Q: What causes end checking in lumber and how can I minimize it?

A: Cracks or checks on the end of lumber result from one of four different causes. The cause can be:

1. Excessive stress in the tree (called growth stresses). These growth stress cracks are typically wider than 1/4-inch and may extend a foot or more up the lumber. They develop in the log or in freshly sawn lumber and worsen during drying.
2. Rapid drying of the ends of lumber, compared to the drying rate of the surfaces. This drying differential results in stresses that crack the wood. Such checks are 90 degrees to the annual growth rings and are straight cracks.
3. Damage from logging. Such checks are across the rings, but will typically be jagged instead of running straight across the rings. Damage can run up the log by a foot or more.
4. Damage due to bacterial degradation. Such damage runs parallel to the rings and may be called shake or wind shake. The wood is quite weak due to the bacterial activity, so the tree will often have this shake develop before it is harvested.

Of these four types of end checks, only No. 2, drying-related checking, can be controlled. The control method is to put a coating on the end of the lumber ASAP after sawing. The coating slows the drying rate of the ends; to do this, the coating needs to adhere well, be thick enough and must stop moisture movement. One of the most popular coatings is a low-melting-point wax in water. The low melting point assures that the wax will evaporate in the kiln and will not enter the manufacturing plant. It is worth noting, however, that once a check forms, the coating does not control the growth of the checks very well; therefore, immediate application (several hours after sawing) is required for best control. Why not buy the coating material for the saw mills to use on your lumber? What excuse could they use for not using it or for putting it on too thin?