Stack height is usually the cheapest storage and freight capacity a plant has, and it is limited by the box rather than the building. Two things decide how high a stack of boxes can go: how the load travels down through the walls, and whether the box on top is actually centred on the box below it.
Cover, locate or stack
A cover lid keeps dust and rain out and does nothing structural. A locating lid has a register that drops into the box below or onto the one above, which keeps the stack aligned and transfers load through the walls rather than through the lid panel. A load-bearing lid is designed to carry the box above across its span. These are three different products and they get quoted as one. Ask which you are buying.
Why misalignment limits stack height
A box offset by thirty millimetres puts its corner load on the middle of the panel below rather than on the corner. That panel was not sized for a point load in the middle, and over a stack of five the bottom box takes a load path nobody designed. In practice, most plants that say their boxes will only stack three high are suffering from a locating problem rather than a strength problem.
The stack load and the rating
Stacked load is different from the load inside the box, and both have to be stated. A box rated for 400 kg of contents is not automatically rated for two boxes on top of it. The number that matters for storage is the stack load at the bottom position, and it should be quoted with the stack height it assumes, because it changes as the stack grows.
Loaded stack and empty stack are different structures
An empty box is light and its walls have nothing to brace against, so it will take less lateral load than a loaded one. Empty boxes stacked in a yard fall over in wind for exactly this reason. If you plan to store empties high, they need either a stiffer wall or a lower stack, and the honest answer is usually a lower stack.
Interlocking options and their real value
Corner registers, moulded feet, a rebate on the lid and pins through the stack all exist. The more positive the interlock, the taller the safe stack, and the more assembly time it costs. In a high-churn plant the extra three seconds a box is a genuine cost, so the design decision is a throughput decision, not only a packaging one.
Strapping and wrapping a stack
Once a stack goes onto a truck, straps hold it together and the top box takes the strap load through the stack. A locating lid spreads that load through the corners; a flat lid concentrates it wherever the strap crosses. If your drivers strap over the top, the lid is carrying stack load whether you specified it for that or not. That interaction is one of the reasons corners have to be designed for the stack.
Testing a stack before you commit
Load the bottom box to its rated weight, build the intended stack height, and leave it for a week in the normal storage position. Then check the bottom box for deformation with a straight edge. This one test answers the capacity question for your actual configuration, and it is more useful than any table of numbers, because it includes your floor, your boxes and your loads.
What the operator needs to know
Two facts decide whether a stack goes up safely: the maximum stack height, and whether the lid locates positively. Both should be marked on the box rather than buried in a document. A painted height line on the wall beside the bay does more than a work instruction, because it is visible at the moment the decision is made.
Where a lid is separate, there should be a stated place for it to live when the box is open. A lid with no home ends up on the floor, gets driven over and is replaced, and replacing lids is a recurring cost that a simple hook or slot removes entirely.
What we can do
We build PP honeycomb board boxes with lids specified as cover, locating or load-bearing according to how you actually stack and ship them. Tell us the stack height and the strap practice, and we will size the lid and the walls for it.