The age-old misconception is that there are many conditions that can cause wood to check. The fact is that there is primarily only one. Checking in solid wood flooring has been and continues to be a prominent claims concern that all-too-often eludes proper identification, evaluation and resolution. Most folks involved with the wood flooring industry understand that checking is a condition related to moisture content changes in wood that results from drying stresses. Commonly misunderstood, however, is the fact that these stresses cannot and do not originate in wood installed within the normal environmental ranges of interior living spaces.
• Surface Checks are failures that usually occur in the wood rays on the flat-sawn faces of boards. They occur because drying stresses exceed the tensile strength of the wood perpendicular to the grain, and they are caused by tension stresses that develop in the outer part, or shell, of boards as they dry. Surface checks can also occur close to a knot, by gum pockets and mineral streaks, and in bacterially infected wood, as such wood is weaker than “normal wood.”
• End Checks, like surface checks, usually occur in the wood rays but on end-grain surfaces. They also occur during the early stages of drying. End checks occur because moisture moves much faster in the longitudinal direction (along the board’s length) than in either transverse direction. Therefore, the ends of boards dry faster and shrink (or try to shrink) sooner than the rest of the lumber; the end result is that stresses develop at the ends.
Hypotheses Through the Years
Various causes have been (and continue to be) offered about why checks develop in wood flooring after installation. The following examples of commonly touted hypotheses for checking in post-installation environments provide useful insight on where we as an industry have traditionally focused:
The wood checked because …
• it was installed in an uncontrolled environment outside of the 35%-55% industry-accepted relative humidity (RH) range
• it was installed within acceptable moisture content (MC) range but dried excessively during the winter months (during seasonal periods of low RH)
• it gained moisture during the summer and swelled excessively, causing checks
• it wasn’t acclimated properly before installation (either too wet, or too dry).
Yet, as mentioned in the definitions above, the fact is that face-checking and end-checking develop as the result of drying the lumber too fast.
Grading standards are seldom utilized at any level in the marketing and sale of wood flooring, even though oftentimes the potential for checking is specifically qualified and quantified in these standards. Their placement on samples and within product literature would present an educational opportunity for sales staff at all levels. In addition, it would actively or passively provide the consumer with a means to acquire this valuable knowledge. Consumers that have obtained product-specific information are less likely to voice complaints about the potential for previously disclosed and occasional defects in the finished floor. After all, wood is an imperfect and natural product. Color, grain, character, and performance variability combine to create and define each wood floor’s unique and distinctive look.
Consideration should be given to the fact that drying defects occur, and that eliminating their existence cannot be expected nor guaranteed. In most good-quality and excellent-quality flooring mills, a very small percentage of boards will continue to be manufactured with a small level of drying defects (both visible and invisible) and will be installed. Manufacturers’ grading standards provide invaluable product-specific information that defines which characters and defects will be allowed, as well as their frequency. It is important that we make this information common knowledge for everyone’s sake.
When checks in the solid wood floor are beyond the manufactured stated limits, claims should be handled swiftly and with clear understanding on the part of all parties that the consumer was not the source for the issue.