Defining and Measuring Knots in Hardwood Lumber

Over the years, I have often questioned the definition of a knot. It has always intrigued me that the NHLA Rules Book doesn’t define them. New technologies can now detect the edges of knots and even determine whether a knot is sound or unsound, which can lead to different interpretations of a knot. The best definition I can come up with is: “A knot is a portion of a branch or limb that has become incorporated in a piece of lumber.”

This definition does little to explain how to measure a knot or classify it as sound or unsound. On page 13, paragraph 47 gives us the formula for average diameter: “TO DETERMINE the average diameter of a knot or hole, add the maximum length and maximum width and divide by two.”

This doesn’t tell us where to start and stop our measurements, so I’ll explain. I have worked around planers, ripsaws, and molders for much of my career, and I have come to realize that a knot, considered a defect, is typically only an issue when it leads to “tear-out” or is unsound. In my experience, tear-out becomes more of a problem once the grain is perpendicular to the board surface, which gives us a pretty good idea of where to measure from and to.

When measuring a knot, determine where the knot’s end grain is apparent and start and stop your measurements at those points. Some knots are easy to identify, while others are not. This is where the inspector must use their best judgment.

The other consideration is deciding between sound and unsound knots. The definition of a “Sound Knot” is on page 86 of the Rules Book: “Sound Knot: A knot that is solid across its face, as hard as the surrounding wood, and shows no indication of decay.”

Normally, there are not many questions about whether a knot is sound or unsound. The question I typically get is about a check or split in the knot. A check is defined as: “A lengthwise separation of the wood that usually extends across the rings of annual growth and commonly results from stresses set up in wood during seasoning.” I believe this is a great explanation of a check that occurs within a knot.

The next thing the inspector needs to determine is whether the knot is still considered solid. This is where we need to use our best judgment to determine whether the check in the knot would cause that area to fail if used for a structural application, such as a brace in upholstered furniture or another product that uses Sound Cuttings.

These are the types of things that make AI-assisted grading a challenge. Remember, NHLA is here to help. Reach out to us if you have questions or need assistance.

By Dana Spessert, Chief Inspector