When it comes to understanding the world of cedar, there are few people we trust more than “Mr. Cedar” himself, Paul Mackie. Paul has been with the Western Red Cedar Lumber Association—known as the “voice of the cedar industry”—for more than two decades, working to represent quality cedar producers and educate people on all things cedar. In this series, he’ll share everything cedar-related, from what exactly western red cedar is to the difference between kiln- and air-dried lumber to installation best practice.

Today’s video is all about how to interpret western red cedar lumber grades. Lumber grading is the process by which the pieces of wood are evaluated. In the case of structural lumber (lumber used for framing houses, or for other structural aspects of building like holding up walls, floors, or roofs), they’re graded to account for things that may have an effect on the lumber’s structural integrity: things like knot size and location, hole size and location, and presence or absence of wane.

How is western red cedar graded?

Because western red cedar is mainly used for “appearance applications” (meaning, applications where strength is not a primary concern and high-strength value is not required), it’s evaluated for different factors than structural lumber would be. The primary factors that contribute to the grade for a selection of western red cedar are: (1) The presence or absence of knots. The higher the grade, the fewer the knots, and some boards will have almost none. Lower grades of tight-knot material will often have knots that are well-scattered and not limited in size (you’ll see larger knots in wider stock and smaller knots in more narrow stock). (2) Grain orientation (for example, vertical-grain in clear cedar).

What’s not taken into account?

Color. Western red cedar naturally presents with a wide spectrum of colors, from tan, to chocolate brown, to straw, or even bright-white, and the color variation is not part of the grading process. While the color variations may give western red cedar the appearance of other wood species like walnut or pine, its durability is unaffected as the lumber’s tone shifts.

What’s unique about western red cedar?

Similar to other “durable wood species,” the durability of western red cedar is found in the heartwood, with significantly diminished durability found in the sapwood. Unlike other durable wood species, western red cedar has a very small amount of sapwood—the tree only has roughly a ¾” ring of sapwood, just beneath the bark. Because of its low sapwood content, western red cedar is rated more durable and resistant to rot, decay, and insects.

What does “STK” mean?

You might have seen it included in lumber grades—and heard a few different answers about what the acronym stands for. The short answer: officially, nothing. Among the interpretations are the phrases “select tight knot,” “sound tight knot,” “some tight knots,” “see-through knots,” and even “South Texas knot.” But “STK” isn’t found in any grade rule book. The grade for tight-knot cedar in the grade rule book (NLGA and WCLIB) is “select knotty.”

Stay tuned for more from Paul as we continue to learn from him on Dunn Solutions over the next few months, and catch up on his previous posts: