Lumber is graded by the various associations of lumber manufacturers having jurisdiction over various species. Two principal sets of grading rules are employed:
(1) for softwoods, and (2) for hardwoods.
Softwoods. Softwood lumber is classified as dry, moisture content 19% or less;
and green, moisture content above 19%.
According to the American Softwood Lumber Standard, softwoods are classified according to use as:
Yard Lumber. Lumber of grades, sizes, and patterns generally intended for ordinary construction and general building purposes.
Structural Lumber. Lumber 2 in or more nominal thickness and width for use where working stresses are required.
Factory and Shop Lumber. Lumber produced or selected primarily for manufacturing purposes.
Softwoods are classified according to extent of manufacture as:
Rough Lumber. Lumber that has not been dressed (surfaced) but has been sawed, edged, and trimmed.
Dressed (Surfaced) Lumber. Lumber that has been dressed by a planning machine (for the purpose of attaining smoothness of surface and uniformity of size) on one side (S1S), two sides (S2S), one edge (S1E), two edges (S2E), or a combination of sides and edges (S1S1E, S1S2, S2S1E, S4S).
Worked Lumber. Lumber that, in addition to being dressed, has been matched, shiplapped or patterned:
Matched Lumber. Lumber that has been worked with a tongue on one edge of each piece and a groove on the opposite edge.
Shiplapped Lumber. Lumber that has been worked or rabbeted on both edges, to permit formation of a close-lapped joint.
Patterned Lumber. Lumber that is shaped to a pattern or to a molded form.
Softwoods are also classified according to nominal size:
Boards. Lumber less than 2 in in nominal thickness and 2 in or more in nominal width. Boards less than 6 in in nominal width may be classified as strips.
Dimension. Lumber from 2 in to, but not including, 5 in in nominal thickness, and 2 in or more in nominal width. Dimension may be classified as framing, joists, planks, rafters, studs, small timbers, etc.
Timbers. Lumber 5 in or more nominally in least dimension. Timber may be classified as beams, stringers, posts, caps, sills, girders, purlins, etc.
Actual sizes of lumber are less than the nominal sizes, because of shrinkage and dressing. In general, dimensions of dry boards, dimension lumber, and timber less than 2 in wide or thick are 1â„4 in less than nominal; from 2 to 7 in wide or thick, 1â„2 in less, and above 6 in wide or thick, 3â„4 in less. Green-lumber less than 2 in wide or thick is 1â„32 in more than dry; from 2 to 4 in wide or thick, 1â„16 in more, 5 and 6 in wide or thick, 1â„8 in more, and 8 in or above in width and thickness, 1â„4 in more than dry lumber. There are exceptions, however.
Yard lumber is classified on the basis of quality as:
Appearance. Lumber is good appearance and finishing qualities, often called select.
Suitable for natural finishes
Generally clear and of high quality
Suitable for paint finishes
Adapted to high-quality paint finishes
Intermediate between high-finishing grades and common grades, and partaking somewhat of the nature of both Common. Lumber suitable for general construction and utility purposes, often given various commercial designations.
For standard construction use
Suitable for better-type construction purposes
Well adapted for good standard construction
Designed for low-cost temporary construction
For less exacting purposes
Low quality, but usable
Structural lumber is assigned modulus of elasticity values and working stresses in bending, compression parallel to grain, compression perpendicular to grain, and horizontal shear in accordance with ASTM procedures. These values take into account such factors as sizes and locations of knots, slope of grain, wane, and shakes or checks, as well as such other pertinent features as rate of growth and proportions of summerwood.
Factory and shop lumber is graded with reference to its use for doors and sash, or on the basis of characteristics affecting its use for general cut-up purposes, or on the basis of size of cutting. The grade of factory and shop lumber is determined by the percentage of the area of each board or plank available in cuttings of specified or of given minimum sizes and qualities. The grade of factory and shop lumber is determined from the poor face, although the quality of both sides of each cutting must be considered.
Hardwoods. Because of the great diversity of applications for hardwood both in and outside the construction industry, hardwood grading rules are based on the proportion of a given piece that can be cut into smaller pieces of material clear on one or both sides and not less than a specified size. Grade classifications are therefore based on the amount of clear usable lumber in a piece.
Special grading rules of interest in the construction industry cover hardwood interior trim and moldings, in which one face must be practically free of imperfections and in which Grade A may further limit the amount of sapwood as well as stain. Hardwood dimension rules, in addition, cover clears, which must be clear both faces; clear one face; paint quality, which can be covered with pain; core, which must be sound on both faces and suitable for cores of glued-up panels; and sound, which is a general-utility grade.
Hardwood flooring is graded under two separate sets of rules: (1) for maple, birch, and beech; and (2) for red and white oak and pecan. In both sets of rules, color and quality classifications range from top-quality to the lower utility grades.
Oak may be further subclassified as quarter-sawed and plain-sawed. In all grades, top-quality material must be uniformed in color, whereas other grades place no limitation on color.
Shingles are graded under special rules, usually into three classes: Number 1, 2, and 3. Number 1 must be all edge grain and strictly clear, containing no sapwood.
Numbers 2 and 3 must be clear to a distance far enough away from the butt to be well covered by the next course of shingles.