Key takeaways
- Corrugated absorbs moisture from the air, and compression strength falls markedly as humidity rises.
- Damp board does not look or feel different until well past the point where strength has gone.
- Board that has been wet and dried does not recover its original strength, because the fibre bonds do not re-form.
- A common moisture failure is not weather but condensation from warm product on a cold trailer floor.
A stack of loaded boxes can fail in a warehouse where nothing has changed. Same boxes, same product, same stacking plan as the previous winter. The only variable is that it is now the humid part of the year.
Corrugated is hygroscopic and nobody mentions it
Board exchanges moisture with the surrounding air until it reaches equilibrium. In moderately dry air it settles at a low moisture content and performs to specification. As the air gets more humid the board takes on more water, and compression strength falls. In very humid air the loss is large.
This happens without any visible change. The board does not feel damp when it has gained a few points of moisture content. It feels like cardboard. The strength has simply left, and the only way to know is to measure it — which is why a pin-type moisture meter is standard equipment for anyone grading or receiving corrugated.
| Conditions | Board behaviour | What it means in practice |
|---|---|---|
| Conditioned, moderate humidity | Performs to its rating | Rated stacking is reasonable |
| Typical unconditioned building most of the year | Small loss of strength | Usually acceptable; keep an eye on stack height |
| Humid summer, no climate control | Noticeable loss | Derate stacking height and load |
| Very humid or condensing conditions | Large loss, rapid creep | Reduce stacking sharply or avoid it |
| Direct wetting | Severe loss | Treat as unrecoverable, even after drying |
It does not fully come back
This is the part that surprises people. Board that has been wet and then dried does not return to its original strength. The fibre bonds that were broken by water do not re-form, and permanent set from any load carried while damp stays permanent.
Dried-out boxes should be judged on their current condition, not on what they used to be. A box that spent a weekend in the rain will not behave like a dry one however convincingly dry it feels on Monday, and it is reasonable for a supplier to grade it lower.
Condensation is the sneakier version
A frequent moisture failure is not weather. It is warm product loaded into a box that then sits on a cold trailer floor overnight. The dew point moves inside the container, water condenses on the board, and the box arrives at its destination compromised while the product inside looks perfect.
Fixes, roughly in order of effectiveness: let the product equilibrate before filling; use a sealed liner; put a slip sheet or pallet between the box and a cold floor; and avoid sealing a warm load airtight immediately before it is chilled.
What to actually do
- Store under a roof. Outdoor storage under wrap can be worse than open air, because it traps humidity against the board.
- Get everything off the concrete. Slabs wick, and the bottom course is where strength matters most.
- Rotate stock. Moisture uptake and creep are cumulative and largely irreversible.
- Derate stacking in humid weather. Apply the cautious figure rather than the comfortable one.
- Buy a moisture meter. It costs less than a pallet of boxes and will settle arguments for years.
Questions