Moisture content, warehouse humidity, and the pallets that shrank
In short
New pallets are frequently built from green lumber at 25 to 30 percent moisture content. As that water leaves, the wood shrinks across the grain, joints loosen, nails back out and boards come off. A recycled pallet has already completed most of that movement, which is why used hardwood is often more dimensionally stable than new softwood — one of the least intuitive facts in this industry and one of the most useful.
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How wood actually moves
Wood holds water in two places: inside the cell cavities as free water, and bound within the cell walls. Losing the free water changes almost nothing dimensionally. Losing bound water — which begins below roughly 28 to 30 percent moisture content, the fibre saturation point — shrinks the cell walls themselves. From there down to service equilibrium, wood shrinks continuously.
Critically, it shrinks across the grain and barely at all along it. A 40-inch deck board does not get shorter. It gets narrower, and it gets thinner. Multiply a small percentage across seven boards and the deck develops gaps. More importantly, the board shrinks away from the nail shank, and the joint that was tight in the yard is loose on your dock.
Equilibrium moisture content, by environment
| Environment | Approximate equilibrium MC | Implication |
|---|---|---|
| Heated warehouse, Ohio winter | 6–8% | Green stock will shrink substantially here |
| Unheated warehouse, Ohio summer | 12–15% | Moderate movement |
| Covered outdoor storage | 15–18% | Seasonal cycling both directions |
| Uncovered outdoor storage | 18–25%+ | Mould risk; value loss |
| Refrigerated storage | 10–14% | Cycling to a humid dock is the real problem |
| Export container, ocean transit | Highly variable | Ship dry or expect condensation |
The number that matters is not the moisture content at delivery but the difference between delivery and service equilibrium. A pallet at 25 percent going into a heated warehouse at 7 percent has a long way to travel, and it will do most of that travelling in your building.
Why used wood is often more stable
This is the counter-intuitive part, and it is genuinely one of the strongest technical arguments for recycled stock. A pallet that has been in service for two or three years has already dried to something close to ambient equilibrium. Its shrinkage has happened. Its joints have either survived that movement or failed already — and the ones that failed did not reach our grading line.
What arrives in your building is wood that has been dimensionally tested by time. Combine that with hardwood construction, which holds fasteners far better than softwood, and a good recycled pallet frequently outperforms a cheap new one in exactly the failure mode people buy new to avoid.
What to specify, and how to check
- Write a maximum moisture content into your specification — below 19% for indoor storage and export, below 15% for cold chain.
- Buy a pin-type moisture meter. They cost very little and settle arguments in seconds.
- Measure in the stringer, not the deck board — the thick member dries slowest and tells you the truth.
- Check a sample from the middle of a bundle, not the outside, which will read drier than the stock.
- Store under cover. A specification is only as good as what happens after delivery.
Appendix: the data behind this
Every piece on this site closes with the slice of our operating record that bears on what it argues. The full record is published at /resources/operating-record.
Inbound moisture content by month
A 10-point swing between July and April, on the same pallets from the same customers. Moisture is the most volatile property of this product and almost nobody measures it.
Read this chart as a table
| Period | Value (percent) |
|---|---|
| Sep | 13.8 |
| Oct | 14.9 |
| Nov | 16.2 |
| Dec | 17.1 |
| Jan | 17.8 |
| Feb | 18.4 |
| Mar | 20.6 |
| Apr | 22.1 |
| May | 18.0 |
| Jun | 14.2 |
| Jul | 12.4 |
| Aug | 12.9 |