Boxes that passed inspection at goods-in are collapsing in the warehouse. The supplier says the specification is fine. The specification may well be fine, for conditions that don't resemble your warehouse. This is the root cause of most corrugated stacking failures I'm brought in to look at, and it's almost never the supplier's fault.
Why BCT figures from your supplier are probably optimistic by 40%
Box compression test (BCT) is the standard measure of stacking strength for corrugated packaging. Your supplier quotes BCT in kilonewtons (kN). The number represents how much vertical force a box can withstand before it fails.
The problem is the test conditions. BCT figures are measured in a laboratory at 50% relative humidity and 23 degrees Celsius, using freshly manufactured board. Those are ideal conditions. A busy UK warehouse in November is not 50% humidity and 23 degrees. A cold storage facility is not. A distribution centre near the coast certainly isn't.
At 85% relative humidity, which is not unusual in a damp UK warehouse, corrugated board loses 40-50% of its BCT compared to laboratory conditions. That figure alone explains a large proportion of stacking failures. The box wasn't under-specified for the lab; it was under-specified for the building it ended up in.
The creep problem that BCT doesn't measure
BCT is an instantaneous test. It measures failure under a rapidly applied load. Your warehouse stacks boxes and leaves them for days or weeks under sustained load. That's a different physical condition.
Creep compression, the gradual deformation of corrugated board under sustained load, occurs at substantially lower forces than the BCT figure. A box rated to 4kN BCT will begin to show creep deformation under a sustained load of around 1.5-2kN in normal conditions. In humid conditions, that threshold drops further.
The practical implication: the safety factor you should apply to your BCT specification is not 2x or 3x as many buyers assume. For normal ambient storage, a safety factor of 4x is appropriate. For humid environments or extended storage periods, 5x to 6x is not excessive.
How to calculate what you actually need
Work out the maximum load your bottom-layer boxes will experience. Take the total weight of goods on a pallet (including the pallet itself), then calculate how many pallets will be stacked on each other at maximum. The load on the bottom pallet equals the weight of all the pallets above it. Divide that by the number of boxes in the bottom layer to get per-box load.
For a pallet with 24 boxes carrying 500kg of goods, stacked two pallets high, the bottom boxes each carry roughly 20-25kg of distributed load. At a 4x safety factor for humidity, you need boxes rated to approximately 800-1,000N BCT minimum. If your spec sheet shows 600N, you have a problem waiting to happen.
Most businesses have never done this calculation. They bought the specification that preceded them, or they bought the cheapest corrugated that looked similar to what they needed.
What to do if your boxes are failing
Measure the humidity in your actual storage environment. If it's consistently above 65%, your BCT requirement needs to be revised upward considerably. Options include moving to a heavier liner grade, specifying double-wall board, or in very high-humidity environments (chilled storage, unheated facilities), specifying board with moisture-resistant liner treatment.
Moisture-resistant corrugated board costs 15-25% more than standard equivalent grades. That premium is almost always cost-effective compared to the cost of collapsed pallets and damaged product. A single pallet write-off typically costs more than six months of premium board pricing on a typical order volume.
If you're uncertain about your current specification, ask your supplier for ECT data in kN/m (edge crush test, which predicts BCT) alongside the BCT figure, and work through the load calculation above. If they resist providing it, that's a separate issue worth resolving.