Glossary
Moisture content
Also called: MC, wood moisture content, EMC
Reviewed
Definition
Moisture content (MC) is the weight of water in wood expressed as a percentage of the wood's oven-dry weight. Pallet specs treat 19% or less as dry; freshly sawn green wood is above the fiber saturation point of about 28% and shrinks roughly 1% in size for every 5% of moisture it loses below that.
On this page
How MC is measured
Moisture content is measured by oven-drying a sample, or with a calibrated moisture meter (test methods ASTM D4442 and D7438). The Uniform Standard does not limit MC, but PDS asks for it at manufacture and assembly and sets the service environment as Dry (equilibrium MC of 19% or less) or Wet (above 19%).
Common MC labels
| Label | Meaning |
|---|---|
| Green | not dried; hardwood cants are often very wet |
| Air-dried | dried in open air, level varies |
| KD19 / KD15 / KD12 | kiln dried to 19%, 15% or 12% |
Why MC matters
- Weight: a GMA-style 48x40 in mixed hardwood weighs about 59 lb green and about 37 lb at 12% MC
- Shrinkage: about 1% of dimension per 5% MC lost below about 28%
- Strength: wood gets stronger and stiffer as it dries
- Nails: smooth nails driven green and used dry lose most of their withdrawal strength
- Mold and buyer rules: Costco requires MC under 19% for containerized shipments; EPAL requires dry wood
See moisture content in the materials section.
Questions
- What moisture content should pallets be?
- For dry service and container shipping, 19% or less is the usual target. Many domestic pallets are built green and dry in use.
- Is moisture content based on wet or dry weight?
- Wood moisture content is the water weight divided by the oven-dry wood weight, so values above 100% are possible for very wet wood.
Sources
- Uniform Standard for Wood Pallets (NWPCA, now Woodpack Global), 2012 edition
- Guide to Understanding the PDS Specification and Analysis (stringer, inches)
- Costco pallet policy