Moisture Levels and Gradients in Commercial Softwood Dimension Lumber Shortly After Kiln-Drying

Typical moisture levels and gradients in kiln-dried commercial lumber were determined in order that the calibration and use of electric moisture meters might be made more reliable. Samples of nominal 2- by 4-, 6-, 8-, 10-, and 12-inch lumber of various species and grades were drawn from recently kiln-dried loads at mills in the South-Central and Southwestern United States. Individual lumber specimens were sectioned at three distances from the end, and wafers were cut in both the thickness and width directions. Some sectioning was done at three elapsed times after the load was removed from the kiln. Moisture determinations were by oven-drying. Specific gravity gradients also were determined. The study revealed large variability in the average moisture content (MC) of kiln-dried commercial lumber. Essentially all material sampled had substantial residual moisture gradients, but no significant density gradients were found. Moisture gradients had decreased in magnitude significantly within 60 hours of removal of the material from the kiln, without appreciable change in average MC. The number of variables involved (size, grade, species, geographic source, drying method) and the wide natural variability of the properties measured would have required enormous samples in order to be statistically defensible. Sampling of such magnitude was essentially impossible. Despite these statistical limitations, it appears that the residual moisture gradients in recently dried commercial dimension can be characterized with enough precision to enable useful adjustments for their effects to be made on the readings of electric moisture meters.

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