Slide your measuring tape across a 2 X 4 and you’ll discover that it doesn’t actually measure two inches by four inches. In fact, it will be ½ inch shy in both directions. In it’s rough state, when the lumber was originally ripped into studs, this same piece was in fact a true 2 X 4. But after drying, it shrank a little. Then it was surfaced (planed) on all four faces, and it shrank a little more.
When you buy standard softwood lumber at your home center, surfaced and jointed on all faces and edges, the industry sells it to you in finished dimensions, but still describes it in nominal dimensions (the size it was before milling)
A piece of softwood lumber with a nominal 1-inch thickness is generally referred to as a board, while nominal 2-inch thick softwood is called framing stock (as in studs, joists and rafters), or dimension lumber.
Softwood lumber is graded by strength and appearance as well as moisture content. For woodworking applications, the three common grades to know are Select, Finish and Common. While boards in the Common grade categories may contain some blemishes and knots, Select and Finish grades are clear or nearly clear of defects.
Be aware, however, that boards within any grade may exhibit some degree of natural distortion (cupping, bowing, twisting), so it’s important to examine each board carefully by sighting along its length and width before you buy.
All construction lumber sold in the US bears an industry grading stamp such as the Western Wood Products Association (WWP) stamp. Nominal softwood lumber is graded similarly, but usually the stamp doesn’t show.
Here’s how to decipher grade stamps:
12 – Identifies the mill. This can be letters or numbers.
1&BTR – This is the grade of lumber, in this case #1 common and better, an excellent furniture grade.
WWP – The grading association that graded the board, in this case the Western Wood Products Association.
S-DRY – The condition of season at the time of surfacing, in this case dry, or seasoned lumber below 19% moisture content. If the stamp read KD-15, it would denote kiln-dried lumber with a maximum of 15% moisture content. Product stamped S-GRN stands for unseasoned (green) lumber containing more than 19% moisture content.
DOUG FIR-L – Indicates the wood species, in this case, Douglas Fir.
SOFTWOOD LUMBER GRADES
Grade Grading Criteria
B Select and BTR Highest quality lumber with little or no defects or blemishes. Nominal sizes may be limited.
C Select Some small defects or blemishes permissible, but still largely clear and of high quality.
D Select One board face usually defect-free.
Superior Finish Highest grade finish lumber with only minor defects.
Prime Finish High quality with some defects and blemishes.
No. 1 Common Highest grade of knotty lumber; usually available by special-order.
No 2. Common Pronounced knots and larger blemishes permissible.
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