Preventing end checks

Q. We are just now processing some 6/4 oak lumber and we find it is full of honeycomb (internal cracks). We can cut back a foot or two and it seems to go away, but that is a big loss. What is going on?

A: When the ends of a piece of lumber dry quickly, they will begin to shrink. The wood several inches back, however, will not be drying or trying to shrink yet. This develops a stressful situation. If the stress gets too high and exceeds the strength of the wood (and wood is very weak in splitting failure), then an end check develops. As the check grows, it becomes what is usually called an end split. That is what we see from the ends of the lumber when we inspect it, although on a humid day, the wood can swell enough to make some or all of the checks smaller or invisible.

What we do not see is that oftentimes the end check grows internally in the lumber (especially with oak and walnut), travel down inside the piece 12 inches or more. We only find this internal damage when we begin processing. So, in your specific case, what you are seeing is severe end checking that developed into honeycomb. It is a drying defect, with the checking being initiated before the lumber was dried under 50 percent MC.

The cure for preventing end checks that only works for freshly sawn, green lumber (or even logs) is to apply a coating onto the lumber (or log) ends. There are widely used commercial end coating products ($4 per MBF cost) that are waxed based in water and can be applied by spraying or brushing. They even have antifreeze. (Of course, they will not stick to ice or mud, so the ends must be clean.)

Coating must be done before any checks develop. The coating also must be thick enough to essentially stop end drying. The commercial products will evaporate when the kiln goes over 130F (approximately), so there is no wax carryover into the manufacturing plant. Avoid using regular paints (not a good enough barrier) or paraffin wax (will not melt in the kiln). As you are seeing, the loss is huge compared to the small costs of applying a coating before drying. I suggest that all thicknesses and all species of hardwood lumber be end coated…it is a small insurance policy.


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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.