Q. We are seeing some checking in our oak and cherry wood after it is finished. Usually these show up in the customer’s home. Can you explain this?

A. A general rule is: Any checking in oak or cherry that is seen in a finished product is a surface check from drying that has reopened. Exceptions can occur especially if the wood product has been mishandled after manufacturing or if the wood is restrained from shrinking. So, let’s examine this statement in more detail and also see what we can do to control checking in drying.

It is possible to dry normal 4/4 and 5/4 lumber with no checks. Appreciate that checks in drying are formed above 50 percent MC. Because almost everyone air dries or predries their lumber at this MC level, and because most folks do not attempt to control drying well in these processes, checking often develops. Once a small check is formed, it is easy for it to grow large, even under subsequently “normal” conditions. Also, in dry lumber or wood a surface check is often invisible, but it is not “healed” so it will reopen when the wood is exposed to very dry conditions.

Note that in air drying, initially the surface of the lumber is often quite wet or at 100 percent RH. The outside air, however, averages 65 percent RH in most of the U.S., with afternoon RH levels often close to 30 percent RH. The difference between 100 percent RH and 65 percent RH (30 to 12 percent MC) is equivalent to over half of the normal shrinkage in wood, close to 3/4. So, the attempted shrinkage in air drying of the surface fibers is actually quite large.

An important point is that wet wood is half as strong as dry wood. Even so, wet wood can resist this high shrinkage stress most of the time.

We sometimes deal with wood that is not as strong as normal. This weaker wood can be caused by bacteria in the tree (which also smells and also creates shake, which is a crack running with the growth rings and not across) or by the formation of tension wood in the tree (which also causes fuzzing when machining and low weight). Weaker wood is much more prone to checking. Weaker wood is hard to identify in lumber until we see checking under otherwise safe or normal conditions. To get around checking, some furniture and cabinet manufacturers will humidify their plant. The humidity keeps the surface fibers “slightly swollen,” so to speak, so any checks are so tightly closed that they are invisible. Of course, when the customer exposes to wood to the normal dry wintertime humidity, there will be some drying and shrinkage at the surface which will reopen the cracks.

Those of us who are older will recall when wood items were finished with varnish and the varnish was actually quite flexible, so small checks were not an issue...the finish did not crack easily. Today's finishes are thinner, provide much less of a barrier to moisture vapor movement, and are more brittle, so opening of checks is indeed more of an issue now.

It is interesting when I deal with cracks in wood products how often there will be finish inside the crack. Obviously, the crack was there before finishing.

In a dry product, the wood is twice as strong as when it was wet and the moisture changes are fairly small compared to air drying, so the shrinkage stress in a product in use is well below the strength of the wood. That is, formation of new checks is not going to happen under normal conditions.

ASK THE WOOD DOCTOR 965

Q. We installed cabinets in a kitchen and another person installed a bamboo floor. I know that bamboo is not wood but maybe you can help anyway. We are trying to help our customer. The bamboo floor is full of small dimples, a little smaller than a dime. If the floor is sanded smooth, then the finish will be ruined. Can you help?

A. You are correct that bamboo is a grass and not wood. However, the bamboo products do behave similar to wood in many respects. The bamboo flooring is very dense...harder than oak. The small strips of bamboo (perhaps ¼ x ¼ inch in cross section or smaller) are glued together, using a fairly large amount of adhesive, under high pressure. This creates the dense flooring product. Now, when a nail or staple is put into the bamboo, perhaps at a little higher gun pressure than with oak flooring, there is no room for the fastener to get into the bamboo. So, the fastener has to push the bamboo out of the way. When the bamboo is pushed out of the way, this creates a bump on the surface. If you check this floor, you will see that the dimples are all associated with a fastener location. (This dimpling can also be seen with dense wood floors as well. We also see it when we put a fastener into the edge grain of plywood, MDF and other composites.)

In your case, the problem is that the installer of the floor did not follow the manufacturer’s installation instructions, which probably included using a thinner nail (18 ga., probably) and not using a staple machine. The instructions also likely included the angle that the nailer is held at and so on. In fact, it is probably the best idea with bamboo to glue it down. For bamboo that might be used for cabinet doors or other similar products, predrilling holes for hinge screws and other fasteners (maybe 90 percent of the root diameter) is the best practice.

In your case, total replacement is the only solution. Repair involving sanding is not a realistic option as the finish would be damaged beyond repair and the finish warranty would be voided, plus an excessive amount of the bamboo would be removed from the surface.