How To: Build a Custom Cat Door in a Window (Spring/Summer Edition)

On Monday you got to see a glimpse of last week's project: building a custom cat door in a window. Well, now here's how we did it!

First, I just want to say that this is a highly customized project, so I'm not going to provide you the measurements we used. It is all going to depend on what type of window you have, and you need to measure your own window anyway, to make sure you have as tight of a fit as possible. This is the 3rd of these types of custom window doors we've made (scroll all the way to the bottom of this post to see past iterations) and each one has been different, but all were based on the same principals.

Which are, make it safe for the cats, and make it so nobody can break in through it. Obviously, if someone wants to break in, they're going to do it, and can just break a window. But you don't want to make it easy for them to get in through the cat door in the window (and by the way, we know for a fact that someone tried unsuccessfully to break in through our very first cat door back in California, but they couldn't, so they broke windows instead).

Also, I should note that this is only stage one of our cat door. This design is fine for spring and summer, warmer months, but we are going to beef it up a bit come fall, to help hold in the heat for winter.


But on with the steps we took to build this latest version! Disclaimer (that my husband asked me to add after proofing this to make sure my instructions all made sense): use a cat door/build this at your own risk. If someone breaks in through it or something, don't sue me. :)

First, we bought our supplies, for the door itself:
2 pieces of 1/2 inch to 1 inch plywood, big enough to fit in your window
Various 1" x 2" slats, as long as your window is wide (the number you need will depend on your window)
High density weather stripping (already had)
Wood sealer/stain/paint
Wood screws that are long enough to go through 1 of your 2 pieces of plywood, through your slats, and about half way through your second piece of plywood
A standard cat door with flap (already had)

And if you need to build a post/ramp:
Quick setting cement
8' long 4" x 4" wood post
12' 1.5" x 4" wood
1' x 2' wood perch
2.5" wood screws
2 L brackets

So, to do this type of cat install, the idea is to sandwich the two pieces of plywood around the frame and window. In our last version, we had newer windows to work with, that had "lips" built into the window frame, that stuck up off the windowsill. So we just put the wood in front of the lip, closed the window a few inches down from the top, and we had a nice frame to work with to make our "sandwich".

Here's a side view illustration of what I mean:

Which worked great in the old window. But our current windows didn't have the little lips sticking up off the windowsill on either side. Instead, they are really old windows, with weird metal pieces that you slide the window up and down on, and with the "lip" being formed by having the window close lower than the windowsill itself. So we had to put the plywood recessed inside the window frame, instead of around it.

Here is a look at our current window, before the cat door was installed:

And here's side view of the recessed sandwiching that we made this time:

And since I'm sure things are still confusing, even with these side view illustrations, lets walk though all of the steps of the build with photos.

Start by measuring your window, with the window open. Measure for the outside piece of plywood that will be recessed within the window frame, allowing for a 3 inch overlap on the top (where you will later close the window to create your sandwich effect). This outside piece won't be quite as wide as your inside plywood, since you are recessing it inside the window frame. Measure for your inside piece of plywood, again, giving yourself 3 inches to overlap the window. Cut both pieces of plywood to fit your window, and double check they're a snug fit.

Then, find the middle of your plywood pieces, and trace where your cat door is going to go. We made sure ours was a couple of inches up from the bottom, to allow for the wood slats that are going to go in the middle of your plywood sandwich. Also, it helps to mark where the screw holes are going to go, and pre-drill them.

Drill out your screw holes, and also drill into each of the corners of your cut out. This makes it easy to get your saw blade in.

Starting in one corner and working your way around, cut out the cat door opening in both pieces of your plywood.

Next, go back and double check that everything lines up. Put your outside piece of plywood in the window, then your inside piece, and make sure your cat door rectangles and screw holes line up.

Then, it's time to figure out your supporting inside slats.

Where the cat door openings are, measure the distance between both pieces of plywood while they are in the window. You will need to find wood slats (we used a combo of 1" x 2" and thinner wood slats, stacked) that add up to the correct width to fill this distance. Once you have your wood slats (saw them down to fit the window width), do another test in the window, with the outside plywood, wood slats along the bottom, and inside plywood all in the window, making sure everything is a snug fit. And then make a second set of wood slats the same width and stacked depth, for the top part of the window.

Then it's time to weatherproof everything. We used a Minwax stain and sealant in one, in a dark brown to somewhat match our deck. Although our long ramp piece and post were already treated and still a little damp. Treated wood can take months to completely dry, so we'll just stain them later this summer once they dry out.

For our purposes, working with a window that was a little over 6 feet off the ground, we switched gears to install the post and ramp next (feel free to skip down to the cat door installation if you want!). We knew we didn't want to attach anything to our house's siding, to avoid letting in any water, so a post was a good option for us. We also did a ramp off of the post and over to our deck that didn't touch the house, so leaves and debris can't collect along the siding.

To have a post sticking out of the ground about 6 feet high, we knew we needed to bury an 8 foot post about 2 feet into the ground, and secure it with concrete. This is what we used:

And our supplies, rubber gloves, dust mask, eye protection and a stirring stick:

We my husband dug a hole 2 feet deep (with the bottom of the hole being slightly wider than the top, to help hold the concrete in place), we inserted the post, mixed our concrete and whomp whomp! It was nowhere near enough.

Apparently you don't get the same volume of concrete as the size of the container. We ended up using 2 1/2 20lb buckets of concrete to fill the hole up to to be about level with the ground. And we still need to brush some dirt over the concrete to cover it up.

Then, to install the ramp from the post over to our deck, we installed two L brackets, one on the post and one on the underside of the deck railing.

It was then just a matter of leaning out the window and attaching the 1' x 2' wood perch for the cats to step out on. We just screwed it into both the top of the post and the ramp. And just like with the ramp, we made sure there was a gap between the perch and the side of the house, so leaves won't fall and build up in front of the cat door.

Alright, now back to the cat door installation itself.

Just like when double checking our measurements and cuts were correct, we first put in the outside piece of plywood, now stained, so it was butted up against the metal slider parts of the window.

And here are the bottom wood slats being pushed into place. Remember, you want a really snug fit, both for safety, and to keep the weather out.

Close the window about 3 inches, so your outside piece of wood is overlapping the window.

See? About 3 inches down. When you're done installing everything, you'll also want to wedge a piece of wood up between the top of the window and the jam, for extra security.

Now, holding the wood slats and the outside piece of plywood tightly together, screw the bottom slats into the plywood. Be sure to recess the screw head a little into the wood, since you'll be fitting your inside piece of plywood over them. Also, very important, only screw from the inside. Having your screw heads on the outside of the window would just make it easy for someone to come along, unscrew them, and break into your house.

Do the same with your top slats, screw them tightly to the plywood, recessing the screw heads. Then add strips of high density weatherstripping to the sides and top, where the window meets the top slats. Technically, you could skip this step if your plywood is snug enough, but extra weather protection never hurts. There will be a lot more weatherproofing in phase 2 though, which we'll do this fall.

(Ignore the piece of wood in the middle, that's just to block bugs from coming in through the cat door hole, since by this point it was dark out.)

Then, before putting the inside piece of plywood in place, use a few tape strips to mark where your screws through the slats are. You want to avoid these when you screw the inside plywood into place! Once marked, screw your inside piece of plywood to the slats.

Finally, install your cat door, as instructed on the package. Basically, put the outside piece on the outside of the outer plywood, your inside piece on the inside of the inside plywood, insert the plastic spacers that come with the door, and screw them into place.

Since we'd used this cat door before, we didn't have the correct length of screws to use this time around (they're an 8-32 size if you're curious, which over 3 inches long is a special order, since anything longer isn't available at Lowes), so we just used an 8-32 rod, cut into 4 pieces and attached with nuts for the moment. The black parts below are duct tape pads to cover the ends of the screws for cat safety, until we get the correct length. We also placed a bookshelf under the window after this photo was taken, so the cats have something to step on before exiting or entering the cat door, just like they have the perch on the outside.

And here are some shots of the completed door and deck access ramp from outside.

In case you're curious, there's room for 2 cats on the perch.

And I'd also like to address something that came up in the comments of my last post. We've had 3 different cat doors since 2007, and never once have we had another animal come though it like a raccoon or squirrel. I suspect this is because of smells, and other animals recognize that this is the cat's territory. But we've also always had the door installed at the opposite end of the house, or on another floor, from where the kitchen is. So animals don't smell food through the cat door and come in to find it.

Other than cats, the only thing that's come through our cat doors is the "presents" the kitties bring us. Over the years there have been dead mice, moles, gophers, birds, snakes, geckos....and on a few occasions the mice and birds haven't been dead. We've become quite the experts at throwing a shoe box over a mouse and setting it free. But other than that, all of our cat doors have been only entered by cats.

So that's it on this iteration of our custom cat door in a window, at least until fall when we take steps to further weatherproof it, including finally staining the post and ramp once they dry out. But here's a quick look back at our previous cat doors.

Built in 2007, this was done with a single piece of plywood inserted into the window frame, and using two wood slats to do the sandwiching around the window. As always, we did our main screws on the inside, so someone couldn't unscrew them and break in from the outside (this is the cat door I mentioned at the beginning of this post that someone tried unsuccessfully to break into), and we had a strong dowel in the window on the inside, to prevent it from opening any wider. The little shelf you see outside the cat door was what the cats used to jump up on from the ground, and it's only attached to the plywood, and just resting on the side of the house. As renters (and just because we're cautious about putting holes where rain could get in), we didn't want to screw anything into the stucco.

Here's the lovely plywood cat door from the back patio:

And this was the door we installed at our last place, and the one I detailed in the first side view illustration. It was the easy "put two pieces of plywood on either side of the window frame" style, since the window had the lips around the entire frame. We did put some wood slats in this one two, where you see the vertical rows of screws below. Not only does this give you more to screw into, but it made the plywood give less when you pressed against it, making it a stronger "sandwich" for the window overall.

For this one the kitties had a built in ledge of brick to jump up on, so it was a pretty easy install compared to the one we just completed.

Has anyone else ever built a custom cat or dog door? Got any good tips to share?