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.
Step 1: Materials & Tools
Plywood (14” x 17” x ~1/4”)
Multiwalled plastic sheet (10” x 13”)
Magnets (2) (I used 3/8” x 3/8” x 1 7/8” magnets I found at Home Depot)
Butt hinges (2) with 12 ½” or 5/8” screws
¼” x 10” oak dowel
~5” strip of galvanized sheet metal & 4 - 3/4” screws
Finishing nails (optional)
Rulers & squares
Chisels and mallet (optional)
Step 2: How Big an Opening?
The only one of these I can help you with is sizing the opening to your dog. Rule of thumb is to make the door two inches wider than your dog at her widest point and two inches longer than the height of her torso, with the bottom of the dog door an inch below the bottom of her torso. It's a pretty good rule, though you can probably shave a little off the width if needed,.
With the two-flap dog door, these measurements are for the opening in the outer flap. Because of the necessary overlaps, I made the opening in my front door a little wider and longer. (The inside of the house is also about 5 inches lower than the outer step, so I added an inch or two for that. With luck you won't need to consider this issue.)
The measurements I ended up with (you'll note nothing quite adds up; however, it all works):
Dog torso: 9” high x 7 1/2” wide
Opening in the main (human) door: 14 1/8” x 10 1/8”
Inner flap: 14” x 10”
Outer flap: 17” x 13 1/2”
Opening in the outer flap: 12 1/4” x 9”
Step 3: Make the Opening
a) A level opening, which insures that your dog door hangs properly.
b) An opening that matches the dimensions of the inner flap (plus about an eighth of an inch or so for clearance).
c) Smooth framing that won’t catch your dog’s fur or scratch them.
d) Provision of support for attaching the dog door hinges and metal plate for the magnetic catch. For example, if your door is hollow-core you'll probably need to glue a block of wood to the inside of the door to provide reinforcement for the thin plywood outer skin, or the screws won't hold.
e) Not too ugly since you are going to see a lot of it. (This wasn't a major issue for me, as my door couldn't get much uglier.)
Step 4: Build the Outer Flap
Cut your ¼ inch plywood to this size.
Mark and cut the opening for the inner flap, using a drill and/or a jigsaw or a coping saw. In fact, I used both saws plus a Japanese ryobi.
Using the drill and either a jigsaw or coping saw, make the opening which will hold your magnet. If your magnet* is a ¼ inch or less thick, you'll want to skip this and chisel out a niche to hold it. The ideal depth places the magnet about the same height above the plywood as the hinge. It should in no case sit level with or below the level of the plywood.
Reinforce the plywood with ¾” thick scrap wood at top & bottom to allow hinges, eye bolts, and the metal plate to be screwed/bolted in place, and to back the hole for the magnet. (The extra little bit of quarter round molding on my outer flap was because I made the opening for the magnet after I'd already added the reinforcement.) The finishing nail & glue approach means you won't need to clamp, but take care that the nails don't stick out the opposite side. Mine did, so I used clamps.
I reinforced the sides as well to help guard against warpage. You want a balance between strength and weight, as your dog won't appreciate a heavy door flap.
Mount hinges to the inside top of the outer flap. Note that the hinge pivot—the part in the center that sticks out—should face outward. Otherwise you'll need to chisel space for it in the main door.
Insert the magnet in the pocket you made for it. A bit of silicone sealant will hold it quite well.
*A note about adhesive flexible magnetic strips: They are weaker than other magnets for their size and the adhesive doesn't work for shit. Or at least it doesn't on wood or vinyl. (Well, to be fair, not much works well on vinyl.) In any case I don't recommend the stuff for this use.
Step 5: Build the Inner Flap
Cut multiwalled plastic sheet to size. Remember that this is about 1/8” less in both dimensions than the opening you cut in the main door, and about 1 1/2” wider and 1” longer than the opening in the outer flap.
Cut 1/2” wide notches in the upper corners to allow room for the eyebolts and dowel ends.
Determine how big an opening needs to be cut for the magnet in the outer face (i.e. facing the outer flap) of the inner flap. If possible, use a magnet that will fit into the width of one channel.
Cut this opening using a utility knife. If necessary, fill this to provide a firm (& reinforcing) bed for the magnet, to raise it to the proper level. I found the 3/8” x 38” x 1 7/8” magnet made this unnecessary.
Mount the magnet, using silicone sealant. The surface of the magnet should be slightly higher than the surface of the plastic
Reinforce all edges of the inner flap with duck tape, folded over so it covers both sides. This is particularly important with the top, to add a bit of strength to the hinge. I cleaned the plastic first with isopropyl alcohol to make sure the tape would stick well. I expect to have to replace the tape from time to time, but it's holding up well so far.
Assemble the hinge:
1) Make sure your dowel is the correct length (the width of the dog door opening minus 1/8” or so).
2) Put the dowel through the top channel of the inner flap.
3) Thread the eyebolts on either end of your dowel. If necessary, sand the ends just enough to allow the eyebolt to pass.
4) Secure the ends of the dowel. I used my smallest bit to drill a hole through the dowel ends and threaded a fine wire through and around the dowel, finishing this by tucking the free ends back into the hole. [I used a length of twist tie wire with the paper stripped off, but use what you've got to hand.]
Step 6: Mount Inner Flap to Outer Flap
Mount one plate on the inside of the outer flap, at the bottom center of the opening. The ragged edges left when you punched the screw holes should face down into the wood. Before you screw it down, lay the inner flap in place to make sure the magnet clears the screw heads. Put the other metal plate aside for later.
Mark the location of holes for the eyebolts. This should be to either side of the top notches, with no more than 1/16” clearance from the plastic. Drill the holes and try the fit. Check how far the eyebolts stick out from the inner surface. You want only enough clearance to allow the flap to swing easily. I found I needed to carve out a bit of a slot in the wood for the eyebolt to fit down into.
Mount the eyebolts to the outer flap.
Make sure everything works: the inner flap moves easily; the magnet mates properly with the metal strip, etc.
Step 7: Mount Dog Door
Mark where the second metal plate should go on the main door. To mount it you can take the dog door off and set it aside, or simply prop the flaps out of your way. Make sure the outer flap magnet mates with the plate properly.
Now for weatherstripping! Check how much of a gap is left between the outer flap and the main door, and between the inner and outer flap. In both cases there will be at least a little. Take the dog door off again and apply weatherstripping. (A cautionary tale here: I used old-fashioned felt weatherstripping, which is what I had on hand. This was much too thick to fit between the two flaps. I glued it to either side of where the inner flap fit, which worked okay. For good measure I added more weatherstripping around the edges of the outer flap. Then I remounted the dog door to the main door and had to shim the hinges and metal plate out so the door would close properly. It does work, but I used up most of my stock of metal washers for the purpose. Cheap thin foam weatherstripping may work better.)
Remount the dog door. Adjust as necessary.
Step 8: Next Steps
A followup project could be to rig a plywood panel that will slide over the dog door from the inside to secure it against intruders . I'm still thinking about that.