Ask a warehouse manager what kills their packaging and they will say forklifts. Our grading data says forklifts are first for catastrophic failure and moisture is first for everything else — the slow degradation that turns a nine-trip container into a four-trip one without anybody noticing a specific incident.
What moisture does to corrugated
Corrugated board is a structural sandwich: two flat liners separated by a fluted medium, with the geometry doing the work. Paper fibres absorb water, swell, and lose stiffness. When they dry, they do not return to their original configuration — the flutes have taken a set.
| Moisture content | Relative edge crush strength | Practical meaning |
|---|---|---|
| 6 – 8% (conditioned) | 100% | Specification condition |
| 10% | ~90% | Normal warehouse; fine |
| 12% | ~80% | Noticeable softening; stacking margin eroding |
| 15% | ~60% | Do not stack; one more cycle at best |
| 20%+ | ~35% | Structurally finished, even after drying |
The crucial point is the last row. Board that has been to 20% and dried back to 9% does not recover its original strength. The damage is permanent and invisible — the box looks fine, feels nearly normal, and fails at a load it used to carry.
Where the water actually comes from
Almost never rain. Four sources, in order of how much damage we see from each:
- Condensation from a cold trailer into a warm, humid dock. A trailer that has run overnight at 20°F pulled into a 65°F dock will sweat, and the condensation lands on the top course.
- Ground contact. Pallets set directly on concrete in a building with no vapour barrier wick continuously. The bottom course of a stack goes first, always.
- Outdoor storage, even brief. 'Just overnight, it's covered' is responsible for a remarkable share of what we reject.
- Washdown and cleaning operations in food facilities, where corrugated is stored anywhere near the wet zone.
If you want one rule: never set corrugated directly on a concrete floor, and never bring a cold trailer straight into a warm dock without letting it equalise.
How to spot it on a box
This is the third item in our nine-point grading sequence and it takes about two seconds once you know what you are looking at.
- Patchy darkening of the liner, often in horizontal bands matching where the stack met the floor or the trailer wall.
- A tide line — a distinct edge where wicking stopped. Below the line is finished.
- Liner that feels slightly furry rather than crisp.
- Corners that have lost their sharpness without any visible impact damage.
- A faint musty smell, which is the one your nose detects before your eyes do.
What it costs
We pulled a year of retired containers and coded the primary cause. Forklift strikes accounted for 31% of retirements but almost all of them were sudden, obvious and attributable to an event. Moisture accounted for 26% and virtually none of them had a known incident behind them.
That asymmetry is the whole problem. Sudden damage gets investigated. Gradual damage gets absorbed into "boxes don't last", and then somebody concludes that reuse does not work for their operation when what actually happened was a vapour barrier.
Six fixes, cheapest first
- Pallets, always. Never board on concrete. This is free if you have pallets, which you do.
- Let cold trailers equalise for an hour before unloading into a warm dock, if your schedule allows it at all.
- Move collection points under roof. Most sites have somewhere; it is a habit problem, not a space problem.
- Rotate stock. First in, first out matters more for corrugated than people think.
- A cheap pin-type moisture meter. Settles arguments with suppliers and with your own team.
- Dehumidification in a packaging store, if you are in a humid region and holding significant inventory. The only item on this list with real capital attached.
The one that surprises people
Shrink-wrapping a pallet of corrugated that is already damp makes it worse, not better. You have sealed the moisture in with the board and removed its ability to equalise with the room. If a pallet comes in damp, unwrap it and let it breathe before you store it.
Our pieces are written by the people who do the work rather than by a content agency, which is why they contain more numbers and fewer adjectives than is fashionable. If a figure here does not match your experience, we would genuinely like to hear about it — two of our published coefficients have changed that way.