This is a long instructable, because it covers not just the construction of the bookcase but also some other features - it has a unique locking mechanism based on a sliding book, multiple secret compartments inside the bookcase (aside from the big one behind it!), and it's fitted with electronics that generate some geeky but fun sound and light effects.
Step 1: Finding a space
You'll know already if there is somewhere in your house suitable for a bookcase to hide a door. For us, it was a weird little space with an angled ceiling and a window. It used to have a balustrade and was entirely useless, but one of the first things we did was wall it off and create an open study nook. That left it open to the living room, which was OK but not ideal, since keeping such a space tidy is a PITA. What makes it suitable for a secret room is that it is cantilevered out over the front door, so unless you're really paying attention when you come into the house, you won't know it's there. It's already surprised visitors, which is ridiculously satisfying.
Step 2: Design
For maximum stealthiness, I was keen to make the bookcase blend in with the rest of our furniture. The plan was simple: I would mimic the design and finish of our existing solid wood bookshelves as best I could, while adopting the hardware kit sold by the Murphy Door company, which is designed to support a door weighing up to 300 lbs.
The plans that come with the kit call for edge-banded plywood, which is not the prettiest of building materials. I made mine out of solid wood edge-glued panels instead: such shelving material is inexpensive, dimensionally stable and of the necessary thickness. The trickiest thing about the new design was creating the illusion of depth on the sides that our existing furniture has, since the hardware needs to be mounted right against the outside edges of the boxes that make up the bifold door. Fortunately, there was an elegant solution to this, that in some ways makes the bookcase stronger (the load bearing side is now directly above the pivot hardware).
I've attached the Sketchup file I used to design the bookcase.
Step 3: Materials and tools
You need 2 sheets of 1/4" sanded plywood (~$25 each), and 6 panels of 3/4 x 15 1/4 x 96 laminated pine project board (~$20 each), and a few lengths of 2x4 construction lumber (or similar). You need the Murphy Door kit (I bought mine from Lee Valley Tools for $170), which contains a neat collection of ruggedly built hardware. I was really impressed with it - it all feels really solid and well thought out. You will also need parts for whatever assembly method you use. I built this with pocket screws and Miller dowels; I like both of these construction techniques because they allow you to build things with wood glue (which is amazingly strong) but without lots of specialist tools and clamps (which I don't have). Total cost for the project was less than $500 (CAN).
While I don't have much in the way of specialist tools, I do have a decent collection of power tools in my garage, including a table saw, a miter saw, hand planer, router, drill etc. However, there is nothing that this project calls for beyond the tools you need to build an ordinary bookshelf, so a circular saw with a good guide plus assembly tools is all you really need.
Step 4: Get cutting
Here's the cut plan for my shelves. Everything is in mm because building in feet and inches does my head in (I make no apology for this; the sooner that Liberia, Burma and the USA fall in line with the rest of the world and adopt the metric system, the better, though I'm not holding my breath). Note that the 3/4" pine panel is 19 mm thick.
The Ikea bookshelves I was trying to mimic have a few routed features: stopped chamfered edges on the front side trim, and a curvy front edge on the shelves. I added the latter before cutting the shelves to length, though I didn't have the right router bit and so improvised. The chamfer I added with a bevelling bit, stopped 50 mm or so before the end of each piece of trim. These little features seem so subtle as to be almost pointless, but their very subtlety is why they work - they add to the appearance of quality with contributing any ostentation.
3/4" pine shelving
#1 and #2: rip one board 191 mm wide out of each piece of shelving. Trim to 2120 mm long. Cut a rabbet to accommodate the 1/4" plywood for the back (~6 mm, but measure it) - make it about 10 mm wide so fixing it to the back is easy. Mark these two pieces "inside left" and "inside right".
Rip one board 185 mm wide out of each left over piece. Trim to 2120 mm long, and mark "outside right" and "outside left".
#3, 4 and 5: Rip all of these into boards 185 mm wide. From these 6 long boards, cut 18 pieces 718 mm long. 12 of these are adjustable shelves, and will probably need to be shortened slightly so that they fit easily (only by a mm or so). I also did some subtle decorative routing on the edges of these 12 boards, and it is easier to do that *before* you cut them to length.
#6: Rip a strip 200 mm wide, and cut to 1511 mm long. Mark "valance". Rip another strip 90 mm wide, and cut to 1511 mm long. Mark "valance back". Rip another strip 70 mm wide, and cut to 1549 mm long, and mark "valance front". From the 70 mm offcuts, cut another two pieces 219 mm long, and mark "valance left" and "valance right".
2x4 spruce construction lumber
I made the trim with ordinary 2x4's and a lot of ripping and some tidy-up with an electric hand planer. This part would have been easier with a jointer and a thickness planer, but it was no big deal. Here are the pieces you need:
Two 718 x 38 x 19 mm. Mark one "right middle" and the other "left middle"
Four 718 x 60 x 19 mm. Mark "top right" "top left" "bottom right" and "bottom left"
Two 2120 x 30 x 19 mm. Mark "back left" and "back right"
Four 150 x 60 x 19 mm. Mark "top right side" "top left side" "bottom right side" and "bottom left side"
Two 2120 x 30 x 38 mm. Cut a rabbet 11 x 19 mm out such that the two arms of the leftover L shape are 19 mm thick. Mark "front left" and "front right".
Cut two pieces 2120 x 746 mm. A LOT of waste here, because you need an entire sheet for each shelf back. However, 1/4" ply is handy for a multitude of other projects, so just set it aside in your scrap wood pile and use it for something else.
Note that if you made the bookcase from plywood, you would need two sheets of 3/4" anyway, and this runs to about $100, compared to $130 for the laminated board + solid wood trim. I reckon the latter is a good deal, given that you don't have to do any edge-banding and that the bookcase looks appreciably more high-end while adding less than 10% to the overall price.
Step 5: Make sides
The outside of each bookcase is made of five pieces of wood, which get assembled before the rest of the case. I used Miller dowels to attach the long strips of trim. Miller dowels need to be cut down with a flush-cut saw after the glue has dried, then sanded flat. This sounds like more of a chore than it really is - it's very quick and you would be sanding anyway.
If you're an expert in the use of dadoes, dowels or biscuits, by all means use them instead; or if you're going to paint it, just use plain old screws and fill the holes.
Before attaching the short pieces, you need to cut out pieces to accommodate the hardware (which is designed to be fixed right next to the outside of the case, so if you want fancy outside trim, you have to pull a trick like this). I glued and screwed these small pieces, since the screw heads aren't going to show.
One of the inside pieces has a long piece of trim attached to it, the other doesn't. Pick the side that you'll see the most for the trim. I attached this using the Miller dowels again, including fake dowels down the other side so it looked like both sides were fixed.
Step 6: Add holes for adjustable shelving
Only one of the shelves in this design is fixed; the rest are adjustable. This is a good thing, because you may love a particular spacing right now, but you may find in the future you want to mount a picture or speakers or something, and you will appreciate the flexibility. On the same basis, make more shelves than you think you might need. I used a little Kreg jig to drill the holes; you could easily make your own jig but I was feeling lazy and got seduced by the Lee Valley Tools catalogue. The adjustable shelves make the bookcase easier to finish, too, because you can stain/polyurethane them separately and they don't get in the way when doing the case itself.
Step 7: Build horizontal members
There are three L-shaped fixed shelves for each half of the door. I joined these using pocket screws, which require the use of a jig and a specially shaped drill bit. Pocket screws pull the two pieces of wood together very tightly, and you can do the joinery without the need for clamps. The roughness of the hole you see is because my drill bit is getting old and blunt. You should end up with six L-shaped shelves, two with 38 mm deep front pieces and four with 60 mm deep pieces. The former go in the middle, the latter at each end.
Step 8: Assemble cases
I tried to document the build using photos so you get the idea of how to assemble it, but there are no real hard and fast rules. I used pocket screws and wood glue to join the sides and 3 horizontal members. Just make sure that the case remains square - you'll notice my use of a square of plywood to measure 90 degree angles and diagonal braces to hold the shelves aligned while the glue dried.
Step 9: Attach plywood backs
This is a quick and easy step - cut the 1/4" plywood sheets to 726 x 2120 mm, and glue and nail to the back of the bookcase. I used a nailgun. Fill the gaps and sand the whole thing smooth.
Step 10: Build valance
This is quick to build and can be entirely done with pocket screws. Note that the final bookcase has crown molding added to the valance, but I didn't add that until after I'd finished installing the bookcase (see later). The 3rd and 4th photos show the hardware being added to the valance - 5 reinforcing brackets on the top, and the guide track underneath.
Note that the top of the valance is hollow. You can take advantage of that later to make a secret compartment on top of your secret door. How cool is that?
Step 11: Finish
Fill any holes or imperfections with wood filler, sand the entire bookcase with fine grit sandpaper, blow/vacuum free of dust, then finish in your choice of paint/stain/polyurethane. I used Minwax Colonial American stain followed by clear satin Varathane finish, to try to match our existing furniture.
Step 12: Install lower track
You need a length of hardwood to that holds the lower track. I used a piece of oak 1550 mm long and nominally 1x4 (but actually only 3/4" x 3 1/2"). I carved a 5 mm deep dado using the table saw, drilled a hole with a Forstner bit to accommodate the pivot hinge, and shaped the edges using a roundover bit in my router, to avoid stubbed toes. I also rabbeted the lower edge of the wood to give a space for the carpet to tuck neatly into. Because this piece of wood supports the full weight of the bookcase, it must be attached directly to the floor. For me, that meant cutting the carpet and adding grabber strips either side. When I did this, the piece of oak was buried too deep in the carpet, so I added a strip of 1/4" plywood underneath to lift it up a little. I'm glad I did - it would have dragged on the carpet otherwise.
I figured out the exact location for the strip by hanging a plumb bob down from the valance, which I just temporarily screwed into place.
Step 13: Install
First of all, get some help. These bookcase doors are awkward to handle. For full instructions, consult the manual. Basically, you attach the pivot hardware, join the two sides of the door with a piano hinge, stand the door up, put the valance on top with 1/4" spacers, mark the drill hole locations for the lag bolts, remove the valance, drill the holes, then replace the valance and lag bolt it into place. Your bookcase should now operate as intended!
Step 14: Secret compartment in valance
I added some crown molding to the top of the valance, to make it look more like the rest of our furniture, and topped the valance with a sheet of 1/4" plywood. I edged the plywood with thin strips of wood so it was brought flush with the top of the molding. The valance is there to stop the bookcase falling over, and is not designed to support heavy objects, so you should only put light things up there - it's not really an extra shelf.
Note that you've now created a large void space that is perfect for hiding stuff in... sneaky! Who would think to look for a secret compartment on TOP of a secret door? All right, apart from anyone who has read this instructable...
There are two more secret compartments that *can* accommodate heavy objects - the tops of each door. Again, easy enough to make plywood lids for these. I didn't bother, partly because I wanted to use one of these spaces for the sound effects.
Step 15: Make a lock
All decent secret door bookcases need an equally secret lock, so I built one out of a hardback book, a bolt and a round steel plate from a an old junction box. The bolt was mounted in a hole in the side of one of the cases. Another hole, this one big enough to accommodate the head of the bolt, was drilled through the other box. The steel plate was cut and glued to the inside of the book in such a way that it captures the bolthead, preventing the cases from opening. Pushing the book in releases the bolthead. The hole in the book that exposes the mechanism is lined with a thin piece of steel from a spaghetti tin, so there is a nice "click" when you push the book in. I drilled a small hole in the back of the case behind the book, so I could attach a nylon cord to the steel plate to allow occupants to unlock the door from inside. This string also allows the occupants of the room to close the door fully from inside.
So what book to choose? Reddit/r/books recently had this discussion, and while there were a lot of fun suggestions, I'd already opted for the least-likely-to-be-chosen-for-browsing option.
Step 16: Light
It would be very easy to accidentally leave the light on inside the study, so I wanted a motion-sensor light. I bought a sensor switch but had to make a box for it out of wood offcuts, since I didn't have a suitable place to mount it on the wall. I used a surplus power cord from a broken scanner to make placement more flexible. It switches on when the door is opened and off again after a few minutes (unless it detects motion, of course).
Step 17: Sound effects
For some extra fun, I added some electronic gadgetry. First of all, when you push the locking book in, it plays the Legend of Zelda "discovered secret" sound (the chimes that play whenever Link solves a puzzle). Then, when you pull open the door, it plays the LoZ "got item" fanfare sound, and a bright light goes on inside the room. This echoes what happens when Link opens a treasure chest. Funnily enough, while most people have no idea what the sounds are, they find them weirdly appropriate. I'm not sure if that is some stored memory, or Nintendo's genius in picking exactly the right sound, or both. Anyway, it's fun to have it there as a geeky in-joke.
So how does it work? The first sound is triggered by the pushing the book in, and the second by the opening of the door. Both use a $7 sound recording module with mic and speaker, and they're powered by 3 AA batteries mounted in a $2 holder.
The sound module was mounted to a board and a big wooden lever triggers the switch on the bookcase. I used the top of a pop bottle to amplify the sound (it makes a huge difference, believe it or not).
The trigger for the second module I designed in SketchUp and printed using a Makerbot Replicator (which I have access to through work). It's basically a little house with a roof and flexible walls that encloses the sound module. When the door opens, it depresses the roof into the switch through an inverted "chimney".
The holes are to allow the sound module to be screwed in place, and the zig-zag sides are to allow compression and springiness. It could of course also be used for the book lock as well, but I kind of like my bulky wooden switch (and it works perfectly), so I left it in place. You can set the switch up in such a way that the sound triggers only when it opens to a certain point.
The creative process of designing something with a 3D printer feels very unusual to someone used to traditional fabrication methods, because you can (almost) completely ignore the "how I am going to make this?" questions, and just design any kind of weird structure you like that will solve the problem you have at hand. I'm not going to pretend for a second that the odd little house is the most elegant solution; it's more a fun little 3D Rube Goldberg-style doodle.
Step 18: Load up with your books
We were pretty short of shelf space, and this project added about 8 m. I mostly loaded it up with paperbacks (>200) plus a few hardbacks and various objects. The door opens very smoothly (though it has a lot of inertia, as you might expect!), and based on the inherent strength of the hinges and the fact we don't have to worry about anything sagging, I think it will last a looonnnggg time. I'm pretty stoked with it. Feel free to ask me questions about the build if anything isn't clear.
Video shows the bookcase in operation; sound quality is pretty terrible (including some random clonking noises in the background), but you'll get the idea of how it works and how easily it opens, even for a kid.