The Number Behind 'Seasoned': Testing Timber Moisture Before It Ends Up in Your Wall

Walk into any yard and you'll find timber described as seasoned, dried, KD, air-dried, or my own favourite, 'it'll be right once it's up'. Some of those are honest descriptions. Some are hopes. The only way to tell the difference is to put a meter on the board and let the number do the talking, because a board that's too wet doesn't complain on the day. It waits until it's fixed, finished and painted, then it shrinks and takes the joint with it.

A label is a claim, not a measurement

Kiln-dried softwood for internal joinery should land somewhere around 8–12% moisture content. Air-dried stock, in a UK climate, is usually sitting nearer 15–20% and will drift with the seasons whatever you do with it. Structural timber graded C16 or C24 is permitted to be wetter than joinery-grade, and often is. None of those figures are wrong, they're just for different jobs, and the problem starts when a rack of one gets used for the other.

So treat the stamp as a starting point rather than a verdict. 'KD' means somebody dried it. It doesn't promise it stayed dry in the rack, on the lorry, or under the tarpaulin at your end.

What the numbers actually mean

Softwood moves mostly across the grain, not along it. A rough working figure: for every 1% change in moisture content, a board shifts about a quarter of a percent of its width. That sounds like nothing until you do it on a 200mm board. Dry that board from 18% down to 9% in a centrally heated house and you're looking at roughly 4–5mm of shrinkage across its width. Enough to open a joint, cup a wide shelf, or leave a visible gap under a skirting that was tight when you fitted it.

Thickness matters less than width, which is why wide boards are the ones that catch people out. It's also why a floorboard and a 50mm batten behave nothing alike even when they come off the same rack.

Testing it properly at the rack

The cheap pin meters most of us carry are fine, provided you use them with a bit of thought rather than waving them about like a divining rod.

• Push the pins in properly. Surface readings on a rough-sawn face are close to meaningless. Drive the pins in, or use insulated pins and hammer them to depth. A reading from the outer 2mm tells you about the weather, not the board.
• Avoid the ends. End grain dries fastest and reads low. Go 300mm in from the end and test the middle of the face.
• Take three readings per board and more on thick stock. Timber isn't uniform. If the spread is wide, treat the high figure as the truth.
• Check the temperature. Most meters assume around 20°C. Cold timber reads higher than it is, so a January yard visit will flatter nothing. If the board is genuinely cold, add a couple of percent to your allowance of scepticism.
• Compare the meter against a known. If you're spending proper money, ask the yard to show you a freshly machined piece and a piece that's been outside. The gap between the two tells you a lot about your meter's honesty.

A capacitance meter, the type you press flat against the surface, is quicker and doesn't mark the timber, but it only reads the top 20–30mm and it hates rough surfaces. Useful for a fast sweep, not for a final decision.

Jobs that shrug it off, jobs that don't

Some timber genuinely doesn't care. Fence rails, shed framing, garden pergolas, raised bed boards and anything permanently outside and unpainted will move about regardless and settle into the weather. Wetter stock here is a nuisance, not a disaster. Decking boards want to be dry-ish at fitting so they don't shrink into wide gaps, but they'll survive a bit of damp.

Other jobs have no tolerance at all. Internal doors, stair components, worktops, floorboards, furniture, and anything that meets plasterboard or a painted finish. Skirting is the classic offender: fitted at 18% in a damp new build, then the heating goes on and every length shrinks away from the wall at the top. Studwork is subtle but real, because wet studs shrinking after the plasterer has been in is how you get hairline cracks above door heads.

As a rough rule: if the timber's going to live inside a heated building, buy it at joinery moisture content and don't let anyone talk you out of it.

The acclimatising bit nobody budgets for

Even good timber needs time to settle to its surroundings. The old guidance of a week per 25mm of thickness is a reasonable starting point for internal work, and it's why joiners who know what they're doing buy doors and flooring well ahead of the job rather than the morning of.

Stack it with stickers between the boards, off the floor, in the room it's going into, out of direct sun. Don't lean it against a radiator to speed things up. That's how you get a board that's dry on one face and wet on the other, otherwise known as a banana.

If you're storing it outside before fitting, keep it covered but ventilated. A tarpaulin wrapped tight over a stack in November is a moisture trap, not a shelter.

Plan for the weather you've actually got

UK timber never reaches a fixed moisture content; it reaches an equilibrium with wherever it's sitting. A heated house in February might pull timber down to 8%, while the same room in muggy August could sit nearer 13%. Fit too tight, or butt boards hard against a wall, and that seasonal swing has nowhere to go.

So leave a bit of allowance, fix wide boards with a couple of millimetres of movement room, and don't nail a floorboard down at both edges. The board isn't being difficult. It's just doing what the meter told you it would.