In 1925 under the auspices of the U.S. Department of Commerce the lumber industry after several years' deliberation adopted simplified practices published as American Lumber Standards. These standards, since revised and improved, constitute minimum basic provisions for manufacturing softwood lumber in standard grades. They are not purchase specifications, however, and should not be used as such. They have resulted in lumber producers adopting standard manufacturing grades under which, it is estimated, about 80 per cent of the softwood lumber now produced is graded. This means that the architect or builder can now specify lumber with assurance under a standard nomenclature and standard sizes. Moreover, the maximum defects, both as to size and as to number, permitted in similar grades for different species are as nearly alike as is practicable in view of the varying inherent characteristics of the different woods.
According to these American Lumber Standards softwood lumber is classified as yard lumber, structural material, and factory and shop lumber.
Factory and shop lumber are intended for millwork plants where they are remanufactured into various products, all serious defects being removed. Consequently it is of no special interest to the architect or the builder.
Structural material is lumber graded on definite strength values as well as on use of the entire piece. Not only are defects limited, but also the location of defects and the slope of grain - a most important consideration in any piece of wood to which engineering stresses are assigned. Its importance lies chiefly in its application for heavy timber construction in bridges, roof trusses, factory-type buildings, and such uses.
Yard lumber includes most of the lumber used for general building purposes. It is defined as lumber less than five inches in thickness and graded in accordance with the number and size of defects, and also, except in the lowest grades, upon the use of the entire piece.
Yard lumber is divided into the "select grades" (of good appearance and finishing qualities and suitable for natural or paint finishes) and the "common grades" (suitable for general utility and construction purposes).
The select grades, used for such purposes as interior and exterior trim, siding, paneling, and finish flooring, are classified as follows:
A Select - practically clear lumber (free from defects)
B Select - has few minor blemishes or defects Both of these grades are suitable for the highest type of work. In some woods the A select grade is not marketed as such, the best grade being known then as B and better and including the clear material which would otherwise grade as A select.
C Select - allows a limited number of small defects which can be covered with paint
D Select - allows any number of defects or blemishes that will not detract from a finish appearance when painted
In general, the A and B select grades are intended for "natural" finishes, where for decorative purposes it is desired to have the grain of the wood visible. The C and D select grades are best adapted for painted finishes which will cover the minor defects permitted in the grades.
The common grades are produced in both "boards" and "dimension."
Boards include lumber less than 2 inches thick, and may be square edged or worked to a pattern in such items as siding, flooring, and ceiling, for which the select grades are also widely used.
Dimension, used as framing material in such sizes as 2 X 4's, 2X6's, 4 X 4's, etc., is manufactured in only three grades: No. 1 common, No. 2 common, and No. 3 common. But, whereas the common grades of "boards" are based on their utility as a covering material, the common grades of "dimension" are based more on allowable defects affecting the strength and stiffness.
In general, No. 1 common lumber is sound and tight knotted - i.e., water tight. In dimension sizes it is widely used for floor joists, rafters, and other parts requiring strength and stiffness.
Number 2 common allows larger and coarser defects, but may be considered as grain-tight material and has other covering qualities. Number 2 common "dimension" is suitable and widely used for studs, plates, braces, etc., where the material is never stressed to its full capacity - in other words, for utility requirements. Number 2 common may also be used satisfactorily for floor joists if the span and loading are such that the strength of the joist will be ample and stiffness is the governing factor. In such cases No. 2 common will be more economical because it will provide practically the same stiffness as No. 1 common material, although not so much strength. Often it will be possible to use a larger-size joist in No. 2 common instead of in No. 1 common, thus getting equivalent strength but greater stiffness, without increasing the cost.
Number 3 common, in some species, is suitable for permanent construction for sheathing and subflooring. In other species, if No. 3 common is used, care should be taken to remove any serious defects or taint of decay.