Instructables
Picture of A Dog Door: The Two-Flap Solution
This past spring I found I needed to install a dog door. I was going to be away on a short trip, and although my neighbor was willing to come over to feed my dog, the weather was still too cold for her to stay outside. A dog door which would allow her to come and go on her own (she freaks when shut inside with no human for company, even for a few minutes) seemed the right answer.

However, cutting a hole in the front door (well, a bigger hole in this case; I already had a blocked-up cat door there from very long ago) is a serious commitment. It affects both the security and weatherproofing of your house. Standard commercial dog doors are a) expensive and b) while they are reasonable at keeping the cold out on still days, they leak around the edges in a stiff wind, which we get a lot of here. I'd seen a two-flap design years ago, and even made a small one that worked okay out of car floor mats, so I decided to scale that up, using a cheap vinyl set of mats from Big Lots.

Long story short, I found that even when reinforced, the mats didn't hold up very long, though they got me and my dog through that weekend trip. However, I had already cut the big hole in the door, so it seemed a shame not to build something that would work over the long term. I slapped a piece of plywood over the opening to keep the weather out and went back to the drawing board.

First of all, the theory. A dog door that overlaps the main (human) door around its edges when closed is inherently more weather-resistant than one that just fills the opening. Only problem is that—unless you have a very clever and agile dog—this only works one way. With the two-flap solution there are in fact two dog doors: (1) the larger one which overlaps the outer surface of your main door, and which can be pushed open from the inside, and (2) a slightly smaller one that fits inside the other and can be pushed open from the outside. Both have magnetic catches that keep them in place when not in use.
 
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I am going to build one similar. I think I will use old projection t.v. protective plastic for the door. Way too much to buy a commercial door in my cheapskate opinion. Thanks for the inspiration.
Rich991 year ago
great idea, gwylan... my dog wants to go out at least 50 times a day, literally (part wolf, i think). i have a sliding glass door in the kitchen, so during the summer i use one of those magnetic screen thingies... i love it!

any chance that you might conjure up a plan for that second flap? i can't put it into my mind how it's built.

i've got to do something for this winter.
gwylan (author)  Rich991 year ago
I'm guessing you mean the inner flap? Unfortunately I didn't take any pictures before adding the duck tape. It is, essentially, just a rectangle of multiwalled plastic, with the rounded edge at the top. The notches in the top corners are to allow space for the eyebolts that hold the hinge pin (long dowel). The dowel runs through the top channel of the multiwalled plastic.

If you don't have a handy piece of multiwalled plastic, a similarly sized piece of plywood or, say, Lexan would do, but you'd need to use regular hinges.

Any clearer?
fazgard1 year ago
Great write up, and descriptions of construction.

I've got a custom dog door for mine and am always looking to improve it.

But, I had to read this three times looking for the 'second flap' - since I only saw the single flap of material.. I see now that there's only one flap, mounted inside the outgoing frame.

In essence, the ingress flap is mounted inside the egress door, correct?

Looks great, think I'll build one similar for mine for the wintertime, thanks for the idea.
gwylan (author)  fazgard1 year ago
Yes. The egress door is basically a framework for the ingress door, rather than a solid flap.

Even though I'd seen doors of this design before and even built a small one, I found working out the particulars a mind-bending exercise. It's very easy to get turned around: the magnetic catch is supposed to face which direction? And then once you've built it, it's all very simple.
Very clever project :)