Causes of side bend
July 29, 2010 | 7:00 pm CDT

Q: What causes side bend during drying? We seem to see more of it nowadays and it affects our yields in the rough mill.

A: This defect, also called crook, is caused when one edge of lumber shrinks more than the other. Normally, lengthwise (also called longitudinal) shrinkage is very small, under 0.3 percent when going from green to bone dry. So this lengthwise shrinkage that causes side bend is unusual.

The most common reason for unusual lengthwise shrinkage is juvenile wood - juvenile wood is wood near the core of the log (within 20 rings).

If the sawyer cuts a log using a pattern that results in one edge of the lumber being closer to the center of the tree than the other, then you will frequently get side bend. The rule is: "Keep the rings centered on the end of the lumber."

Crooked and knotty logs also cause side bend, especially if a bad sawing pattern is used. The grain on one edge of the lumber can be swirled quite a bit, while the other edge has straight grain.

There is no control in the kiln whatsoever -- it is a sawyer/supplier problem.

Have something to say? Share your thoughts with us in the comments below.

Profile picture for user genewengert
About the author
Gene Wengert

Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 45 years. He is extension specialist emeritus at the University of Wisconsin-Madison.