Friday, 11/21/2008

Softwood Lumber Grading

Softwood lumber is categorized by intended use - construction or remanufacture. Construction lumber is graded and sized after sawing, surfacing, etc. It is commonly available in lumber yards and serves as the primary resource for many projects. Lumber for remanufacture will undergo secondary processing to create a product that may differ markedly from the original piece. Some examples include stock for pencils, ladders, poles, boxes, etc.

Lumber intended for general construction is subdivided into stress graded, non-stress graded, and appearance categories. Stress graded and non-stress graded lumber are required to have consistent structural integrity. With appearance lumber, visual quality of the piece is most important and structural integrity is a secondary requirement.

The term "yard lumber" is often applied to the non-stress graded and appearance lumber that is sold by retail lumberyards. With such lumber, grading is done on the better side of a piece after drying and surfacing, and grades are designated by specifying the allowable size and number of defects (e.g., knotholes). This contrasts with hardwoods where most grades are determined from the poorer side of each piece on the basis of a specified number of clear cuttings. Another distinction is that hardwoods are typically graded prior to drying and surfacing.

Non-Stress Graded Lumber

With non-stress graded lumber, pieces are graded primarily for serviceability but appearance is also considered, especially in the higher grades. Imperfections such as knots and knotholes are allowed to become larger and more frequent as the grade drops. The primary product is boards that are less than 2 inches in nominal thickness and 2 inches or more in nominal width. Common nominal widths are 2, 3, 4, 6, 8, 10, and 12 inches. Lengths are usually from 6 to 18 feet in increments of 2 feet. Three to five different Common grades may be applied to boards in this group depending upon the species and the lumber manufacturing association involved. In descending order of quality, the grades are:

No. 1 (Construction) - Moderate-sized tight knots. Paints well. Used for siding, cornice, shelving, paneling, some furniture.

No. 2 (Standard) - Knots larger and more numerous. Paints fair. Similar uses as No. 1.

No. 3 (Utility) - Splits and knotholes present. Does not take paint well. Used for crates, sheathing, sub-flooring, small furniture parts.

No. 4 (Economy) - Numerous splits and knotholes. Large waste areas. Does not take paint well. Used for sheathing, sub-flooring, concrete form work.

No. 5 (Economy) - Larger waste areas and coarser defects. Cannot be painted. Applications are similar to No. 5.

Stress Graded Lumber

This category includes most softwood lumber that is nominally 2 to 4 inches thick, referred to as "dimension" lumber. Examples include posts, beams, decking, studs, rafters, joists, timbers, and other structural lumber where working stresses will exist. Important qualities for stress-graded wood are strength, stiffness, and uniformity of size. Stress ratings may be determined either visually or mechanically to derive working values for properties such as bending stress and elasticity. A single set of grade names and descriptions are used throughout the United States although the allowable properties vary with wood species.

Appearance Lumber (Finish, Selects)

Appearance lumber is often non-stress graded but a separate category exists due to the greater importance placed on appearance. This group includes most softwood lumber that has been custom milled to a pattern or otherwise surfaced on all four sides (S4S). Examples are trim, siding, shingles, flooring, casing, base, stepping, and paneling. The highest grade of appearance lumber is Finish, which is subdivided into grades composed of letters or combinations of letters (B & BTR, C, D) or names such as Superior or Prime, depending on the grading agency. The next level down is Selects which has grade designations composed of numbers, letters, and names of combinations of them (B & BTR, C Select, D Select).

To further complicate matters, some differences in grade designations exist for different wood species and products. Cedar and redwood, for example, have different grade designations than other softwoods due to the marked difference in color between heartwood and sapwood in these two woods. Typical redwood board grades are Clear All Heart, Clear, and Select. Other woods, such as western white or Idaho white pine (IWP) are graded as Supreme-IWP, Choice-IWP, and Quality-IWP. Some appearance lumber may also carry annotations such as FG (flat grain), VG (vertical grain), or MG (mixed grain).

In spite of all of these complexities, you will usually encounter four grades of Select boards in lumber yards. These are denoted A, B, C, and D with A being the highest grade, and D the lowest. In practice, grades A and B are combined into one grade known as B and better (B & BTR). The below table briefly summarizes each of the standard grades of Select appearance lumber:

A Select - No knots, splits, or other visible defects. Used for fine furniture, exposed cabinetry, trim, flooring.

B Select - A few, small defects but nearly perfect. Used for fine furniture, exposed cabinetry, trim, flooring.

C Select - Small tight knots. May be nearly perfect on one side. Used for most furniture, shelving, some trim and flooring.

D Select - More numerous knots and other small blemishes. May be used for some furniture, shelving, some trim and flooring.

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