Framing lumber, unlike money, does grow on trees, so there’s always the chance it will have some natural defect, such as a large knot or a long split, that can reduce its overall strength. But not all defects are obvious. That’s why every stick of lumber coming out of a sawmill is appraised by trained inspectors and given a grade stamp. This cryptic tattoo indicates that a piece of wood meets established standards for strength and stiffness, and it also offers useful information about the type of wood and how much moisture it contains. Follow along as we decode the stamp and show you how to get the most lumber for your money.
1. Species
Denotes the wood’s species or a group of species with similar strengths. (See “Wood Code,” below). While all species are graded at the same four levels of strength and appearance, they are not equally strong. A joist made of Douglas fir, for instance, will cover a greater span than a hemlock joist of the same size and grade. But using bigger joists of a cheaper species might save you money without loss of strength.
2. Grade
Establishes lumber quality based on a grader’s visual inspection. The ranking goes from “Select Structural” (highest strength, best appearance) to No. 1, No. 2, and No. 3 (lowest strength, not pretty). As the numbers go up, quality and price go down. “Stud” is a fifth designation, meaning it’s good for a load-bearing wall. This Old House general contractor Tom Silva usually buys stacks of No. 2 lumber, which provides good strength at a reasonable price. Those stacks also include some No. 1s and Selects, to boot. For demanding applications, such as interior load-bearing walls, he’ll pay extra for these stronger grades.
3. Moisture Content
“Green” unseasoned lumber fresh from the mill is labeled either air-dried (AD) or surface-green (S-GRN). Its moisture content of 19 percent or more makes it inexpensive, but it can move unpredictably as it shrinks and dries. You’ll get less movement with kiln-dried (KD) or surface-dry (S-DRY) lumber, which has a moisture content between 16 and 19 percent. (KD-HT means the wood was also heat treated to kill pests, a requirement for imported lumber.) Tom saves money by ordering S-GRN wood when framing a new house, figuring that all the wood will air dry at the same pace. But in an existing house where the framing is already dry, he’ll use more stable, and more costly, KD lumber, or even MC-15 lumber, which has a moisture content of 15 percent or less. The two remaining designations, while not very useful to a homeowner, enable grading agencies to enforce their standards.
4. Certification Mark
Identifies the accredited agency or association that oversees grading accuracy, in this case the National Lumber Grades Authority.
5. Mill
Gives the trademark or number of the mill of origin.
Wood Code
Abbreviations for framing-grade softwoods, from strongest to weakest
Doug Fir, D Fir- L: Douglas fir (L=western larch)
SYP: Southern pine
Hem, Hem-fir, H-F: Hemlock or fir
S-P-F: Spruce, pine, or fir