Why board and water do not mix
Corrugated gets its strength from the geometry of its fluted medium and the stiffness of its fibers. Water attacks both. As board absorbs moisture, the fibers soften and the flutes lose their spring, and the whole structure gets weaker.
The important part is that this happens gradually and invisibly. You do not need a spill or a leak. Sitting in a humid warehouse for weeks can do it on its own.
The humidity-strength relationship
Stacking strength — what the industry measures as edge crush, or ECT — drops as relative humidity climbs. A box rated for a certain load in a dry environment can hold substantially less once it has equilibrated to high humidity.
This is why a load that stacked fine in winter can start leaning or collapsing in a humid summer warehouse, even though nothing about the boxes changed except the air around them.
“The same box can lose a large share of its stacking strength just by sitting in humid air.”
Where moisture sneaks in
None of these involves visible water. They are all just ambient moisture finding its way into fiber that is happy to absorb it.
- Concrete floors, which wick and release moisture — always store on pallets.
- Exterior walls and dock doors, where condensation forms.
- Seasonal humidity swings in unconditioned space.
- Cold-to-warm transitions that cause condensation on cold board.
How to protect your boxes
The fundamentals are simple and cheap. Keep boxes up off the floor on pallets, pull them away from exterior walls, and store them somewhere with reasonably stable humidity. If you buy in bulk, rotate stock so nothing sits for months absorbing whatever the season brings.
For genuinely humid environments or long storage, ask about moisture-resistant liners and treatments — but good storage habits solve the majority of moisture problems on their own.
Reading a box that has been damp
A previously wet box is not automatically scrap, but it needs a careful read. Feel for softness, look for water rings and delamination, and check whether the corners still hold square. If the structure has gone limp, its rated strength is gone with it.
This is exactly what our graders screen for, and it is why moisture history is one of our four core grading signals. A dry box and a once-wet box can look identical and perform completely differently.