The four ways moisture gets into a warehouse everyone calls dry
Boxes get wet in buildings with no leaks. The four mechanisms — slab wicking, condensation, door exchange and wash-down — and how to find each one in an afternoon.
By Marisol Vega, Grading line lead · 6 min read · Published June 10, 2025
Have a question this raised?
Tell us what you have or what you need. A real person in Houston reads every one of these — usually within a couple of hours during dock time.
- 1. Slab wicking
- 2. Condensation on cold loads
- 3. Dock door humidity exchange
- 4. Wash-down and floor cleaning
- The afternoon audit
Corrugated gets wet in dry buildings through four mechanisms: slab wicking from concrete, condensation on cold loads brought into warm humid air, humidity exchange at dock doors, and floor wash-down running under pallets. None involve a leak, and all four are found by walking the building rather than reading a hygrometer.
- Category
- Operations
- Published
- June 10, 2025
- Written by
- Marisol Vega, grading line lead
When a customer's returns start grading badly for moisture, the first response is always that the building is dry and there are no leaks. Both things are usually true. Here are the four mechanisms that do not involve a leak.
1. Slab wicking
Concrete is porous and sits on ground that holds water. Moisture moves up through the slab continuously, and a box set directly on it is in contact with a surface that is quietly damp even when it looks and feels dry.
How to find it: tape a square foot of clear plastic sheet to the slab, sealed on all four edges, and leave it overnight. Condensation underneath in the morning means the slab is passing moisture. Do this in several places — a slab is not uniform, and the corner by the old loading area is often the worst.
The fix is a pallet, always, without exception. It is the cheapest single improvement available in any warehouse.
2. Condensation on cold loads
A pallet coming out of a cooler, or off a refrigerated trailer, or in from a cold night, is below the dew point of the warehouse air. Water condenses on it exactly as it does on a glass of iced tea in July, and it condenses on the boxes as well as on the product.
How to find it: look for water rings on the floor under staging positions near the cold dock, and boxes with damp bottoms and dry tops. That signature is condensation, not a leak.
The fix is dwell time and air movement: let loads temper in a staging area with fans before they go into storage, and do not wrap them tight while they are still cold.
3. Dock door humidity exchange
Every time a door opens in August, the building exchanges conditioned air for Gulf Coast air. In a busy dock the air within thirty feet of the doors is essentially outdoor air all day, and the boxes stored there are equilibrating with it.
How to find it: put a cheap hygrometer at the door, one in the middle of the building and one at the back. If the spread is more than about 15 points, your door zone is not the same building as your storage zone.
The fix is zoning: nothing stores within the door zone. Staging yes, storage no. Lids on anything that must sit there.
4. Wash-down and floor cleaning
Scrubbers and hoses put water on the floor, and water runs. It runs under pallets, sits in the low spots and is invisible until somebody lifts a pallet and finds the bottom course ruined.
How to find it: walk the building right after cleaning and look at where water pools. Every warehouse floor has low spots; almost nobody has mapped theirs.
The fix is sequencing and elevation: clean when storage areas are clear, and keep corrugated storage off the floor path entirely.
The afternoon audit
- Three hygrometers: door, middle, back. Read them over a week.
- Plastic sheet slab test in four locations overnight.
- Walk the floor immediately after wash-down and mark the pools.
- Lift the bottom course on five random pallets and look underneath.
- Check for tide lines on stored stock — a tide line means it already happened.
That is one afternoon and it usually finds the problem. In the twenty or so of these we have walked with customers, it has been slab wicking or door zoning almost every time, and it has never once been the roof anybody was worried about.
- ECT
- Edge Crush Test. Pounds per linear inch a board edge withstands before buckling. The best single predictor of stacking performance.
- Tide line
- A wavy discoloured band showing a box stood in water. Permanent strength loss even after drying.
Marisol Vega — Grading line lead
Written on the yard at 1050 E Richey Rd, Houston. Everyone who writes here works the grading line, the baler, the routes or the quote desk — there are no guest posts and nothing on this blog is syndicated.