Step 1: Design
Step 2: A-Frame
1) Take 2 pallets & attach in an A shape: touching @ the top & spread @ the bottom.
2) Secure w slat boards on either side w 2 nails each. Obviously, the boards need to be short at the top & longer @ the bottom. Using whole pallets & precut scraps, this was a good build for my 10-yr-old son.
If you only have 1 pallet to use, u can lean it up against a tree & strap w a ratchet strap. I used 7 pallets to build my "mini lumber yard" so I did both styles.
Step 3: Shelf Braces
1) Drill a hole w a hole bit big enough for the rebar to slide through. (I used 1/2".) Style & diameter of rebar varies. Punch completely through top slat just inside the 2x4 then angle down into the bottom slat as far as u can. U want a hole for the rebar to rest in at a slight up angle (~8*).
2) Space your holes wide enough & level enough to hold the boards. 2 holes per shelf is all you need. Space shelves ~18" apart vertically. On my short racks, I have 2 shelves on each side of the A-Frame. On the tall rack, I have 4 shelves on each side. I sort lumber by width, length, & thickness.
3) I cut rebar to 12" long & slid them in. Additional securement not needed. The rebar acts like a lever: hole in the back keeps it up while boards in front try to push it down.
Warning: Don't cut rebar too long! My first attempt was for 24" rebar shelves, but the weight that far out bent the rebar & spilled the boards. 12" is fine for the weight.
4) If there is no rear board to brace against, a single 10d framing nail can be used. Hammer it ~1/2 way into the 2x4 at the right angle to hold the rebar at the desired upward angle. Hammer it in angled downward to give it more resistance.