I had a large pile of 3/8" plywood pieces and a need for some more shelves. Since the pieces were all under 24" I looked for an easy way to make them.
Step 1: History of the Idea
For years I've been making pigeon hole shelves like these. Just long strips of plywood with slots half way through. Slots on the back for horizontal shelves and slots on the front for verticals. This is how the dividers in liqueur boxes are made. Also note the anti-freeze jugs for drawers, just cut the front out and you have a gallon of storage with a handle. Works with 1/2 gallon milk jugs too. I made these back in 1982 from junk plywood paneling.
Step 2: Basic Parts
So here are the parts for this modular shelf idea. The large piece can either be a shelf or a vertical support. The two small pieces are fillers for the edges where there isn't another shelf. I used a table saw to rip the plywood to the depth of the shelf, here I made them 12" deep. Then I cut the width / height of the shelf boards, they are interchangeable. I also cut the 2.25" x 12" filler pieces. Using a dado blade in the table saw I cut a 3/4" slot 6" deep on both sides of the shelves and up the center of the fillers. If you can cut so the wood grain runs from side to side they will be somewhat stiffer, but with short shelves between supports it isn't bad.
Step 3: Corner Joint
Here you can see where the shelf and vertical meet at the corner. The two small spacers make the corner tight where there isn't another vertical or shelf.
Step 4: Edge Joint
Along the edges you will only need one filler, here it is vertical. This joint does take up some of the shelf space, but it is so easy to assemble! No nails, screws or glue. I assembled all the pieces with the shelf laying on its back. I used a rubber mallet to drive all the slots together, only took minutes. Then I flipped the assembly over to finish with the fillers that load from the back. Finished! You could paint these or stain and varnish, just remember to test the dado width to allow for the paint thickness.
Step 5: End View
Here is an end view. Ignore the extra slots, they were left over from the plywood's last project.
Step 6: Final Notes
So all you need to do is work with the materials you have and what size shelves you need. The space / height is about 3" less than the plywood you cut. Since I have 22" scrap the next set I'll make will be 11" deep, still a nice size for books. There are no diagonal braces shown. I hope to wedge my shelves between the walls. If your shelves are wobbly you could screw some pipe straps on in a "X" shape from corner to corner. Good luck! Carl.