Homemade magnetic dog flap nirvana

"Sometimes I have to pee in the middle of the night. Sorry."

We've got a very large dog and a correspondingly large doggie-door. The rubbery flap that came with the door, never great, warped and wore out over time so the metal magnetic contacts that held it shut no longer lined up. We found the manufacturer of the door was no longer in business so replacement flaps weren't an option. Worse still, the door size was in between the sizes for other doors, so replacement wouldn't be easy.  Our options seemed to be buying a new kitchen door, putting an even bigger hole in the existing kitchen door, or building a frame inside the existing frame to hold a smaller doggie-door.

After hours of fruitless googling, with banshee winds howling through the kitchen nightly, I racked my brain to figure out how to replace the manufactured flap with a homemade version, and wandered the aisles of the hardware store until finally inspiration struck. With about $20 of materials, a few hours of tinkering and some false starts, I made a magnetically-sealed flap that was far superior to the original: It worked reliably and sealed as tight as a drum! I was really proud of the replacement flap but had blown the chance to document how it was made for the next despondent googler.

A year later, when our new kitten came home and we locked the dog out of the house for a while, she chewed up my lovingly hand-crafted door and we were back at square one. Ungrateful bitch! On the other hand I was happy because I knew just how to replace it and now I had the chance to document how it was done... You want to know, am I right?

The original door came with a rubbery two-part flap: For going one way, an inner flap that's the size of the hole, and for going the other way, an outer "frame" flap that overlaps both the inner flap and the metal frame. The flap sections each had a few weak magnets spaced around the edges to seal the inner flap to the outer, and to seal the outer flap to the frame. The seal was reasonable, but not great. Over the course of a few years, the door warped, got chewed on a few times, and finally just wouldn't seal correctly anymore because the flaps were too misshapen for the magnets to line up. Unfortunately I've long since disposed of the original so I haven't got a picture of it.

My replacement flap keeps the same basic idea of an inner rectangular flap and outer framing flap, but uses a much more reliable seal made of magnetic tape that goes all the way around the edges.  I'll jump ahead a bit to show you how (awesomely) it works.

"INCOMING!"

Coming into the house, the inner flap pushes the outer flap away from the doorframe and they both swing up. When they flap back down, the magnetic strips around the outside edge of the framing flap align and attach with magnetic strips on the doorframe's edges.


Here's a video. Note how satisfyingly solid and precise that seal is!

Only got her to stick her head out far enough for a treat, too cold today!

Going out of the house, the inner flap pulls away from the outer framing flap (which is magnetized to the inside of the doorframe) and only that part swings up. When it flaps back down, another set of magnetic strips around its edges reattach the overlapping portion to the stationary outer framing flap.

Again, a very satisfying slap demonstrates the nice tight seal:

Note it is also cat-proof, although this may be temporary. (She has been observed carefully studying Penny's use of advanced canine technology.)

OK, back to the building details... Here you're looking at the ruined post-kitten-introduction-chewing version I'm re-replacing from the house-interior side, to help you see a little more of how it works. I was figuring out what I was doing as I went along, so this one was a little rough around the edges.

Here's the list of raw materials:

• Magnetic tape in two widths, 1" and 1/2"
• Vinyl floor runner, large enough to cover the hole twice over to make two flaps
• Contact cement (because the adhesive on the magnetic tape sucks)
• And of course, duck tape

Not pictured are the only tools you'll need: a box cutter and scissors.

You'll note I did not include a tape measure or ruler. It's because they actually aren't necessary! Everything gets sized, cut, glued, etc. right inside the frame so there's no need to measure much of anything and you'll end up with a perfect fit. (This is a dramatic improvement since building of the first one, which was tape-measure-city yet not as exact.)

I've removed the old flap from the doorframe. You can see three strips of 1" wide magnetic tape already around the side and bottom edges. (If you're starting from scratch, you can start by applying some contact cement to the back of some magnetic tape strips to make your
frame look like this.)

I cut a sheet of the carpet runner large enough to cover the whole interior of the doorframe. (Use the box cutter rather than scissors to make this easy.) This will become the outer, "frame" flap... Start keeping track of which one's which!

Here I've cut another sheet of the carpet runner that's just as tall, but only as wide as the actual hole through the doorframe.

Here I'm measuring out four strips of 1" magnetic tape the same length as the height of the hole through the door. You can cut the tape with heavy-duty scissors like these. I'm not actually fastening anything here, just getting the right size.

Key bit here: I'm about to use the contact cement to glue one of these magnetic strips in place, but before I do that I stick it face-to-face with another strip so the two magnets are on the inside, with the sticky sides outward. The way the strips are made, there are alternating stripes of of north/south polarity down the length. You'll find they only align exactly one way; if you find them pushing against each other or otherwise resisting aligning, turn one around.

I want this thing to last so I don't trust the adhesive under the strip's tape backing, which can be pretty weak. Instead I brush on contact cement and wait 10-15 minutes for it to be ready for bonding (per directions on the bottle). Here the cement-brushed side is up.

The two strips are flipped over and pressed down in place, against the right edge of the hole. Then the same is done for the left side.

Now we remove the paper covering the adhesive on back sides of the top strips that are not yet glued to anything. Brush them both with contact cement and wait a bit. Note that the two pairs of strips are still perfectly aligned...

...which is important when we bring back the wider flap, line it up at the top edge, and smooth it down in place, pressing hard on the portion covering the strips. This was the main bit, everything from here out is easy.

Just peeling up the wider flap here to give you a look at how it's all lining up. (You can see that the wider flap is just begging for another strip of magnetic tape to be applied to its outside edge to match up with the ones on the frame. Patience, it's coming!)

We're going to do the same glue-in-place-already-stuck-together thing with a pair of matched strips at the bottom edge of the narrower flap, positioned just inside the doorframe edge. However, note we've switched to the smaller 1/2" magnetic tape here. The reason is that the dog will be nosing it open at the bottom, and we want the bond between the flaps to be a little weaker so she's able to peel them apart. Once she's got her nose into the gap at the bottom edge, the sides will peel apart easily.

I skipped showing gluing of the top of that pair (where my fingers are on the paper above), but you can guess how it went... Remove paper, brush cement, wait, push top flap down hard. Once that's done, you can peel away the top/wider flap and set it aside. In this picture we can see that the extra vinyl at the bottom edge of the narrower flap is no longer necessary. Run the box cutter along right under the new thin magnet strip and get rid of the excess.

(Updated: I missed this step when I posted this originally! You'll need to cut a rectangular center hole out of the wider flap now. The hole is bordered on three sides by magnets at this point, so it's easy to see where to run the box cutter there. For the top edge, you need to cut straight across at the height of the doorframe hole, but be very careful not to cut all the way through the sides. Sorry I didn't get a picture of this, but it should be pretty clear.)

The flaps are held onto my doorframe by some screws through holes in the top, which go into threaded holes on two plates, shown here. (This method is part of the original door, not something I did for this project.) The stock door stayed on the screws really poorly due to weird slots from the top edge of the rubbery flap down to the screw hole and periodically came loose, usually when it was raining and windy outside. Grrrr...

We can reuse that mechanism for this replacement flap. However, the top side of the wider flap we just cut a hole in is pretty flimsy. So before we punch holes through the flaps for the screws, it's duck tape time. I folded strips around the top side of the wider flap a few times, then put a folded strip lengthwise along the edges of that portion to reinforce them. I similarly reinforced the narrower flap, then punched holes and put the screws through them into the plates. It seems like the duck tape is enough reinforcement because they've shown no signs of coming loose (even after the pup was actively pulling on them).

Finally, here's a shot of the finished product in situ. I did minor trimming with the scissors around the inside bottom edge to ensure the wider flap is clearing the edge of the frame easily, but only took about a millimeter off (probably the thickness of the duck tape at the top edge offsetting it down).

The fit is exact and there is absolutely no air leakage. The vinyl material stays relatively warm when it's cold outside. I could probably insulate the flaps better (eg glue flannel or thin foam sheets onto them) but it's probably not necessary where I live.

The vinyl floor runner material happened to have textured stripes on it which (in my opinion) makes it look nicer. They also help mask scratches and dirt that naturally accumulate over time with doggie use. If you aren't looking at this thing very closely, you'd never know it wasn't originally part of the door!

And that's it, mission accomplished. You're welcome, Internet! It's the least I could do to pay you back for all the times you've helped me... You deserve it.

Updated: A friend wanted to know just how much feline-force the containment field could stand. I made this little video to demonstrate:

Updated 2017/08/17: I noticed today that Crystal Reimche made a great video demonstrating her own homemade magnetic dog flap nirvana! Check it out for a more in-depth explanation and demonstration of how the inside-flap-outside-frame works. If you try this and it works for you as well, let me know and I'll link to you here too!