Material Grading – Knots: Wednesday’s Word(s) Of The Week

Knots in wood are the locations of former tree branches. If you’ve ever worked with wood, you probably know how hard the knot material is. However, the knot interrupts the primary, longitudinal fibers in wood, so it is always considered a weak spot structurally. 


There are specific rules which govern the maximum size of knot allowed in lumber to qualify for various grades. “302” tension plies have the smallest maximum knot size, then N1, N2, and N3. The allowable knot size rules vary by species, the lumber size (larger lumber can have bigger knots) and whether the knot is on the lumber’s edge (center knots can be larger than edge knots.)

As an example, for 2×8 Southern Pine, the maximum centerline knot sizes are:
302 – ¾”
N1 – 2¾”
N2 – 3½”
N3 – 4½”


Because the allowable knot size depends on the lumber size, if a 2×12 is ripped to create two 2×6’s, they will need to be re-graded. A N1 2×12 would not automatically create two N1 2×6’s.

*Wednesday’s Word Of The Week is a feature on Unalam’s Wood Times Blog. Each Wednesday our structural engineer, Rik Vandermeulen, will discuss a new term associated with glulam manufacturing. He will do this until we run out of words. If there is a timber or glulam term that you have heard of and want to know more about, let us know in the comments.