Who gives a shake?

Q. What is the difference between a check, a crack, honeycomb, a split, wind shake, and ring shake?

A: This is a good question, as once we have the definitions clearly stated, we also have the potential to attempt to prevent the damage in the future for some of these defects. All the terms or defects mentioned in your question are wood breaks or failures.

Appreciate that the ambient humidity will affect the visibility of many of these failures.  With even a little higher humidity than usual, the wood can swell and close part or all of the failure. Such closing however does not heal the wood, so low humidity will reopen the failure.

A check is a small failure (commonly called a small crack by a layperson) on or within a piece of wood. A check runs along the length of the grain (up and down direction in the tree), sometimes spiraling or at an angle to the sides of the piece. A check crosses the annual growth rings, so does not run “with” or parallel to the rings. A check, due to its location, can be an internal check (meaning inside a piece of wood), a surface check (meaning on the surface), or an end check (meaning on the end and within several inches of the end). Almost all checks are formed because the wood is dried too quickly at high moisture contents due to excessively low humidity (the level varies with species, grain such as flat- versus quartersawn), maybe at too hot of a temperature or excessive air flow.

The idea of “too quickly” reflects the strength of the wood, which can vary due to wood species, growth rate (growth ring spacing), bacterial activity in the living tree, juvenile wood (generally the wood within 15 to 20 growth rings from the center of a log called the pith). and lower than normal wood density.

Unfortunately wood is weak in splitting failure, so,once a check is framed, it is easy to make it worse (bigger) even with normal drying conditions. (It is a good thing that wood is weak in splitting failure if you are splitting firewood manually, however.)

Honeycomb refers to a check that is inside a piece of wood; that is, an internal check. Another older name is bottleneck checks. For some species, like maple, honeycomb checks are quite small; in other species, like oak, deep and wide. Virtually, all honeycomb is a surface check first that then worsens in the middle stages of drying because drying is too fast for the wood.

A crack is a very large check. Usually, with a crack, the opening or failure of the wood runs from one face to the other side. Cracks can be due to excessive drying, but most often are the result of stress within the tree, called growth stress. Such growth stresses can cause an immediate crack in the end of a log when the tree is being processed in the woods. Other times, a little normal stress in drying added to the large growth stress is enough to cause a failure. As a good rule of thumb, any crack wider than 1/4” is definitely a growth stress crack, perhaps worsened in drying.

Planer split
Sometimes, when the wood is dried too low in MC or at too hot a temperature, the wood becomes brittle. As a guideline, do not dry softwoods under 10% MC and hardwoods under 6.0% MC; this is both during drying and after drying is complete. When the dry wood is machined, such as in a planer, especially with dull knives, a long check or crack develops. This crack is called a planer split.

Unlike the other failures which run across the growth rings when, shake runs parallel to the rings. Virtually all shake is caused because of a bacterial infection in the living tree. The bacteria secreted an enzyme that destroyed part of the chemicals that hold the wood together. So, the bacterially infected wood is maybe 25%, or sometimes more, weaker than normal. When the wind blows, the tree develops these shake failures; hence the name wind shake. As the shake failure is parallel or follows the grain, the name ring shake is also used. It is safe to say that once the tree is cut, no further bacterial activity occurs. (This bacterial infection is associated with a foul odor, wet pockets in dry wood, and several other processing defects, as mentioned in this column before.) Bacterially infected wood should be eliminated from processing at the sawmill, but it can be hard to see at this point. Shake is a grading defect for both softwoods and hardwoods.

Q: Every so often, we have a nail head that pops up above the surface. What causes this and what can we do?

A: It is easiest to discuss this issue by using a few examples. Consider a 6” wide, 18” long (although length is not an important factor), and 3/4” thick piece that you will nail through the face and into a 2” framing member. You nailed this assembly when the wood was all quite dry. Also, you drove the nail in deep enough so that the top of the head was even or maybe slightly below the surface.

Now, it is springtime and the humidity around the joint is increasing, causing the 3/4” piece to swell slightly in thickness. With this swelling, we have a few options.

If the nail has a large head, and the nail is NOT driven a long way into the framing member, the nail’s head will stay at the same location with respect to the surface and the end of the nail with the sharp tip will be pulled out slightly. At this point, the assembly looks fine. But, now comes winter and the wood shrinks. With this shrinkage, the wood is not gripping the nail well enough to drive it back into the framing member, so, the head stays where it was in the swollen wood, but since the wood dried and shrank in the winter, the head is left sticking out.  This cycle can be repeated yearly for a while.

There is also a second effect. When the nail head was pounded into the surface, some fibers were squished to make room for the head. (Remember that wood fibers are like miniature soda straws, so they can be squished by eliminating the hollow space in the middle.). These squished cells have a desire to rebound back to their original size. The force is even greater when moisture is added to the wood. So, there is already quite a bit of forcing trying to lift the head out of the wood…the swelling mentioned in the previous paragraph adds to this force.

If you read the scenario above, note that this nail popping required a large head and not deep penetration into the framing member. So, the potential cure is using a smaller head and longer nail, with maybe an increase in diameter, to make the nail more “immovable.”

If the previous example confused you, consider a slightly different example: A nail driven through the face of the 3/4” piece into the framing member fairly deep, as before, plus you drill a nice size pilot hole in the 3/4” piece. Now the 3/4” piece does not hold onto the sides of the nail. So, it is the head that does all the work holding the two pieces together. What happens if this assembly was put together in the humidity of summer? Then, when winter dryness and shrinking happens, the nail does not move, but the wood pulls away, exposing the head. This issue will be worse if the moisture change is large.

So, what is the real culprit in all these example? Answer: Moisture change. Anything we can do to minimize moisture change will cure and likely eliminate the problem Make sure we have dry wood when nailing. In most homes and offices, 6.8% MC IS IDEAL. You need to measure the MC yourself with a meter costing over $250 and made in the USA. I do not know how many times I see moisture issues and the person uses a number on an invoice as an indicator of the MC…invoices do not measure MC; meters and smart people do.

Q: Every so often, someone putting a hinge on will strip the screw. Actually the wood strips, so the screw is still okay. We keep trying to address this issue with the employees. But my question is: “What options do we have to repair a stripped hole in wood?”

A: This is the first time I have had this question in this column. I’ll bet we have some great answers from our readers, so I hope they share their ideas. I was taught two procedures.

Small hole
I was taught that what we need to do is put some solid wood inside the hole and then reinsert the screw.  I recall that we had a bunch of wooden toothpicks available and put one or two in the hole and reinserted the screw using care not to “lose” picks. This works well, but only if the original screw did not split the wood. With a split, we probably should put some PUR adhesive in the hole. Normally the hole with toothpicks is too tight for adhesive to have much benefit.

Larger hole
A second option is to drill the hole bigger with a sharp bit, maybe 3/8” diameter. Wiggle the bit a little so the hole is actually a bit larger to make room for adhesive. Now insert a 3/8” oak or maple dowel in this hole, making sure it is a loose fit by a few 1/1000”. If the fit is good, then put some PUR in the hole and insert the dowel. A little squeeze of excess adhesive is good. After the glue dries, then drill a pilot hole and insert the screw.

Screw design, etc.
Obviously, screw design can be an issue. Small threads will not grab the wood very well. Overall diameter is likewise important. Double the diameter and get much more than double the strength. Likewise, longer is stronger. A pilot hole prevents splitting and actually, without a split is stronger than a normal,screw without a pilot hole. Finally, the wood density affects stripping. Softer, lighter weight species strip more easily. There are some commercial products available for repair of stripped holes, but I have no experience with them.


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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.