Lumber Kiln Drying Time and Schedule Duration

Drying time goes roughly as the SQUARE of thickness, so 8/4 stock takes about four times as long as 4/4 and a mill quoting it at twice the 4/4 schedule has committed to a delivery it cannot make. Red oak 4/4 from 85% to 8% moisture content, on a schedule averaging 3.5 points per day above fibre saturation and 1.6 below it, is 15.7 days to reach 30% and 13.8 days for the last 22 points -- 29.5 days of drying, 33.5 including equalizing and conditioning. THE LAST 22 POINTS TAKE ALMOST AS LONG AS THE FIRST 55, which is the fibre saturation effect: above it free water moves easily, below it bound water moves by diffusion and the rate collapses. A single linear average of 2.61 points per day hides that entirely, and a schedule quoted on an average badly understates its own tail. Now the same species at 8/4. At a thickness exponent of 1.8 the scale factor is 3.48, so drying runs 103 days -- over three months against 29.5, and 1.74 times what proportional thinking gives. It is also why 4/4 and 8/4 should not share a charge: the schedule that is safe for the thick stock is wasteful for the thin, and the one that suits the thin stock will degrade the thick. EQUALIZING AND CONDITIONING ARE NOT OPTIONAL on stock that will be machined -- equalizing brings the charge to a uniform moisture content, conditioning relieves the drying stresses that otherwise cause boards to cup, pinch the saw, or move after machining -- and the kiln is occupied for the whole of it. A time estimate from schedule rates the user supplies. It does not produce a drying schedule and must not be used as one: schedules are species, thickness, and grade specific, are published by the Forest Products Laboratory and by kiln manufacturers, and specify dry-bulb and wet-bulb temperatures step by step rather than a rate. Running faster than the schedule causes surface checking, honeycomb, collapse, and casehardening, and none of it is visible until the wood is machined. The thickness exponent is an approximation that varies by species. It does not address air drying, pre-drying, sticker and airflow requirements, kiln sample practice, or the moisture meter corrections needed for species and temperature. The applicable drying schedule, the kiln manufacturer, and the mill's own kiln samples govern.

days = the moisture-content points to remove / the schedule's rate in points per day, computed separately above and below the fibre saturation point; the alternative thickness scales the drying time by (thickness ratio) raised to an entered exponent, commonly 1.5 to 2.

The schedule-rate time relation and the thickness scaling by name, with the Forest Products Laboratory dry-kiln schedules named as what actually governs. The applicable drying schedule, the kiln manufacturer, and the mill's own kiln samples govern.

Division on the schedule rates the user enters; no published drying schedule is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: mc_initial_pct, mc_final_pct, fsp_mc_pct, rate_above_fsp_ppd, rate_below_fsp_ppd, equalize_condition_days, thickness_in, alt_thickness_in, thickness_exponent, days_above_fsp, days_below_fsp, drying_days, cycle_days, linear_rate_ppd, thickness_scale, alt_drying_days

• Time goes as thickness to about 1.8 so 8/4 is roughly four times 4/4, not twicedrying practice
• The rate collapses below fibre saturation bound water moves by diffusion, and the tail of a schedule is the slow partFPL dry-kiln schedules
• This is not a schedule schedules are species, thickness, and grade specific and specify temperatures step by stepFPL / kiln manufacturer