Pallet moisture content and the 20% line

The 20 percent line

Moisture content (MC) is the weight of the water in a board as a share of the wood. The trade treats 20 percent as the ceiling for safe storage: below it, decay fungi cannot take hold, and above it, the risk rises with every point. A pallet left in an open Orlando yard in the wet season can pass the line quickly.

Who cares

• Food distribution, which wants sound, dry, clean pallets.
• Pharmaceutical and medical freight, same.
• Nurseries shipping plants, where damp pallets carry rot.
• Anyone whose receiving dock inspects pallets.

What heat treatment changes

Treating wood to a core temperature of 56 °C for 30 minutes is what ISPM 15 asks for, and it says nothing about the wood's moisture afterwards. Pallets loaded into vans while still hot are susceptible to mould. Treated is not the same as dry, and dry is not the same as treated.

Keeping under the line

Store pallets under cover, off the ground, with air moving through. The storage guide explains how. In the dry stretch around November, when Orlando gets about 1.79 inches of rain, a stack dries naturally.

Measuring it

Moisture content is the weight of the water in a board as a share of the wood. A resistance meter with two pins gives a reading in seconds, so a yard can check ten boards from different stacks in the time it takes to talk about it. Read several boards on each pallet, because ends and middles dry at different rates, and note the reading against the line: under 20 percent will not support decay fungi, above it is a risk.

Numbers worth knowing

• Used pallets tested in the Forest Service survey sat at 10 to 12 percent.
• New pallets tested sat at 17 to 20 percent, already near the line.
• A genuine EUR pallet is specified to be built from dry wood only.
• Orlando's three wettest months deliver about 45 percent of the year's rain.

Moisture by month in the yard

Why 20 percent

Wood holds water in two places: in the cell cavities and in the cell walls. Once the walls are full, at roughly 28 to 30 percent moisture, extra water sits as liquid in the cavities, and that is when decay fungi grow fastest. Decay can start with less water than that, and the trade draws its safe line lower, at 20 percent. Wood in air at about 90 percent relative humidity settles at roughly 20 percent moisture, so a humid Florida night can hold an open stack near the line without any rain.

Reading a meter honestly

Pin meters read the electrical resistance between two points in the wood, and they are most accurate between about 6 and 30 percent. The reading depends on the species and on temperature, and shallow pins read the surface, not the core. Take readings at the middle of several boards, press the pins in along the grain, and note which pallets sit in the middle of the stack, where they dry last.