Spec guide
Stacking and load ratings, without the optimism.
- Common default
- 2 high loaded, 4-wall
- Biggest threats
- Humidity and storage time
- Ratings
- Typical; confirm with supplier
Ask about a stacking plan
Send the box, the load weight, the stack height and the storage conditions in a quote request, and a partner supplier follows up.
The answer, first
Take the box compression rating, then reduce it for every condition that applies: long storage, high humidity, used stock with edge wear and misalignment between stacked boxes. Those reductions compound. A box with a high laboratory rating can carry a small fraction of that as a realistic stack load in a humid warehouse after months of storage, which is why two high is the sensible default and three high is a decision, not an assumption. Ask the supplier for the rating and the safety factor they assume.
What edge crush actually predicts
Edge Crush Test measures the force a one-inch-wide strip of board withstands on edge before buckling. Multiply that through the McKee relationship (board strength, board thickness and box perimeter) and you get an estimate of box compression strength: the total downward force the empty box resists before it collapses.
Two things follow. First, strength lives in the vertical edges, because the corrugations act as columns; a punctured or crushed corner removes a disproportionate share of the total. Second, a bigger box is not a stronger box: perimeter helps, but height hurts, and a tall box is more vulnerable to the slightest misalignment.
| Condition | Effect on strength | Why | Notes |
|---|---|---|---|
| Short storage | Small reduction | Creep: board deforms slowly under sustained load | Rotate stock where you can |
| Long storage | Larger reduction | Creep continues and accelerates with any humidity cycling | Rotate stock instead if possible |
| High humidity | Large reduction | Fibre bonds weaken as board takes on water | Often the biggest factor in unconditioned space |
| Moderate humidity | Noticeable reduction | Measurable loss well before board feels damp | Common in summer warehouses |
| Used stock, light wear | Little or none if dry and square | Cosmetic wear does not touch the vertical columns | Check the bottom edges |
| Used stock, heavier wear | Moderate reduction | Cosmetic wear plus edge history | Avoid stacking heavily worn boxes loaded |
| Misaligned stacking | Large reduction | Load must pass corner to corner to work | A very avoidable error |
| Pallet overhang | Large reduction | Bottom edge unsupported, carrying load it was not designed for | See the pallet compatibility guide |
How the effects stack up
Picture a 5-wall 48 × 40 × 36 box stored for a couple of months in an unconditioned warehouse in summer, on good pallets and well aligned. Storage time takes one bite out of the laboratory figure, humidity takes a second, and any wear takes a third. By the time all three are applied, the realistic stack load can be well under half of the number printed on the datasheet.
If each loaded box is heavy, a second box on top may already exceed what is left. The stack does not fall over immediately. Creep failure is slow and looks like gradual bulging before it looks like a collapse, which is exactly why people believe three high is fine until the day it is not.
Five habits that buy back strength
- Align corner to corner. Load travels down the vertical edges. A box set noticeably off centre loses a meaningful share of its capacity.
- Keep it dry. Roof, dehumidification in summer, and never store on bare concrete without a slip sheet or pallet.
- Rotate stock. Creep is cumulative and largely irreversible. First in, first out is a structural policy, not just an inventory one.
- Use full pallets. Overhang is a very common cause of bottom-edge crush.
- Cap the top box. A telescoping cap spreads point loads from whatever inevitably ends up on the top of a stack.
A useful rule of thumb: if the bottom course is bulging, the stack is already failing. It has simply not finished yet.
Questions
Stacking, answered.
How high can I stack gaylord boxes?
Why do stacks fail in summer and not in winter?
Does a used box stack as well as a new one?
Reading a stack
Seven warning signs, in the order they appear.
Creep failure is gradual and then sudden. Everything before the last item on this list is a chance to intervene.
- 01
The bottom course looks slightly fuller than the ones above
Barely perceptible. This is the board starting to deform under sustained load and it is the earliest sign you will get.
- 02
Vertical edges on the bottom box are no longer sharp
Rounded corners mean the column structure is deforming. At this point the box has permanently lost capacity.
- 03
The stack is no longer plumb
A visible lean across three boxes. Usually blamed on the pallet, often the board.
- 04
Visible bulging on one face
The face that is bulging is carrying more than its share, which means the load was never centred.
- 05
Creaking during handling
Board does not creak when it is within its envelope. Anybody who has worked a floor knows this sound.
- 06
Flaps no longer sit flat
The top of the box has deformed enough that the closure has changed shape. Late-stage.
- 07
It is on the floor
There is no warning sign after this one.
Practice
Ten habits that buy capacity back for free.
- Align corner to corner
- Keep storage dry
- Rotate stock first-in, first-out
- Match the pallet; measure it, do not trust the label
- Use a pallet with a solid top deck for heavy loads
- Cap the bottom box to spread point loads
- Centre the load inside the box, not just the box on the pallet
- Keep stacks away from exterior walls
- Do not stack a heavily worn box loaded
- Derate in summer using the worst-month humidity, not the specification
Planning a stack? Send the details.
Box, load weight, height, storage time and conditions in a quote request, and a partner supplier follows up. Confirm ratings with the supplier before you build the stack.