Understanding and Preventing Bowing in Air-Dried Wood

A look at some of the factors that can introduce bow into stacked lumber during drying. October 1, 2009

I had a load (500BF) of 4/4 Poplar FAS/1FAS that wound up with a 1-3 in bow over the length (8 - 12 ft). The wood was stuck within two days of getting into the facility during Dec (very cold) on 3/4 in sq sticks placed every two feet. The stuck bunks were placed on 4x4's every 2 - 4 ft that had been leveled relative to each other while waiting for placement in one of the kilns. The bunks entered the kiln around 25%MC and after drying to 6 -7% I unstuck the bunks and took additional readings, all being in the 5 -8% range.

Oddly, when I was unsticking these boards I did not notice any bowing even though I was unsticking by hand. The flat packs were banded and shipped. The customer stated that the packs "sprung" after the bands were cut and he noticed that roughly 60% of the boards had this bow. I'm not sure why I wouldn't have seen this and I'm not sure why they should have bowed as the bunks were level and stacked thereby placing weight on the majority of the boards.

Forum Responses
(Sawing and Drying Forum)
From Gene Wengert, forum technical advisor:
Bow is usually caused by lengthwise shrinkage that is different on one face than the other. Such shrinkage will only occur after drying only if the MC is changing. However, you state that the MC was correct, so we can eliminate that cause.

A stack that is not level as it gets higher (called pile bend) due to sawing thickness variation will result in bow on the upper layers. As you did not notice warp when you unstacked the lumber, we can eliminate pile bend.

Another possibility is that the lumber was planed and so we are dealing with internal stresses (also called casehardening). Casehardening stress only shows up when machining the lumber. But you do not mention planing, so we can eliminate this one too.

Finally, yellow-poplar is known to have growth stresses in the tree and these stresses can cause bowing, side bend and large end splits. However, you should have seen these when you unstacked. Such stresses are not modified by drying, but will always be there. It is my guess that this is what you have and you just missed seeing it when you handled the lumber. To test this, have the customer rip a few pieces down the middle and see if the two halves are both straight. Note that once dried, no further bow can occur, even with poor handling of the piled lumber, as the wood is too strong to bend permanently at this MC level.

From the original questioner:
Thank you for the responses. The customer did not say if he noticed the bow only after planing, I will question him on this. I have often worried about the wood stacks becoming progressively more bent as they get higher and I try to adjust the stacks to be as flat as possible at each level. I have asked to have some boards ripped and will note what I find out.