How Long Does Timber Take to Dry? A Diffusion Bound Tested Against the One-Month-Per-Inch Rule

Two versions of a drying rule circulate widely. The first says that air-dried timber needs one month of seasoning for each inch of thickness; the second, more cautious version says one year for each inch. This paper tests both versions against a diffusion model of a board drying from both faces, computing the time for the core of the board to reach a stated moisture target as a function of thickness, initial moisture content, and a stated range of diffusion coefficients. The study is simulated, not measured, and no board described here has been load-tested or dried under observation for this work; because the model omits surface resistance and other real delays, every computed time is a lower bound — the fastest drying the stated diffusion process permits — and that bound character is central to the conclusions. Diffusion time grows with the square of thickness, not in proportion to it, so any rule linear in inches must fail somewhere: doubling thickness from 50 mm to 100 mm multiplies the bound by 4.01. For the mid-range diffusion coefficient, drying from 60% initial moisture to a 20% core target, the bound is 0.19 months at 25 mm, 0.76 months at 50 mm, 3.05 months at 100 mm, and 6.86 months at 150 mm. The one-month-per-inch version over-estimates the bound for thin and permeable stock and under-estimates it for thick, refractory stock taken to a tighter target: refractory hardwood of 150 mm to a 15% core target has a bound of 27.77 months against the rule's 5.91 months. The one-year-per-inch version over-estimates the bound in every case computed, by more than an order of magnitude for thin stock. The corrected scheduling rule originated here replaces inches with inches squared: minimum months equal a class coefficient times the square of the thickness in inches, with coefficients of 0.079, 0.197, and 0.491 for permeable, mid-range, and refractory stock to a 20% core target, rising to 0.127, 0.318, and 0.796 for a 15% target — a floor for planning, to which real yard delays must be added, never a promise that the wood will be ready.