If You Give a Mouse a Cookie–A DIY Tutorial on Window Trim and How One Thing Always Leads to Another

We’ve all heard the story of If You Give a Mouse a Cookie. Then he wants some milk, then a straw, then a napkin….And eventually you wind up giving him another cookie and on and on the story goes. Seriously this is my life. I start one thing then that gives me an idea to do this, then I have to do that, then what if we did this, but then I’d have to do something with that. It’s a vicious cycle.
So, making the above introduction relevant, we got new windows in our house….which of course leads me to thinking “now we have to redo all the window trim.” Which will then lead me to “now we have to do something different with the curtains.” Which will then lead to “I think I want a window seat there.” See how this goes?
Good news for you is that this provides me an opportunity to give you a tutorial on window trim…maybe that’s good news, maybe not ha!
So to begin, let’s discuss my previous window. They were old, single pane windows like I’ve never seen before. I can almost guarantee that your windows will be easier to trim than mine were because these are so weird. Most windows will sit back from the drywall some and provide a little place for a sill. Mine did not…in fact, mine had this jamb that was acting as the trim. That’s it.

Normally you would have this piece around the outside of your window, but it would be flush with the drywall…mine wasn’t. By the way, first pic is old window, second pic is new–the old widow sat back a little, but the new one comes all the way to the front of the jamb, so I couldn’t cut this off to be flush with the drywall. So we had to make this work.

By the way, I know this is like rule number one of how not to hang curtains. I just haven’t figured out yet what I want to do here. See second paragraph.

I wanted my trim to match what I did with the kitchen windows when we remodeled. You can check those out here.
If you have a normal window (where your window sets back from the drywall some), you’ll need a 1×6 or a 1×4 (depending on how deep you want your sill) the length of the window opening PLUS about 10″ or 11″. If your windows are like mine and come flush, use a 1×4 or smaller.
You’ll also need:

(2) 1x4x length of window opening+7″

(2) 1x4x height of window opening

Crown moulding and half round moulding x the length of window opening plus a foot or so for angled cuts and any screw ups while cutting (it happens)

Wood putty and sandpaper

Primer and paint

Miter saw

Jig saw (optional)
I recommend measuring and cutting as you go as not all windows will be perfectly square so the measurements above are only guidelines so you know how much wood to get.
First, remove your old trim with a pry bar and a hammer. (I didn’t have to do this since my window didn’t have any to begin with.) if you have a piece of wood where the sill should be, remove it also. Itll look something like this: (this was my kitchen, not the one I’m doing now)

Next, install the sill. I apologize, I don’t have good pictures here because I didn’t do it this way for this window, but I did for my kitchen.  Measure the window opening and add 7″ for the side trim pieces, plus another 1 1/2″ to 2″ for EACH side for an overhang.
If your window sets back a little, you will need to trim notches off the ends so it will sit back flush to the window. Center your board to your widow and make a mark where the opening is and measure the depth from the front of the drywall to the window. Using a jigsaw, cut the notch so it will slide into the window opening and the end pieces will sit flush against the drywall.

These don’t show it well, but I had to notch out every place between widows so it would sit flush to the window and then leave some overhang on each end.  Once you have your window sill in place, use a nail gun to nail the sill on. (And don’t worry if you have some small gaps or if everything isn’t perfectly flush…some times houses aren’t square and you can get it perfect no matter how hard you try. You can caulk this later.)
If you have a widow like mine, you take the 1×4 sill and drill pocket holes along one end.  Then, screw it into the bottom window jamb. Be careful to go slow so you don’t crack the jamb.

I had a gap between my sill and the wall, so I needed to fill that later.
Once the sill is on, attach the sides. Measure from the sill to the top of the window opening on both sides. Since things aren’t always square, these may be slightly different. Cut 1x4s to these lengths. Attach these so that they are resting on the sill and they are flush with the opening of the window. Mine had to be even with the jamb since mine are weird. Yours should be over the jamb on the sides. Nail them on with a nail gun.

Next, add the apron. This is the piece that goes under the sill. It should be the length of the window opening plus 7″. Cut another 1×4 this length. Using a nail gun, attach it and make sure it is centered.

Then attach the top. Measure the distance between the outsides of the trim pieces at the top and cut a 1×4 that length. Make sure it fits, but don’t attach it yet.
Cut a corner off your crown moulding. To cut crown, place the piece upside down on the miter saw and make sure the flat parts are square against the bottom and back rests of the saw. Turn the saw to cut a 45 degree angle.

Do not cut it like the picture shows !!! Turn it 45 degrees in the other direction!! Sorry, I was cutting a different piece when I took the picture.
Remember when cutting crown that is going on the outside corners, the top should be longer than the bottom. Once your corner is cut, place it on the 1×4 for the top trim piece, lining up the corners. The inside of the corner of the crown should be flush with the corner of the 1×4. Mark the uncut end of the crown where the corner of the 1×4 hits it. Turn your miter saw to cut the opposite 45, and cut the crown where the INSIDE corner of the bottom of the crown is at the mark you made. The piece should measure the length of your top trim piece from inside corner to inside corner on the bottom of the crown.
Now attach the top trim piece to the wall with a nail gun and attach the crown to the top of the 1×4 making sure to line up the corners.

Cut the small pieces for the corners so the bottom from end to inside corner is 3/4″. Put glue on the pieces and tape them in place.

I need my table saw to cut the half round moulding and it is currently out of order. If you are lucky enough to find any half round precut, you can add that across the bottom of the top piece to cover any gaps between the top piece and the sides. Mine will have to wait 🙁
If you have any gaps between your sill and the wall, you can slide a small spacer piece between the top and glue and tape it.

Once the glue is dry, remove the tape and putty all the nail holes. Once that is dry, sand it smooth. Apply two coats of primer, then caulk all the joints where one board butts against another. Once the caulk is dry, apply two coats of paint and you’re done!

Window selfie! See me in there?

Ignore my gap between the top and sides…my window jamb was off square and it’ll eventually be covered by half round so that it looks more like this:

I hope I didnt lose you anywhere. It gets a little hairy in the wording, but it’s really not too hard. And about the half round, you can order it online or check your local lumber yard. Lowes has stopped carrying it. You can cut a 3/4″ dowel rod down the middle or use 1x board, a router and a table saw to make your own. Or you could just caulk it and leave it.


Now I’m on to figuring out how to do my curtains, deciding how/if I want to put a window seat in, then deciding whether I want to put built ins or a fireplace next to the window seat
If you give me a project, then I’ll have another one, then another….
Stay tuned 🙂


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