Moisture content (MC) is the mass of water in wood expressed as a percentage of the wood's oven-dry mass. For pallets it is the single most important timber property after species: wood kept at or below about 20% MC resists mould, stain and decay, while green wood above that level shrinks, loosens nails and grows fungi within days in warm weather.
How moisture content is defined
Wood science uses the dry basis: MC (%) = (wet mass − oven-dry mass) ÷ oven-dry mass × 100. Because the reference is the dry wood rather than the wet piece, freshly sawn softwood sapwood can show values well above 100%. In living trees and fresh logs, water often makes up more than half of the total weight.
Water sits in wood in two forms:
- Free water fills the cell cavities (lumens). It leaves first and easily during drying, and its loss changes weight but not dimensions.
- Bound water is held inside the cell walls. Removing it is slower and takes more energy, and it is what causes shrinkage.
The boundary between the two is the fibre saturation point (FSP), the MC at which the cavities are empty but the cell walls are still saturated. For most species it lies between about 25% and 30%. Above FSP, wood is dimensionally stable but heavy and vulnerable to fungi; below FSP it shrinks as it dries and swells as it takes up moisture.
Green, shipping-dry and dry: the key thresholds
| MC range | Common term | What it means for pallets |
|---|---|---|
| Above ~30% (often 40–150%+) | Green | Heavy, no shrinkage yet, high fungal risk in warm conditions |
| 25–30% | Around FSP | Shrinkage begins; mould still likely |
| 20–25% | Partly dried / typical air-dry limit | Borderline; air drying alone usually stops here |
| ≤20% | Shipping dry | Stain, mould and decay fungi cannot grow while it stays dry |
| 12–18% | In equilibrium with outdoor air | Typical seasonal range outdoors in temperate climates |
| 6–10% | Kiln or heated-indoor dry | Furniture and interior joinery levels, rarely needed for pallets |
The USDA Forest Products Laboratory's Wood Handbook states that keeping wood at or below 20% MC gives reasonable protection against fungal damage, and that freshly cut wood piled in warm, humid weather can discolour noticeably within five or six days. That is why the 20% line, not FSP, is the practical target for pallet and packaging timber. See mould and blue stain on pallets for the biology behind it.
Equilibrium moisture content
Wood constantly exchanges moisture with the surrounding air until it reaches its equilibrium moisture content (EMC), which depends on relative humidity and, to a lesser degree, temperature. Outdoor monthly averages in much of the United States fall between roughly 12% and 18% EMC; dry, centrally heated buildings can bring wood down to 6–7%. A pallet dried to 15% and stored in a humid port warehouse will gain moisture again, while one shipped wet into a dry, heated distribution centre will shrink in service.
Why moisture content matters for pallet performance
Shrinkage and dimensional change
Below FSP, wood shrinks unevenly in its three directions. Typical green-to-oven-dry values are:
- Tangential (along the growth rings): about 5–10%
- Radial (across the rings): about 2–6%
- Longitudinal (along the grain): only 0.1–0.3%
Tangential shrinkage is often about twice the radial figure, which explains cupping of flat-sawn deck boards. For a pallet, this means a 22 mm (0.87 in) deck board nailed green can lose a measurable fraction of its thickness and width as it dries, gaps open between boards, and overall dimensions drift out of tolerance. Automated warehouses and racking systems, which rely on tight dimensional limits, are particularly sensitive (see pallets in automated warehouses).
Fastener performance
When wood dries around a nail, the fibres shrink away from the shank and lose their grip. Nails driven into green timber that later dries typically hold far less than nails driven into wood that was already dry, which leads to loose boards, nail "pops" and protruding heads that damage goods. Screw-shank and ring-shank nails, discussed in pallet nails and fasteners, reduce but do not eliminate this effect.
Weight, freight and handling
Water is dead weight. A standard EPAL Euro pallet weighs roughly 25 kg when produced to specification; a pallet built from green timber can weigh considerably more, which adds freight cost, especially in air freight, and burdens manual handling.
Mould, stain and hygiene
Above about 20% MC, mould and sap-stain fungi colonise surfaces and sapwood, particularly when wet pallets are stretch-wrapped or loaded into closed containers. Food, beverage and pharmaceutical shippers often specify maximum MC values in their pallet purchasing specifications for this reason.
ISPM 15 heat treatment is not drying
A common misunderstanding is that ISPM 15 heat treatment (core temperature 56 °C for at least 30 continuous minutes) dries the wood. It does not: the schedule is designed to kill pests, and a pallet can leave an HT chamber with almost the same MC it entered with. A heat-treated pallet can therefore still be green and still go mouldy. The difference is explained in kiln dried vs heat treated.
Measuring moisture content
| Method | Principle | Standard | Strengths | Limitations |
|---|---|---|---|---|
| Oven-dry (gravimetric) | Weigh, dry at 103 ± 2 °C to constant mass, reweigh | EN 13183-1, ASTM D4442 | Reference method, any MC range | Destructive, takes hours to a day |
| Resistance (pin) meter | Electrical resistance between two pins | EN 13183-2, ASTM D4444 | Measures at chosen depth, inexpensive | Needs species and temperature correction; less reliable above FSP |
| Capacitance (pinless) meter | Dielectric response of a surface layer | EN 13183-3, ASTM D4444 | Fast, non-destructive | Density-dependent; reads only an upper layer |
| In-line scanners | Microwave, radio-frequency or capacitive sensors on the production line | — | 100% inspection | Calibration per species and thickness |
Practical rules for pallet plants:
- Measure at depth. Surface readings understate MC in freshly dried boards. With insulated pin electrodes, readings at roughly one-quarter to one-third of the thickness are more representative.
- Correct for species and temperature. Meters are calibrated for a reference species and temperature; mixed pine, spruce and poplar stock needs the right correction setting.
- Sample statistically. Check several boards from different parts of each pack, including the centre, rather than a few from the outside.
- Calibrate regularly against oven-dry samples, especially when switching species or suppliers.
Controlling MC in practice
Producers bring timber below 20% through air drying, kiln drying or a combination of both. After drying, the job is to keep it dry: store packs under cover with airflow, avoid stretch-wrapping warm or damp pallets, and do not stack fresh pallets tightly in closed trailers. Buyers should state a maximum MC and a measurement method in purchase specifications, because "dry" means different things to different suppliers.
Frequently asked questions
What moisture content should pallet wood have?
A practical target is 20% or less, measured at depth, because stain, mould and decay fungi cannot grow in wood that stays at or below that level. Many buyers in food and pharmaceutical supply chains specify similar or stricter limits.
Can moisture content be more than 100%?
Yes. MC is calculated relative to the oven-dry mass of the wood, so if a piece holds more water than dry wood substance, the value exceeds 100%. This is common in green softwood sapwood.
Does heat treatment to ISPM 15 dry the pallet?
No. The 56 °C / 30 minute schedule targets pests, not moisture. Some water is lost, but a heat-treated pallet can remain green unless it is also kiln dried.
Are pinless moisture meters accurate enough for pallets?
They are useful for quick screening, but they read only a surface layer and depend on wood density. For acceptance testing, pin meters with species correction, verified against oven-dry samples, are more reliable.
Sources
- Chapter 4: Moisture relations and physical properties of wood (Wood Handbook, FPL-GTR-282, 2021) — USDA Forest Service, Forest Products Laboratory
- Wood Handbook, Chapter 14: Biodeterioration of Wood — USDA Forest Products Laboratory
- Air Drying of Lumber (FPL-GTR-117) — USDA Forest Service, Forest Products Laboratory
- Fundamental Aspects of Kiln Drying Lumber — Oklahoma State University Extension
- Wood drying — Wikipedia
- EPAL Euro pallet — European Pallet Association (EPAL)