1″ x 2″ pieces extend 1/2-inch past the end of the 1″ x 6″ on each end. I nail the pieces together through the face of the 1″ x 2″ and into the edge of the 1″ x 6″.

Those pieces are nailed together in such a way that the back of the assembled header piece is flat (i.e., the back of the 1″ x 6″ is flush with the edges of the 1″ x 2″ pieces on the back of the assembled header piece).

Once assembled, that large header piece of casing sits right on top of the side casing pieces, and I nail it to the wall making sure that the ends of the 1″ x 6″ line up with the outside edges of the side casing pieces, which makes the ends of the 1″ x 2″ extend past the outside edge of the side casing by 1/2-inch.

Step 7 – Cut and attach the apron.

And finally, I use a piece of 1″ x 4″ lumber to make the apron. For this piece, I take the same measurement as I did for the top header 1″ x 6″ (i.e., I measure from the outside of the right side casing to the outside of the left side casing), only this time I measure at the bottom right above the window sill. Again, don’t just assume that the top measurement will be exactly the same as the bottom measurement. Even if you try to have everything perfectly square/level/plumb, measurements can be off just a bit. So always take separate measurements for each piece that needs to be cut.

So measuring just above the window sill, I measured from the outside edge of the left casing to the outside edge of the right casing, and used that measurement to cut a 1″ x 4″ piece of lumber to use as the apron. Then using a bubble level to check for level, and making sure that the ends of the apron lined up perfectly with the outside edges of the side casings, I nailed that piece into place.

And with that, the installation is done! It still needs wood filling, sanding, caulking and painting, but the installation is done.

So again, here’s how my window looked before being trimmed out…

And here it is with it’s pretty new casing that looks way fancier than it really is. 😀

I’m so glad I went with this style of trim on my door and windows because I’ve trimmed out every single door and window in this house myself. I love that I can get this custom look with a wide casing and a fancy-looking header without having to cut even one single miter cut. It’s deceptively simple.

Helpful sources and products:

• Table saw – If you’re looking for a really good, reliable table saw, but don’t have a lot of room to store it, I highly recommend the one I have — the Dewalt 10-inch Compact Job Site Table Saw. It’s heavy enough to be durable, but small enough to be portable. And you can use the guide fence to rip widths up to 24.5 inches, which is pretty amazing for a small, portable table saw.
• Nail gun and compressor – If you need a small, portable air compressor and nail guns, I recommend the set that I have — the Porter-Cable 3-Tool Combo that includes a 6-gallon pancake air compressor, a 16-gauge nail gun, an 18-gauge nail gun, and a staple gun. While my most-used of those tools is the 16-gauge nail gun, I do use the others as well. The 18-gauge nail gun is great for installing smaller trim. The staple gun is great for some upholstery projects (easy wrap-and-staple projects, like upholstered headboards or dining chairs), and for things like attaching house wrap or securing barriers under hardwood flooring. While I use the staple gun the least, it’s still very nice to have on hand.

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  Kristi Linauer

Addicted 2 Decorating is where I share my DIY and decorating journey as I remodel and decorate the 1948 fixer upper that my husband, Matt, and I bought in 2013. Matt has M.S. and is unable to do physical work, so I do the majority of the work on the house by myself. You can learn more about me here.