Recently I was faced with having to haul away an immense pile of debris from tearing down an old, decrepit garage. At the same time, I needed to put up a lot of shelving in the new garage without spending a lot of money. I figured out a way to save both some of the haulaway cost of the debris and the cost of new shelving material by re-using some of the badly-weathered plywood wall and roof sheathing from the old garage as shelving boards. The trick to turning the ratty, thin (1/2" thick) plywood into strong, custom-sized shelving is to laminate two layers together with construction glue. Here are a few tricks I learned in the process.

Step 1: Define Your Project

There are a few things to consider when planning your shelf project. It's important to make sure you have enough material to do the job. Although 1/2" thick material is the most common and usable for this, many other configurations are possible. Two layers of half-inch yield a nice, sturdy 1" thick shelf that spans 32" (two studs) with a reasonable load capacity. You can also laminate either 2 or 3 layers of 3/8" plywood, depending on span.
Another nice thing about this technique is that finished shelf size is almost unlimited. Shown here is a set of 14 foot shelves I put together earlier from 2 layers of 1/2" scrap plywood. The one-piece boards span 32" between supports and, as you see, can carry a lot of stuff.
The raw material I used would have been thrown away by a sane person. Cracks, splits, warps, nail holes, surface deterioration: it's all good. As long as it's not rotted and has some structural integrity left, it'll work OK. Figure out how much usable area you can get out of your stash, keeping in mind you'll be cutting it into strips the width of your finished shelves, and that you'll need twice the area to start with (assuming 2-ply lamination). Also plan on cutting with, rather than across, the surface grain for maximum strength.

Step 2: Tools and Supplies, and Safety Notes

This Instructable assumes you have at least a rudimentary knowledge of woodworking and/or carpentry, and are comfortable using a circular saw. Safety glasses are a must, of course, especially when using the saw. Another hazard peculiar to the nasty plywood is splinters. Make your life more pleasant and wear work gloves when handling the plywood. Splinters seem to jump out of weathered wood and into my hands if I so much as look at it sideways.

Stuff you'll need:

Safety gear: eye protection, work gloves, ear protection, dust mask.
Circular saw with a carbide-tip blade: a fine-tooth plywood blade is a plus.
A platform to cut the plywood on; a pair of sawhorses and 3 8' long 2x4s works nicely.
A pair of 4" C-clamps.
An 8' long straightedge, such as a piece of 1x3 or MDF molding.
A tape measure, a square, and a pencil.
A claw hammer.
A way to drive drywall screws: cordless drill or screwgun.
A box of 1-1/4" or longer coarse-thread drywall screws.
A caulking gun.
Construction or subfloor adhesive. I used a large tube per 16 square feet of lamination.
Clamping mass. I used a dozen concrete blocks for 1' x 8' shelves.
Finishing materials of your choice. I used leftover exterior house paint over a coat of primer.
Shelf brackets of your choice. I used triple shelf brackets ("Z-brackets") for their low cost and quick assembly.
And of course, lots of crappy-looking plywood.

Step 3: Select and Prepare the Plywood

Select the best of the lot. Begin giving some thought to how the cut pieces will assemble into finished boards. A few rules of thumb:

Stagger the joints between segments generously. Keep lots of separation between top layer and bottom layer joints, ideally more than the spacing between shelf supports.
Keep longer pieces underneath. If your shelves are more than 8' long, a bottom-layer joint is inevitable.
Make all cuts with, rather than across, the outer grain of the plywood, or in the direction of greatest strength.
Raggedy corners and edges can be hidden on the lower layer and toward the back.
Face the worst sides towards each other during glue-up, leaving the nicer sides on the outside.

Lay out the selected pieces and remove all nails and roofing paper, taking care to collect all the removed nails. Each loose nail is a potential flat tire (or tetanus shot!). Scrape off any large chunks of foreign material.

Step 4: Cut Plywood Into Strips

Set up your cutting platform and cut the plywood into strips as wide as the finished shelves, or a bit wider if you plan on ripping the shelves to a clean, finished width. If you don't have a preferred method of cutting plywood, here's how I do it.
I set up a pair of sawhorses and place 3 8' 2x4 studs on top to support the plywood. Lay the plywood on top with 2 studs supporting the large part and one stud outside the cut line. This way, both pieces are relatively well-supported when you finish the cut.
Measure the distance between the blade and the edge of the base of the saw. Add this number (1 1/2" for mine) to the desired width of the cut piece (12" for me). This will be the measurement at which you position the straightedge for cutting (13 1/2" for me.) Using the tape measure and pencil, make marks near either end of the plywood, and one near the middle. Place the straightedge on these marks and clamp it in place. Check against the center mark for bending of the straightedge and correct if necessary. You may need to fasten the straightedge to the plywood with a drywall screw if it is especially unruly.
Position the outer stud along the approximate centerline of the piece to be cut off. Set the saw blade depth to 1/4" - 1/2" greater than the plywood thickness. With the greatest of mindfulness, make the cut by guiding the saw along the straightedge. Repeat the process until all your plywood has been cut into strips. Stand back and marvel at how the junky plywood is starting to look more useful already.

Step 5: Lay Out Pieces to Plan Glue-up

Now it is time to give some thought as to how you'll arrange the pieces for glue-up. Lay out the cut pieces and pair up for top and bottom layers, optimizing the layout for best usage of material while bearing in mind the rules of thumb outlined earlier. It's easiest to lay out all the shelves at the same time on a large flat area. Large holes and cutouts can be plugged by cutting filler pieces out of leftover stock with a jigsaw. You can let one or both ends run long and trim to length after glue-up, or trim individual pieces beforehand.

Step 6: Glue the Boards Together

Begin by laying out the bottom layer of the first shelf on a bed of 2x4 studs, ugly side up. Apply a generous amount of adhesive to the plywood, then place the top layer on, ugly side down. Press down to make good initial contact (carefully walking on it works nicely), then use drywall screws to pull the ends of each piece to its adjoining layer. Do the ends of each individual piece, and don't forget about the ends of the bottom layer. Screws run through from the top will do the job nicely. Add screws to any badly-warped areas along the length. The screws will protrude through the glue-up into the studs below; this is a good thing, helping to flatten the finished board.
Build another board on top of the first board, repeating the above process. Again, the drywall screws will attach to the board underneath, keeping the top board flat. The whole stack will tend to flatness this way.
When the last board is assembled on the stack, place lots of mass on top of the stack for clamping. I used a dozen concrete blocks as shown and got good results. More is always better. Let the whole thing set up overnight.
As I was finishing, it occurred to me that, with the use of lots of long drywall screws, clamping may be unnecessary. The screws aren't wasted as they can be re-used, but there would be a lot of screwing and unscrewing involved. The results might even be better, with fewer small gaps between layers.

Step 7: Unclamp and Finish the Boards

Remove the clamping mass, remove and recover the drywall screws, and trim the boards to length using the circular saw and square. The boards now look pretty sturdy.
At this point, you'll need to decide on finishing options. Unless they're going in a garden shed, you'll want to do something, if only to avoid getting splinters. I used leftover exterior house paint over primer, which worked well to tack down the loose fibers. Exterior semitransparent stain would also work well if you choose a dark color to hide the ugly. Sand the sharp corners first. Also, if you want to rip the boards to a clean finished width, now is the time to do it. If you want to get fancy, fill and sand any holes and gaps along the edges and surfaces. But really, I was pleased at just how serviceable these shelf boards were with just a bit of edge sanding and paint.

Step 8: Hang Your Shelves!

Using your choice of shelf brackets, hang 'em up! I used triple-shelf brackets, AKA "Z-brackets". These used to be available at all the big-box stores years ago, but they're hard to find now for some reason. I ended up buying a case of 36 online from Ace Hardware. For my money, these are the quickest and most cost-effective way to get a bunch of utility shelves up in a hurry. They're made of folded galvanized sheet steel, and they're really flimsy until you get them up with shelves on them, at which point they become magically strong. I've used these in 3 garages now, and never had a failure. And as you can see, when it comes to loading them up, I don't baby them in the least.
This is the second set of laminated shelf boards I've made; the 14 footers shown earlier were the first. Besides being cost effective, this technique can rightly be considered green for keeping plywood scrap out of landfill, and using leftover paint just sweetens the deal. Virtually any size or thickness of shelf board can be made this way, and the finished quality is up to the maker. Scrap plywood makes perfectly serviceable utility shelving with a minimum of effort and cash, but you can use this technique with new material to make some really nice outsized shelving that couldn't be cut full-thickness from a 4' x 8' sheet. And a proper finish would allow the end product to pass any spousal-approval inspection. I've thought about making some one-piece, "round-the-corner" shelves this way.