Pallet lumber moisture: green vs kiln-dried, shrinkage, weight and mold
Green pallet lumber can hold 50–80% moisture; kiln-dried (KD) lumber is dried to about 19% or less. Drier wood is lighter, stiffer and far less prone to mold, but green wood nails more easily and costs less. Heat treatment (HT) for ISPM 15 kills pests; it does not dry the pallet. Specify moisture separately.
Moisture is the pallet property buyers most often forget to specify, and it causes most of the complaints: mold on arrival, pallets heavier than quoted, loose nails and boards that shrank after delivery. It is also the property most often confused with heat treatment.
Green vs kiln-dried: what is the difference?
Green lumber has not been dried after sawing. Hardwood pallet cants are often 50–80% moisture content (MC), measured as the weight of water divided by the weight of dry wood. Kiln-dried (KD) lumber has been dried in a kiln to a target, most often 19% (KD19) and sometimes 15% or 12%. Air-dried lumber sits in between and depends on climate and time.
| Condition | Typical MC | Mold risk | Weight | Typical use |
|---|---|---|---|---|
| Green | 30–80% | High in warm or closed spaces | Heaviest | Short one-way domestic trips, fast-turn pallets |
| Partly air-dried | 20–30% | Moderate | Medium | Local reusable pallets |
| KD19 | ≤ 19% | Low | Lighter | Food, pharma, retail, export containers |
| KD15 / KD12 | ≤ 15% / ≤ 12% | Very low | Lightest | Sensitive products, long storage, indoor display |
The Uniform Standard for Wood Pallets does not limit moisture content. It is a buyer choice, so it must be on the spec.
Heat-treated is not kiln-dried
This is the most common mistake in pallet buying. ISPM 15 heat treatment (HT) requires the core of the wood to reach 56 °C (133 °F) for 30 continuous minutes. That kills insects and larvae. It does not dry the wood to any moisture target, and a green pallet that is heat-treated is still a green pallet.
| Stamp or claim | What it guarantees | What it does not |
|---|---|---|
| HT (in an IPPC mark) | Core reached 56 °C for 30 min; export compliant | Any moisture level |
| KD, KD19 (lumber grade stamp) | Dried to a moisture target at the mill | ISPM 15 compliance |
| KD-HT | Both, when done in a kiln under an HT program | Moisture after the pallet has sat outside |
A kiln-drying schedule usually exceeds the HT time and temperature, which is why many lumber mills stamp KD-HT. But the HT mark on a pallet is the only evidence of ISPM 15 compliance. See heat-treated vs kiln-dried and heat-treated pallets.
How moisture changes pallet weight
Water is heavy. A green pallet can weigh 40–60% more than the same pallet at 12% MC. Published PDS examples for 48 × 40 in (1219 × 1016 mm) pallets, including nails:
| Lumber | Weight as built (green) | Weight at 12% MC | Difference |
|---|---|---|---|
| Mixed Eastern hardwood, 13.5 board feet | 59 lb (27 kg) | 37 lb (17 kg) | −37% |
| Oak/high-density hardwood mix, 17.0 board feet | 75 lb (34 kg) | 48 lb (22 kg) | −36% |
| SPF, 13.9 board feet | 45 lb (20 kg) | 30 lb (14 kg) | −33% |
This matters for freight cost, parcel and LTL weight limits, manual handling and air freight. See pallet weight.
Shrinkage: why green pallets change size
Wood starts to shrink when it drops below the fiber saturation point, about 28% MC. The rule of thumb used in PDS is about 1% loss in width and thickness for every 5% drop in MC below that point. Length barely changes.
Worked examples from PDS physical property reports:
- A 5/8 in (0.625 in, 16 mm) deckboard shrinks about 0.014 in (0.36 mm) in thickness drying to 19%.
- A 5-1/2 in (140 mm) lead board shrinks about 0.123 in (3.1 mm) in width drying to 19%.
Effects on the pallet:
- Gaps widen. Deckboard gaps on a green GMA pallet can open by 1/8 in (3 mm) or more across the deck as it dries.
- Nails loosen. Plain-shank nails driven in green wood that then dries lose most of their withdrawal strength. The Uniform Standard's connection equations cut smooth-nail withdrawal to 25% in that case. Threaded nails are not penalized, which is one reason threaded nails are standard. See pallet nails and fasteners.
- Boards cup, twist and check. Fast drying in sun or wind can distort thin deckboards and open end checks.
Moisture and strength
Dry wood is stronger and stiffer than green wood. PDS asks for moisture at manufacture and for the service environment: dry (equilibrium MC of 19% or less) or wet (above 19%). A pallet designed for a dry service environment will be rated lower if it is used wet, for example in a cooler or outdoors. If your pallets live in freezers, washdown areas or yards, say so on the spec.
Moisture and mold
Mold needs moisture, warmth and time. Wood above about 20% MC in a warm, closed space such as a shipping container, a stretch-wrapped load or a truck in summer can grow visible mold within days to weeks. Keeping pallets at 19% MC or below is the main prevention. Details, including treatments and cleanup, are in pallet mold.
Buyer specs that set moisture limits:
- Costco (Structural Packaging Specifications, Section 5): when containerized, pallet moisture content cannot exceed 19%.
- EPAL: requires Euro pallets to be dried to reduce mold risk; check the current EPAL technical regulations for the value.
- Many food and pharmaceutical buyers write KD19 or KD15 into their pallet specs.
How to measure pallet moisture
- Use a calibrated wood moisture meter (pin or pinless) with a species correction.
- Measure several boards and at least one stringer or block per pallet, away from the ends, where wood dries fastest.
- Record the average and the highest reading.
- For disputes, use oven-dry testing to ASTM D4442.
A single surface reading on a pallet that sat in rain says little. Readings from the core of a stringer tell you more about mold risk in a sealed container.
Moisture in cold and wet environments
Pallets used in coolers, freezers, washdown areas or outdoor yards pick up moisture however dry they were at delivery. Wood exchanges moisture with the air until it reaches equilibrium moisture content (EMC), which rises with humidity. In a humid cooler a kiln-dried pallet can climb back toward 20% or more. For those uses, either accept that wood pallets will be wet and specify accordingly (threaded nails, a wet service environment in PDS, mold control), or consider plastic. See plastic vs wood pallets.
Kiln-dried after assembly vs before
There are two ways to get a dry pallet:
| Method | How it works | Pros | Cons |
|---|---|---|---|
| Dry lumber, then assemble | Shop buys KD lumber or cut stock | Dimensions stable at assembly; nails driven into dry wood | Dry hardwood is harder to nail; costs more |
| Assemble green, then kiln-dry the pallet | Pallets are built and then dried in a kiln (PDS calls this "conditioning") | Uses cheaper green stock; can combine with HT in one kiln cycle | Parts shrink after nailing, so gaps open and plain nails loosen |
Either can meet a KD19 spec at delivery. What matters to you is the moisture when the pallet arrives, how long it stays dry in storage, and whether the fasteners suit the method: threaded nails tolerate drying after assembly much better than plain-shank nails.
Specifying it for a quote
Add these moisture fields to your RFQ:
- Moisture at delivery: green acceptable, KD19, KD15 or KD12
- Where it is measured and how (meter type, number of readings)
- Whether the whole pallet is kiln-dried after assembly (PDS calls this "conditioning") or only the lumber before assembly
- ISPM 15 treatment, stated separately (HT or none)
- Service environment: dry, cooler, freezer, outdoor, washdown
The designer shows weight and board feet for any design; the board feet calculator gives the same figures for a quick check.
Frequently asked questions
Is a heat-treated pallet kiln-dried?
Not necessarily. ISPM 15 heat treatment only requires the wood core to reach 56 °C for 30 minutes, which removes little moisture. A kiln-dried pallet has been dried to a target moisture content. A KD-HT stamp means both were done.
What moisture content should a pallet be?
For food, pharmaceutical, containerized export and long storage, 19% or below is the usual target, and some buyers ask for 15%. Costco's pallet policy limits containerized pallets to 19%. Green pallets are fine for short, dry, one-way trips.
How much lighter is a kiln-dried pallet?
A typical 48 × 40 in mixed hardwood pallet weighs about 59 lb (27 kg) green and about 37 lb (17 kg) at 12% moisture, based on published PDS examples. That is roughly a third lighter.
Do green pallets shrink?
Yes. Below about 28% moisture, wood shrinks roughly 1% in width and thickness for every 5% of moisture it loses. Boards narrow, gaps widen and nails can loosen, especially plain-shank nails.
How do I check pallet moisture?
Use a pin or pinless wood moisture meter set for the species, and read several boards away from the ends. ASTM D4442 (oven-dry) and ASTM D7438 (meter calibration) are the reference methods.
Sources and references
- Uniform Standard for Wood Pallets (Woodpack Global, formerly NWPCA)
- ASTM D4442, Standard Test Methods for Direct Moisture Content Measurement of Wood (ASTM International)
- Costco Structural Packaging Specifications, Section 5 Pallets (Costco Wholesale)
Related
- ComparisonsHeat treated vs kiln dried pallets: what the stamps meanHeat treatment (HT) heats the wood core to at least 56 °C for 30 minutes to meet ISPM 15 for export. Kiln drying (KD) lowers moisture content, typically to 19% or less, to reduce mold, shrinkage and weight. Choose HT for international shipping, KD for moisture control, and KD-HT when you need both.
- GuidesPallet mold: causes, prevention and what to do with moldy palletsPallet mold grows when wood stays above about 20% moisture in warm, still air: green pallets in containers, wrapped loads or covered yards. The fix is dry wood: specify kiln-dried pallets (19% or less), store them dry and ventilated, and check moisture before loading. HT alone does not prevent mold.
- GuidesHeat-treated pallets (HT): what they are, when you need them, and costA heat-treated (HT) pallet's wood core was held at 56 °C (133 °F) for 30 minutes under ISPM 15, then stamped with the IPPC mark and HT. Most countries require it on wood pallets crossing their border. It adds roughly $1–3 per pallet (2026 US estimate) and kills pests; it does not dry the wood.
- GlossaryMoisture content (pallet lumber)Moisture content (MC) is the weight of water in wood expressed as a percentage of the wood's oven-dry weight. Pallet specs usually call for 19% or less when dry wood is needed; green lumber is above about 28%.
- GlossaryGreen lumber (pallets)Green lumber is freshly sawn wood that has not been dried, with moisture content above the fiber saturation point of about 28%. Many hardwood pallets are built from green lumber because drying adds cost.
- GlossaryWhat does kiln-dried mean for pallets?Kiln-dried (KD) lumber has been dried in a heated kiln to a target moisture content, most often 19% or less (KD19). Kiln drying reduces weight, shrinkage and mold risk, but a KD stamp alone does not show ISPM 15 compliance.
- GlossaryKD-HTKD-HT indicates wood that is both kiln dried (KD), dried to a target moisture content, and heat treated (HT) to ISPM 15. Only HT is an ISPM 15 treatment code; KD describes moisture and does not by itself make a pallet export compliant.
- GuidesHow much does a pallet weigh? Weights by type, material and moistureA standard 48 × 40 wood stringer pallet weighs about 30 to 60 lb (14 to 27 kg). New green hardwood pallets sit at the top of that range and dry ones near the bottom. A EUR pallet weighs about 25 kg (55 lb), pooled 48 × 40 block pallets about 60 to 70 lb, and plastic pallets 14 to 70 lb depending on type.
More in Materials
- GuidesPallet wood species: hardwood vs softwood, and which to specifyMost North American pallets are made from mixed Eastern hardwoods (oak, maple, poplar and others) or softwoods such as Southern yellow pine and SPF. Density drives strength, stiffness, nail holding and weight. Specify a species class or group plus a grade rather than a single species.
- GuidesPallet lumber grades: what Standard, Utility and Economy meanPallet lumber is graded by the defects each part may contain: knots, wane, splits, decay and slope of grain. The Uniform Standard for Wood Pallets sets minimum quality for reusable and single-use pallets, and PDS names grades from Select to Economy. Specify a minimum grade per component.
- GuidesPallet nails and fasteners: types, sizes, MIBANT and nail countsMost wood pallets use threaded nails: helical (screw shank) for dense hardwoods, annular (ring shank) for softwoods. A typical deck nail is 2-1/4 in long with 0.112–0.120 in (11.5–11 gauge) wire. The Uniform Standard requires at least 2 nails per joint on boards up to 5-1/4 in wide and 3 on wider ones.
- GuidesPlastic pallet buying guide: styles, resins, ratings and costChoose a plastic pallet by how it is handled and stored: nestable for cheap one-way use, stackable for floor stacking, rackable for open-beam racking. Compare static, dynamic and racking ratings only when the test standard and support match. New US prices run about $40–200 (2026 estimates).
Last reviewed 2026-09-24. 1,341 words.