Keeping lumber flat during drying

Advice from Professor Gene Wengert on drying lumber in order to produce flat, true boards. December 12, 2000

When drying, any weight on the top of the stack is better than none. In fact, it is the first few pounds that do the most good.

However, to get really flat lumber
-- Align stickers (and any 4x4 bolsters) perfectly (24" on center) or else go to closer sticker spacing (12"), and then wandering is OK.
-- Dry as fast as possible for the species and thickness. The dry outside fibers are so strong that they will help hold the board flat.
-- DO NOT EVER let partly dry lumber regain moisture, as this wets the surface fibers, making them weak and unable to hold the lumber flat. (Keep rain off the lumber, do not mix MCs in the kiln, do not be too conservative in the kiln, etc.)
-- Add weight to the top of the pile (up to 10" of concrete) to help hold the lumber flat. (Watch for sticker indentations on softer species.)
-- Use correct sawing techniques to prevent the effects of lengthwise shrinkage of juvenile wood, cross grain, etc. In other words, saw parallel to the bark and not to the pith; rotate 180 degrees from face #1 to face #2.
-- Appreciate that flat-sawn lumber from near the center of the log has a natural tendency to cup.
-- Use straight logs.

Professor Gene Wengert, forum technical expert

On sticker spacing, would closer than 12" on sweetgum and sycamore help? What is the limit to how close you can go?

We have some sycamore air drying that is on 16" spacing with only some warping. I had some sweetgum kiln dried that was on wide spacing and has warped a lot. Is there any thing i can do to correct this now that it is dried?

I put lower quality and mis-cuts on top of high quality, well cut wood.

I only use planed white wood for stickers, and take much care with placement. Low quality wood gets stickers as far as three feet apart with high quality as close as one foot. All stickers are aligned in any one pile--this may seem like too much work, but pays off.

Any wood that will be ripped in the middle, like those with center wood, may also be used for weight, as the cup will be ripped out.

I would not go closer than 12". I would think that good 16" spacing will have no more warp than 12". Warp is next to impossible to remove from dried wood.

Professor Gene Wengert, forum technical expert

In dry kilns that have a high airspeed available to them, a useful technique is to use little or no heat, low humidity, and very high air velocity in the very first few days of a kiln charge, before being placed on a normal drying schedule. This technique creates stresses in the shell of the boards, which helps hold the lumber flat while drying. These stresses are relieved at the end of the charge using normal equalizing and conditioning schedules.

Stickers on 12" centers are definitely a plus on gum and sycamore. Dry aggressively.