Description
This article is from the Woodworking FAQ
Collection 1, by multiple authors.
22 How is lumber graded?
The following was provided by David R. Mount
(dmount@bigcat.missouri.edu) and added to the FAQ with his
permission.
*********************************************************************
The "Encyclopedia of Wood" from Sterling publishing is a good
source of this information, a synopsis of which I'll provide here.
As I recall (don't have it in front of me) Bruce Hoadley's book
"Understanding Wood" has a comparable treatment.
I'll give a brief summary of dimensions and grading as follows:
Softwood Construction Lumber
Most softwoods (though certainly not all) that are made into finished
boards are intended to be construction lumber. This material is
generally sold according to it's "nominal" dimension, typically 1 by
something or 2 by something. Mininum thickness for planed, dry 1-by
material is 3/4"; dry means average moisture content of 12%, maximum
of 15%. Minimum thickness for 2 by material (planed and dry) is
1-1/2". For wet or "green" lumber (this includes formerly dry
material that has been allowed to take up water and has not re-dried)
minimum surfaced thickness is 25/32" and 1-9/16" for 1-by and 2-by
material, respectively. For widths, the rules are the same for
both 1-by and 2-by material. For widths up through 7 inches, the
minimum dry, planed width is 1/2" less than the nominal dimension.
So, dry 1x6 and 2x6 material should both be 5-1/2" wide. For
widths of 8" and above, the width should be 3/4" less than the
stated dimension (e.g., 1x10 should be 9-1/4" wide).
Grading softwoods is complex (though not as bad as hardwoods) and it
depends on the use (boards versus structural lumber), and the type
and distribution of defects (tight knots, loose knots, spike knots,
wane, shake, etc.). There are several different grading authorities,
but the board grades most common in the U.S. are (in decreasing order
of "quality") finish or select, #1 (common), #2 (common), #3 (common),
and #4 (common). Structural lumber generally uses different
terminology including such things as #1, #2, and #3 structural,
superior, stud, utility, and several others. Ask your lumber dealer
to explain the system by which their lumber is graded; there is a
lot of variation from dealer to dealer. The "Encyclopedia of Wood"
has pictures of the board grades; Hoadley's book may also, I
don't recall.
Hardwoods
Standard thicknesses for hardwood lumber are as follows (in inches):
Nominal (green) Dressed (dry and planed)
Thickness Thickness
-------------------------------------------------
2/4 5/16
4/4 13/16
5/4 1-1/16
6/4 1-5/16
8/4 1-3/4
Notice that 2/4 lumber is officially listed as only 5/16" planed. In
practice, hardwood stores I've been to generally sell material they
call "2/4" that is actually surfaced to either 1/2" or 7/16". I
don't know why. In my experience, the thicker sizes are typically
sold as I've listed above.
Hardwoods are generally sold as random widths, unlike construction
lumber that has width controlled as tightly as thickness. In
general, according to the Encyclopedia of Wood, dry hardwood is
sold as the next highest inch down to 3/8" shy of the nominal
dimension for less than 8 inches and 1/2" shy for greater than
8 inches. In other words, a 6" wide board may be as narrow as
5-5/8", a 10" board as narrow as 9-1/2". In practice, hardwood
dealers vary in how they figure width into the board foot
calculation; some go to the nearest inch, some to the nearest half
inch, some to the exact dimension. Don't be shy to ask a dealer
how he/she makes the calculation; you have a right to know.
Grading hardwoods is *really* complex. Grades are determined by
number, width, and length of clear (free of knots and other defects)
cuttings that could be obtained from the board, which is in turn
determined by the number and location of defects, primarily knots.
The top three grades are "firsts", "seconds", and "select(s)".
Hardwood dealers typically sell these grades together as "firsts
and seconds" (known as FAS) or "select and better" (SAB). In
short, if all the defects were cut out of these boards, they
should yield between 83.3% and 100% of pieces with at least one
clear face. The specific grading rules are much more complex,
but this is the general idea. Note that there has been a movement to
relax the requirements somewhat for black walnut. I also believe that
many dealers sell wood as "FAS" that actually contains some select
material; it's not a big deal usually, unless you need big wide long
clear boards and are buying sight unseen.
After these top three grades follows #1 common and #2 common. These
grades allow a lower percentage of the board to yield clear cuttings
(BTW, "cuttings" means smaller boards) and allows the cuttings to be
narrower and shorter than in the better grades. #1 common should
yield 66.6% to 100% clear cuttings (depending on the board size)
while #2 common will yield 50% to 66.6%.
A couple notes about #1 and #2 common boards. First, they are often
not displayed right out front by a hardwood dealer; if you are
interested in lower grades, ask about them. #1 and #2 boards can
sometimes be a better buy; they are generally 20 to 70% cheaper
than FAS boards so the cost per square inch of clear lumber may be
lower if obtained from lower grade boards. Of course, this requires
that you can use smaller pieces; you also have to figure in the cost
of your time to cut around the knots. On the other hand, in more
casual pieces I often leave some knots in the finished piece if
they won't cause structural problems. Some of the most beautiful
figure occurs near knots. Each person will weigh these variables
differently.
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