Spec guide
Stacking and load ratings, without the optimism.
- Safe default
- 2 high loaded, 4-wall
- Humidity penalty
- Up to 50% of strength
- Storage penalty
- Up to 40% over 90 days
Check a stacking plan
Send the box, the load weight, the stack height and the storage conditions. We will tell you if it is inside the envelope.
The answer, first
Take the box compression rating, then multiply it down: ×0.6 for storage beyond thirty days, ×0.5 for high humidity, ×0.8 for used stock in grade 3A, and ×0.7 for any misalignment between stacked boxes. Those multipliers compound. A box rated at 2,400 lb of compression in a laboratory can be carrying a realistic 400 to 600 lb of stack load in a humid warehouse after three months — which is why two high is the sensible default and three high is a decision, not an assumption.
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 | Multiplier | Why | Notes |
|---|---|---|---|
| Storage 30 – 90 days | × 0.65 | Creep — board deforms slowly under sustained load | Below 30 days, use × 0.85 |
| Storage beyond 90 days | × 0.50 | Creep continues and accelerates with any humidity cycling | Rotate stock instead if possible |
| Relative humidity 80 – 90% | × 0.55 | Fibre bonds weaken as board takes on water | The single largest factor in most yards |
| Relative humidity 65 – 80% | × 0.75 | Measurable loss well before board feels damp | Most unconditioned warehouses in summer |
| Used stock, grade 2A | × 0.95 | Essentially full strength if dry and square | Grade 1A: no derate |
| Used stock, grade 3A | × 0.80 | Cosmetic wear plus minor edge history | Do not stack 4A loaded at all |
| Misaligned stacking | × 0.70 | Load must pass corner to corner to work | Worst single avoidable error |
| Pallet overhang | × 0.70 | Bottom edge unsupported, carrying load it was not designed for | See the pallet compatibility guide |
Worked example
A 5-wall 48 × 40 × 36 box with a laboratory compression figure of 2,400 lb, in grade 2A, stored 60 days in an unconditioned Kansas City warehouse in August, on good pallets, well aligned:
- 2,400 × 0.65 (storage) = 1,560 lb
- × 0.55 (humidity) = 858 lb
- × 0.95 (grade 2A) = 815 lb of realistic stack capacity
If each loaded box weighs 1,600 lb, a second box on top already exceeds that. 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 even two inches off centre loses roughly a third 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 the most common cause of bottom-edge crush we see in inbound loads.
- Cap the top box. A telescoping cap spreads point loads from whatever inevitably ends up on the top of a stack.
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?
Reference
Stacking decisions, tabulated.
The same arithmetic as the derating table, run for the combinations we are actually asked about. Find the row closest to your conditions.
| Grade | Storage | Humidity | Alignment | Realistic capacity | Safe stack |
|---|---|---|---|---|---|
| 1A | Under 30 days | 50% | Aligned | 2,040 lb | 3 high under 1,000 lb/box |
| 2A | Under 30 days | 50% | Aligned | 1,938 lb | 3 high under 950 lb/box |
| 2A | 60 days | 65% | Aligned | 1,111 lb | 2 high under 1,100 lb/box |
| 2A | 60 days | 85% | Aligned | 815 lb | 2 high under 800 lb/box |
| 3A | 60 days | 65% | Aligned | 936 lb | 2 high under 900 lb/box |
| 3A | 60 days | 85% | Aligned | 686 lb | 2 high under 650 lb/box |
| 3A | 90 days+ | 85% | Aligned | 528 lb | 1 high, or unloaded only |
| 2A | 60 days | 65% | Misaligned | 778 lb | 2 high under 750 lb/box |
| 2A | 60 days | 65% | Pallet overhang | 778 lb | Fix the pallet before stacking anything |
| 2A | 60 days | 85% | Misaligned + overhang | 399 lb | Do not stack. Fix both problems. |
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 lean of an inch or two across three boxes. Usually blamed on the pallet, usually 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 — worth roughly 30%
- Keep storage dry — worth up to 45%
- Rotate stock first-in, first-out
- Match the pallet; measure it, do not trust the label
- Seven or more top deck boards above 1,500 lb
- Cap the bottom box to spread point loads
- Centre the load inside the box, not just the box on the pallet
- Keep stacks eighteen inches off exterior walls
- Never stack a 4A box loaded
- Derate in summer using the honest humidity figure, not the specification
Send us your stacking plan before you build it.
Box, load weight, height, storage time and conditions. We will tell you honestly whether it holds.