Books are heavy and most shelving materials, especially low-cost engineered wood materials like particle board, will slowly sag under their weight. Here's a simple method for making cheap particle board shelves that look good and keep a straight horizontal span despite long-term heavy loading.

I've also discussed a simple design for an open-frame free-standing bookcase using these shelves. Over the past 20 years, I've built many bookcases and workbenches this way, and none has any visible sag. Of course, your mileage may vary....

The short wood fibers (sawdust, wood chips, sawmill shavings) used in making particle board (AKA chipboard, fiberboard, pressed board) make it notoriously fond of sagging. Our trick is use of a long and relatively large dado -- a rectangular slot that allows a rear 2X4 support to prevent the particle board shelf from deforming.

Step 1: Materials and Tools

Although you can make the shelves any of a wide variety of sizes, for this Instructable, we'll focus on making a roughly 4' long shelf suitable for use in a bookcase. Lumber generally comes in lengths near 8' or longer, but there is no need to drag such long lumber home to cut it. Wherever you buy the lumber, have them cut the lumber to length; an 8' piece becomes two nearly 4' pieces. The parts you'll need per shelf are:

A roughly 4' long slab of 3/4" thick raw particle board (shelving blank) 10"-12" deep
The same length of 2X4 lumber (which is typically really 1 1/2" by 3 1/2"); hand pick a nice piece of construction-grade material
Wood glue
Flat black latex wall paint (easiest to find around Halloween)
Wood stain and/or polyurethane in the color of your choice

The tools you will need (and should use with appropriate care) are:

Router table/fence with 3/4" straight bit or a table saw
Sandpaper or power sander
Brushes for painting and staining

Step 2: Into the Blackness

Using either the router with a quarter round bit or a sander, round over what will be the top and bottom edges of the front of each shelf. The curve isn't important, the point is simply to smooth the sharp and somewhat rough looking front edge of the pressed board.

Now you're ready to paint the front, sides, top, and bottom of the pressed board pieces black and set them aside to dry. I generally give them 2 coats, sanding lightly before the second coat.

The black paint is not just to seal the particle board and provide a nice contrast with the stained wood that will be around it; flat black paint converts the rough and rather ugly texture of the pressed board into something that looks more like a high-end solid surfacing material. The photo shows how nice the black surface looks compared to the unpainted, unrounded, raw back edge. Further, the latex wall paint will seep into the wood significantly, so that minor scratches that happen later will still look black. In short, flat black paint can hide a multitude of sins.

Step 3: A Tight Slot

Using the 3/4" bit on the router, set the guide to cut at least 3/4" from the edge (see the figure in step 5). Now take the 2X4 and cut a 3/4" wide by 3/4" deep slot lengthwise as shown in the figure. You'll probably want to do that in 3 or more passes, going a little deeper each time. If you don't have a router with a fence or a router table, you can use a table saw to nibble out this slot.

The precision of the width of this slot is critical, so take your time and keep things well aligned from pass to pass. The width of this slot should precisely match the thickness of the raw pressed board. A tight fit is good, a loose fit is very bad.

Sand the slotted 2X4 and stain it if you wish. It is much easier to do this before everything is together. You can seal it with a nice polyurethane now too, or you can add a clear polyurethane top coat to the whole thing after it is assembled.

Step 4: Put It Together

The back edge of the black particle board now gets glued into the slot in the 2X4. This should be a very tight fit, because the black paint raised the "grain" on the particle board a little, but that's ok.

Lay the slotted 2X4 on the floor with the slot facing up and run a small amount of wood glue down the slot. Then start to insert one corner of the particle board and use your weight to slowly force the board all the way in and then flat. A slight rocking motion sometimes helps.

Normally, you'd expect to clamp this so the glue can dry without slipping. However, once the raw back edge of the board has contacted the glue, it will swell slightly, so it is in effect naturally self clamping.

Clean-up any excess glue using a moistened paper towel and let the assembly dry.

Step 5: Why It Works

The result is a handsome black shelf with a stained wood "stop" at the back.

One might think that we have just made the particle board as rigid as the 2X4 -- but this is far more effective than, for example, screwing the same 2X4 under the board as a support. Why? The slot prevents the board from bending over its entire back edge, rather than just at a few screw points (and particle board doesn't work well with screws). It also is very effective in spreading the load on the shelf. Further, the fact that the 2X4 is in the 4" tall orientation, rather than 2" tall, makes it more rigid.

If you're truly paranoid or just like how it looks, you also can glue and screw a 2X2 (or even another 2X4!) under the front edge, but it's really the rear support keeping the pressed board from bending that makes this shelf sag free. The figure shows a side view.

Now you could use these shelves with nearly any supporting structure, but they are stiff enough that they don't need a lot of support area....

Step 6: The Bookcase

So, here's the style of bookcase I've been making and using for about 20 years. There's really not much to it beyond what we've already covered. A couple of upright 2X4 posts go on each end, and the shelves tie into them with some very simple slotted joinery. Basically, the rear slotted 2X4 is 1 1/2" shorter than the shelf, and the shelf is supported by 3/4" deep slots on each of the four 2X4 posts.

Note that you don't really need any fasteners for any of this, although I've sometimes used a few screws primarily to help hold things tight while glue is drying. Screws are really only effective when the thread is holding in a piece of 2X, not particle board. I glue the shelves into the post slots, but bolts could be used instead to allow disassembly.

If you're paranoid, or you're making a workbench which is significantly deeper than a shelf, or you just feel like it, simply add a 2X2 front support which is 1 1/2" longer than the particle board, and set it into the 2X4 side posts as shown in the first figure. The second figure shows a close-up of the bookcase shown at the start of this Instructable -- which uses a 2X4 in the front, but doesn't have it set into the posts.

This general approach also can work replacing the particle board with other sheet construction materials such as plywood or OSB. The texture of the finish is not as nice using OSB.