The various grades can first be divided into two broad categories:
Remanufacture grading is applied to wood used in industry. There are, for example, pencil stock, barrel stave, ladder rail, and stadium seat stock grades, each emphasizing particular qualities needed by a certain industry. Many such grades are encountered only in a particular part of the country, and in any case you won't find them in the average lumber yard.
A second class of remanufacture grades is industrial clears, used for kitchen cabinets, for example. Because appearance is important, the grading of industrial clears somewhat resembles that of hardwood.
The final class of remanufacture grades is Factory or Shop Grades, which may find their way to a retail lumber yard. The best grade is called Factory Select or Select Shop; the remaining grades are usually numbered in order of decreasing quality, No. 1, No. 2, and No. 3. To know what the grades mean you need to know which lumber association’s rules are being applied, but as a typical example, Select Shop might mean cutting 70% clear both sides or 70% B or better on one side, with the percentage clear dropping to 50% for No. 1 and to 33¹/₃% for No. 2.
Construction grades can be divided into three categories:
Appearance grade is mainly for board and moldings. The grades are designated by letters, A, B, C, D, and combination grades such as "B and better” (B&BTR, sometimes called B&B) and “C and better” (C&BTR) are usual. Appearance grades may also be called “Select,” “Clear” or “Prime,” depending on the species.
Special designations are used with certain species, such as “all-heart redwood." Not all grades are available. Like hardwood grading, appearance grades emphasize the appearance of the best side. As a general rule, even D contains no defects that would detract from the appearance after the wood is painted. C is considered appropriate for "high-quality exterior and interior trim, panelling, and cabinet work, especially where these are to receive a natural finish." B has minor defects like pin knots, depending on the species.
Nonstress-graded lumber is used where structural failure is unlikely or its consequences not catastrophic. Not that strength is unimportant, but grading is based on size and appearance. The grades may be numbered or terms like “construction,” (equivalent to No. 1) “standard,” (No. 2) and “utility” (No. 3) may be used. Typical uses for No. 1 are as siding, shelving and panelling. The lower grades are used for wall and roof sheathing, subfloors, and concrete forms. The lower the grade, the more knots, knotholes, and other defects. Certain products have their own nonstress-graded grades; for lath, for example, there is a No. 1 and a No. 2 lath grade.
The goal of stress-grading is that all the lumber in a single grade will have similar mechanical properties; it is principally used for dimension lumber such as two-by-fours. Unlike the appearance and nonstess grades, a single set of standards for stress-graded lumber applies across the United States, the National Grading Rule–which helps to ensure that a wood-framed house built to a particular set of plans won't collapse no matter where in the country it’s built. Stress grades are either visual or mechanical. The visual grades are assigned by visual inspection; the inspector assesses how much the defects that can be seen detract from the strength the wood would have if it was defect-free. The mechanical grades are assigned by testing the wood in a machine; lumber so graded may be marked MSR (machine stress rated).
The table below gives the names of the visual stress-graded grades and some idea of their mechanical properties. The bending strength ratio compares the strength requirement for the grade with that of a piece of wood of the same size and species having no visible strength-reducing characteristics.
|Classification||Grade Name||Bending strength|
wood 2 – 4 inches thick and 4 inches wide
|Structural Light Framing
wood 2 – 4 inches thick and 2 – 4 inches wide
Structural Joists & Planks
wood 2 – 4 inches thick and 6 inches or more wide
wood 2 – 4 inches thick and 2 – 4 inches wide
So if you're putting up a shelf in the garden shed to hold that old VW engine you're planning to restore some day, it's probably not a good idea to use Utility grade two-by-fours, even though you don't care what the shelf looks like.
The American Lumber Standard requires softwood to have a moisture content of 19% or less, which is indicated by the mark S-DRY, for surfaced dry. Some other markings are
KD, kiln dried (in southern pine, KD indicates 15% maximum moisture);
S-GRN, surfaced green, moisture content of more than 19% when the lumber was surfaced
PAD, partially air dry.
Lumber grades often indicate which sides have been surfaced. “S4S”, for example, means surfaced 4 sides and “S&E” means surfaced on 1 side and 1 edge.
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Last revised: 11 August 2004.