Grading of lumber is not an exact science, since it is done by looking over the lumber and deciding to which grade it belongs. However, this inspection is done by men highly trained in judging lumber. Standards may vary somewhat in different sections of the country. American Lumber Standards for grading lumber have been set up by the lumbering industries, the contractors, and the Bureau of Standards of the United States Government. These standards are usually followed in the grading of lumber.
Lumber contains knots, pitch pockets, and
other defects in varying amounts. The number
and size of these defects determine the
usefulness of the lumber for many purposes.
The American Lumber Standards set up three groups of lumber according to its principal uses: yard lumber, structural timber, and factory or shop lumber.
Yard lumber is that which is used for general building purposes; it is graded into two general classifications, select and common. There are four different grades of select lumber—A, B, C, and D. Grades A and B are suitable for natural finishing, while grades C and D are for painted surfaces. Common lumber has five different grades-1, 2, 3, 4, and 5. Numbers 1 and 2, common grades, are suitable for use with-out any waste. Numbers 3, 4, and 5 are low grades of lumber with many knots, blemishes, and knotholes.
Structural timbers, those 5" or over in thickness and width, are graded according to the strength and use of the entire piece.
Factory or shop lumber is intended for use in making articles where such defects as knots and knotholes may be removed in the manufacturing process. This lumber is graded on a basis of the proportion of us-able area for cutting a number of pieces of specified minimum size and quality.
Sources: William H Johnson and Louis V. Newkirk. General Woodworking. New York: Macmillan, 1946, pages 6-7; Walter E Durbahn, "What You Should Know About Lumber", American Magazine 157 April 1954, page 67.