Moisture content in wood is quoted as a percentage, and almost everyone assumes it is a percentage of total weight. It is not, and the difference is why the numbers look strange.
The definition
Moisture content is the weight of water in the wood divided by the weight of the wood with all water removed — the oven-dry weight. So a board containing equal weights of wood fibre and water has a moisture content of 100%, not 50%. Freshly cut green softwood can genuinely exceed 100%, which sounds impossible until you know the denominator.
Checking it with a scale and an oven
This is the actual laboratory method, scaled down. Cut a small sample from the board, well away from the ends. Weigh it — that is the wet weight. Dry it in an oven at around 103 °C until the weight stops changing, usually overnight. Weigh it again — that is the oven-dry weight. Moisture content is (wet minus dry) divided by dry, times one hundred.
A pin-type moisture meter is faster and perfectly adequate for yard work, but it measures electrical resistance between two pins and so reads whatever is between them. It is species-dependent, temperature-dependent, and reads the surface more than the core. For a dispute, do the oven test.
Why 19% is the magic number
Mould and decay fungi need free water in the wood cell, which requires moisture content above roughly 20%. Below that, the water is chemically bound into the cell walls and biologically unavailable. So 19% is a safety margin under the biological threshold, and that is why kiln-dried specifications cluster there.
It is not a drying target for stability — that depends on the equilibrium moisture content of wherever the wood will live, which in a heated Mid-Atlantic warehouse is more like 8 to 12%. Wood will move toward equilibrium regardless of what it left the kiln at.
HT is not KD, again
Heat treatment to ISPM-15 requires 56 °C at the core for thirty minutes. That kills pests. It may remove some moisture incidentally, and it absolutely does not guarantee a moisture content. A heat-treated platform can be wet enough to bloom within a week. If your programme is mould-sensitive, specify kiln dried to a percentage, and specify it separately from treatment.
The practical consequences for a buyer
- Green lumber is heavier. On a weight-limited lane that is payload you are losing.
- Green lumber moves as it dries. Dimensions change, joints loosen, boards cup.
- Recycled platforms are usually the driest material available, because years in service beat any kiln schedule. This is a genuine and under-appreciated advantage of reclaimed stock.
- A stack stored badly will regain moisture regardless of what it was dried to. Drying is not a permanent state.
Why the denominator is oven-dry weight
It seems perverse until you think about what you are trying to measure. You want to know how much water is in this wood relative to the wood itself, and the only stable reference is the wood with all the water gone. Use total weight as the denominator and the same amount of water gives you a different percentage depending on how much water is already there, which makes the number useless for comparison.
The practical consequence is that green softwood can exceed 100% moisture content — more water than wood by weight — which sounds impossible and is simply a statement about the denominator.
Equilibrium moisture content, which matters more
Wood does not stay at whatever moisture content it left the kiln at. It moves toward equilibrium with the air around it. In a heated Mid-Atlantic warehouse in winter that equilibrium is around 6 to 9%; in an unheated shed in August it is 12 to 15%; outdoors under cover, higher still.
So kiln drying to 19% is not a permanent state, it is a starting condition. What it buys you is a platform that begins below the biological threshold and has somewhere to go before it crosses it. A platform delivered at 25% is already there.
Why reclaimed stock is usually the driest material available
Years in service, with repeated wetting and drying cycles, bring wood to a stable equilibrium far more thoroughly than any kiln schedule. The cells have already done their moving. This is a genuine and under-appreciated advantage of recycled platforms: new green-lumber pallets delivered into a humid plant cause more mould problems than reclaimed stock does, which surprises people every single time it comes up.
The four specification mistakes
- Specifying HT and expecting dryness. Heat treatment is a phytosanitary measure. Specify kiln dried to a percentage if you need dryness, separately.
- Specifying a moisture content without a measurement method. A pin meter and an oven test can differ by several points. State which governs.
- Specifying 19% for an application that needs 12%. Nineteen is the mould threshold, not a stability target. For millwork and cladding you want lower.
- Specifying a moisture content and then storing the delivery outdoors. The wood will regain moisture and the specification becomes decorative.
The check anyone can do
Cut a sample from the middle of a board, weigh it, dry it in a domestic oven at its lowest setting overnight, weigh it again. Wet minus dry, divided by dry, times a hundred. It takes two minutes of work and an oven you already have, and it settles disputes that otherwise run for weeks.
Jonah, millwork · Pallets Recycle, 1113 Ingram Ave, Richmond VA · published March 18, 2025 · 899 words. Corrections welcome — tell us if we have this wrong.