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 2x8 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 2x12 is ripped to create two 2x6's, they will need to be re-graded. A N1 2x12 would not automatically create two N1 2x6'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.