Dealing with open splits in panels and more woodworking FAQs
August 12, 2022 | 10:27 am CDT

Q: We have some panels made with thin solid wood pieces, about 1/2″ to 3/4″ thick. We are seeing a lot of large open splits after finishing that can run a foot up the piece. Can you help us?

A:  A split requires considerable force to be initiated. Once started, however, wood is quite weak in splitting failure, so a split can increase in length quite easily.
Where does this force to split wood initially come from?

The force for splitting can come from within the tree itself, with some species developing three main splits in the log about extending from the butt up to three feet up the stem. These splits can occur between the time the saw cuts the stem and the tree hitting the ground.

Many species of eucalyptus have this internal stress within the tree at a high enough level to form these instant splits. They are not too common with North American species, but do exist sometimes.

In North America, these small splits are usually eliminated when the lumber is cut to its marketed length. Lumber that was not double-end trimmed (DET) will have these small splits present and they can get worse in drying.

A second major course of splits is caused because the end grain on a piece of lumber dries maybe 15 times faster than the face grain farther away from the end.

This fast drying creates attempted shrinkage, which develops large forces that exceed the wood’s strength. For this reason, many species require a wax end coating to stop the rapid drying of the ends. These drying splits tend to be under 1/4" wide, but can extend 12" up the piece.

Such drying end splits occur more easily and are larger when the tree itself is grown in a wetter site and then has a bacterial infection that weakens the wood. Such drying splits also occur when there are stresses within the tree, but such stress is not large enough to crack the wood without some other force; the two causes add together.

A special situation exists for hardwoods (leaf trees). Sometimes the stress inside the tree results in the formation of a special, very weak type of wood. Technically, it is called tension wood. It might weigh only 1/2 or 3/4 of the normal weight of that species. It might have only half the strength. In fact, if also over-dried, sometimes the pounding of planer knives on the wood is enough to break such brash wood, forming what is often called a planer split or roller split.

Finally, drying at a high temperature (over about 160 F) or drying in excessively low humidities to moisture contents under 9% MC for softwoods (needle trees) or under 6% MC for hardwoods (leaf trees) makes the wood more brittle.

Appreciate that repair of a split is quite difficult, as it is likely that there is residual stress in the split piece. So, repair, and exposure to low humidity, or even impact from a planer knife, a hammer, or even dropping the piece of lumber can add enough force to reopen the repaired split. With such residual stress, the warping of wood is common when the moisture changes.

Hint: When spits are seen after finishing, use a 10x magnifier glass to look inside the split. If you see stain or finish inside the split, you know that the split existed before finishing.

Q:  We are switching from red oak to white oak. Are there differences in grading and drying this wood? We will be using 4/4 mainly.

A:  The two groups of oak, red and white, each consist of about 20 species that can end up in the commercial marketplace. Therefore, the color within the red oak and the white oak groups can vary with species. Also, within the group, some species shrink and swell with humidity changes more than others. This means that it is extremely important to have the lumber at the correct MC, (usually 6.8% MC average and all pieces within 1% MC of the average) or else you can expect defects, such as warp and open glue joints, during and after manufacturing. In my experience, over-drying of white oak creates machining issues, so avoid temperatures over 160 F and kiln humidities under 5.25% EMC.

Overall, white oak species tend to have more tiny knots, called adventurous buds or pin knots. Depending on the grader’s instructions, the grader may not consider them negative defects when grading.

As a generalization, white oak from fast-grown sites (growth rings wider than 1/4" apart) has a tendency to check on the surface and in the interior (honeycomb) when drying using standard oak drying practices. So, unless experienced in drying this wood, avoid this white oak.

The demand for white oak barrel staves certainly means that the best quality lumber is hard to find. Lower-grade oak, red or white, tends to have more likelihood of checking around knots. This checking means that defecting in the rough mill must be a little farther away from the knots.

Gluing of oak, red or white, is the same – rather difficult and unforgiving if things are not perfect.

Q:  What makes a closet door warp? We know it was flat when set and when installed.

A: The most common reason (maybe 98% of the time) that a closet door (and also a cabinet door) warps in use is a combination of two factors – moisture content (MC) change of the wood and slope of grain (SOG) of the wood. Individually, these two factors separately are not much of a concern, but together they spell TROUBLE.

MC Change. When the MC changes several percent in wood, a decrease in MC causes shrinkage and an increase causes swelling. Without slope of grain, these changes in size, especially if they occur slowly, can be absorbed by the wood and will be even, so there is no issue. Generally, we prefer that hardwoods (leaf trees) have a moisture content of 6.8% MC, as this matches the average MC in use in most houses (about 38% relative humidity). We can tolerate plus or minus 1.0% MC variation. Softwoods (needle trees) do best at 10.0% MC plus or minus 1.0% MC.

Slope of Grain (SOG). Normally, the grain (wood cells) in a tree runs vertically. However, due to genetics or growth factors, sometimes the cells within the tree are not vertically aligned but twist or spiral.

This slope is real obvious around a large knot or when sawing flat lumber from a crooked log. When there is SOG in a piece of the wood, shrinkage or swelling is no longer even and is also different than other pieces in the door. Without a moisture change, no problem in having SOG.

There are a few species of wood, including hard maple, mahogany, and sweet gum, and softwood lumber cut from within ten growth rings of the log’s center, that will have a lot of SOG.

MC AND SOG. The wood with MC change and slope of grain can be in the stiles and rails, or in the insert or center pieces. Usually, the stiles and rails are the culprits, but you can cut the center out of the door and then see which section is warped — the center or the outer frame.

For expensive doors, you can likely afford to monitor the SOG. A split always follows the grain, so any splits in the lumber show the SOG. Likewise, regions around knots, especially large knots have SOG. When cutting lumber, try to avoid getting too close to knots.

Also, lumber with warp, especially twist or side bend, is likely a SOG piece. All this seems to be an extra effort and might even reduce the yield of parts from lumber. So, remember that SOG is not an issue unless the MC changes. I suggest that you purchase a quality moisture meter (over $250 and made in USA) to check incoming lumber’s MC. (Obviously, employees using the meter need to know and use the precise operating instructions). It also helps to store hardwood lumber at approximately 38% RH and softwood at 55% RH; temperature is not a factor.

Of course, if you dry the lumber yourself, work with your kiln operator to achieve the desired MC. If you purchase lumber, look for a supplier who appreciates your MC and SOG needs, and a supplier who hopefully checks the MC before the lumber is put on a truck.

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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.