The moisture range before the glue spreader
There is no single moisture percentage that makes every timber bond repeatable. The release range depends on the timber product, the adhesive and the applicable product standard. A production line should therefore sample boards from each defined lot, condition them to the approved range, and record the readings and decision before adhesive application. Moisture content can shift as wood exchanges water with surrounding air, so a result taken at kiln exit may no longer represent the board at the glue spreader. [1] [3]
- Wood moisture changes with temperature and relative humidity, so a kiln reading alone does not establish the moisture condition at bonding. [1]
- The Forest Products Laboratory’s 1963 electric-meter guide recommends random sampling; for large lots it suggests at least 10% of boards with a resistance meter or 20% with a radio-frequency meter, with minimum counts for lots under 100. [2]
- EN 14080:2013 includes separate requirements for moisture measurement and moisture at bonding for the glued laminated products in its scope; its limits should not be generalized to every timber product or adhesive. [3]
- Pioneer Adhesives lists 6 to 12% wood moisture for Pioneer Wood Glue under its stated bonding conditions, illustrating why production release limits must be checked against the specific adhesive instructions. [4]
Set the release limit from the product and adhesive
Start with the standard for the product being made, then check the adhesive supplier’s current instructions for the intended species, process and service conditions. EN 14080:2013 covers specified glued laminated products and has dedicated provisions for moisture measurement and moisture at bonding. It is not a universal limit for all timber assemblies. Pioneer Adhesives lists wood moisture of 6 to 12% for Pioneer Wood Glue, a water-based industrial adhesive described as D3 classification. That range belongs to the product’s stated bonding conditions, not to structural glulam generally. Record the chosen minimum and maximum, the permitted difference between pieces where applicable, and the source documents and revisions behind them. [3] [4]
Sample the lot, not just the easiest boards
A moisture average can conceal wet or dry boards that fall outside the process window. Define a lot narrowly enough that its boards share meaningful conditions, such as species, thickness, drying batch and storage history. The U.S. Forest Products Laboratory’s 1963 publication Electric Moisture Meters for Wood (FPL-08) advised random selection so every board has a chance of inclusion. For a large lot it suggested a minimum sample of 10% when using a resistance meter or 20% for a radio-frequency meter; for lots under 100 boards, it recommended at least 10 specimens for resistance meters or 20 for radio-frequency meters, or the whole lot if smaller. These are practical recommendations from a 1963 Forest Products Laboratory guide, not a current standard requirement or a substitute for the product standard or a factory’s validated control plan. [2]
Measure consistently and investigate outliers
Use a meter suitable for the species and board dimensions, apply the manufacturer’s correction factors, and keep the instrument in working condition. The same 1963 guide notes that readings vary both because moisture differs within a lot and because the electrical relationship used by the meter varies. It recommended checking a single board at three locations to reduce errors from uneven moisture along its length and width. Keep the same defined measurement locations and procedure across checks; when a reading is near a release boundary or conflicts with the lot pattern, hold the board and investigate with a suitable reference method rather than averaging away the exception. [2]
Condition boards until the bonding state is stable
Wood takes up or releases moisture as air temperature and relative humidity change. The Forest Products Laboratory’s Wood Handbook describes moisture exchange as dependent on the surrounding air and the wood’s current moisture content. Store conditioned timber so the lot does not re-equilibrate with uncontrolled air before bonding, and recheck after a material change in storage or production delay. Treat planing, transfer and queue time as part of the process: the recorded result should represent the timber close to the point where adhesive is applied, not only its earlier state in the kiln or warehouse. [1] [3]
Make release a recorded production decision
A useful batch record links the timber lot to the measurement date and time, species and dimensions, meter identification, sampling locations, individual readings, corrections applied, approved range, outliers and the release or hold decision. Record conditioning actions and any remeasurement after a hold. Keep the adhesive identity and technical data sheet revision with the batch record so a later investigation can distinguish a moisture excursion from a change in adhesive or process conditions. The standard and adhesive instructions establish the controls; the record shows whether this batch met them. [3] [4]
Check the board at the spreader
Before a batch enters bonding, confirm that the sampling plan covers the lot, readings fall within the product-specific range, differences between pieces meet any applicable requirement, and boards have not been exposed to conditions that invalidate earlier checks. If the evidence is incomplete or a result is out of range, hold the affected material, condition or segregate it, and measure again under the approved procedure. The production target is not a convenient average; it is a traceable release decision tied to the timber, adhesive and product being made. [1] [2] [3] [4]