Construction-grade lumber usually comes from softwood trees, often fir or pIne. Softwood lumber comes in one of three general grade types--stress-graded, non-stress-graded and appearance. Most lumber sold in yards or large home-improvement centers is visually stress-graded lumber, which comes in five classes, with 13 sub-classes. Only one sub-class has the grade name "construction," but all 13 of these stress-graded lumber grades have construction applications.
Who Decides the Rules for Grading Lumber?
The U. S. Department of Commerce has stipulated that a National Grading Rule Committee composed of professionals in lumber technology shall establish grade descriptions for dimensional lumber. The Commerce Department reviews and approves these descriptions. Other lumber associations have also established grading rules.
The Five Classes of Stress-Graded Lumber
The five classes of stress-graded lumber are light framing, structural light framing, structural joists and planks, and appearance. Appearance-class stress-graded lumber shares a name with non-stress-graded lumber, but has different properties.
How is Lumber Classified?
A qualified lumber grader takes a look at a piece of lumber and estimates its bending strength from its appearance. The decision relies on a visual assessment, but the classification rates bending strength.
Light Framing-Grade Lumber
Most wood sold in lumberyards falls within the light framing classification in one of three grades: construction-grade, with 34 percent bending strength; standard-grade, with 19 percent bending strength; and utility-grade, with 9 percent bending strength. Construction-grade and standard-grade lumber are safe for residential building. Utility-grade lumber has less than 1/3 the bending strength of construction-grade lumber; also, yards often sell it wet, which means it will warp as it dries.
Structural Light Framing Grade Lumber
Structural light framing-grade lumber comes in four sub-grades, in descending order of bending strength: Select structural (67 percent), usually called "select" by lumbermen; 1 (55 percent); 2 (45 percent); and 3 (26 percent). Those without special construction needs beyond normal balloon-frame construction (wood studs on 16-inch centers) won't need select structural-grade; some builders use it where it will show because it has few knots, a straight grain and looks good.
Studs Grade and Structural Joists and Planks Grade
Structural joists and planks grade has four grades with names and bending strengths almost identical to the four grades of structural light framing grade, but comes only in larger dimensions. Studs, the classification, has one grade, stud grade, with a bending strength of 26 percent. Similar to standard-grade, with slightly greater bending strength, it often comes precut to exactly 92 1/4 inches. Add a floor plate and a top plate and the height totals exactly 96 inches, the height of an 8-foot stud wall.
Another Grading Criterion
Construction lumber comes graded either S-GRN, with more than 19 percent water by weight--green lumber, in other words--or S-DRY, with less than 19 percent moisture. Experienced builders can sometimes get away with using the cheaper S-GRN lumber by putting it up almost immediately before it dries and warps--once locked into a stud wall or other structure, it can only warp so much. But most will pay a little more and get S-DRY grade.