Understanding framing lumber grades
The species of lumber and lumber grades available will often vary depending on which region you are in. In California, for example, the predominate species of dimensional lumber used for home building is Douglas Fir-Larch (DF-L). As you move east, lumberyards stock both DF-L and Hem Fir (HF). Once you get into the Southwest, you may use Spruce-Pine-Fir (SPF). All of these are considered softwoods and are graded under the American Softwood Lumber Standard PS 20.
Once it was possible to simply order 2×4 and 2×6 for walls, and 2×12 for headers. The species that was in stock locally was all I ever needed, but today architects are looking more closely at the performance characteristics of the lumber that goes into the homes they design.
Specifying lumber grades
Today, tradesmen must carefully read the plans and specifications to identify the various species and grades of materials that are specified. You can’t simply assume the lumberyard will send you the correct materials. If you frame a home using the wrong materials, you’ll be held responsible for the error. If it’s simply a matter of changing out a couple of headers that should have been built with a stronger material, it isn’t a big problem. But if you frame a whole house with SPF that was specified to be framed with DF-L, you may have to take it down and build it again. Clearly, you need to understand grade stamps.
The first efforts to standardize lumber sizes and establish uniform quality began in the United States in 1924. In 1953, the Secretary of Commerce appointed the American Lumber Standard Committee (ALSC) to oversee implementation of the American Softwood Lumber Standard.
As wood is produced in a sawmill, a lumber grader, certified by an ALSC approved grading agency, visually inspects samples or quality. The grader checks the size and number of knots and other imperfections in the wood, and assigns a value or “grade” to the material being produced.
Dimensional framing lumber is not graded for appearance. Even #1 grade lumber may have defects that make it visually undesirable for an exposed application. When lumber is used as finish trim it should be ordered in an Appearance grade.
When you order specific lumber grades, such as HF #2, you’ll get #2 &+ (and better). This means all the lumber bearing this grade stamp should be a minimum of #2 quality. Some pieces may be higher quality. You may find individual pieces of lumber in a unit of #2 &+ graded as #1 or even Select Structural, but the minimum quality will be #2.
It’s common to find framing plans for homes that have a general note stating: “All framing materials shall be Hem Fir Standard U.N.O. (unless noted otherwise)”, and then have specific labels for certain headers or beams that call for the use of a stronger material, such as DF-L #1. Specifying a species of lumber can become a problem when an architect designs a house plan in California, where DF-L is common, and the home is constructed in Texas, where SPF is the species most yards stock. To avoid these complications some designers specify an Extreme Fiber Stress in Bending Value (Fb) instead of naming a species and grade.
For example, the specifications might read, “All framing materials shall be minimum Fb 850.” In this case, use a lumber values chart to determine the appropriate material. Fb 850 is the fiber bending stress value for HF #2 or Douglas Fir-South (DF-S) #2. Lumber with a higher rating, such as DF-L #2, which carries a rating of 900 Fb, could be substituted, but SPF #2 (775 Fb) would not be acceptable.
Wood has a sponge-like cellular structure that absorbs moisture under humid conditions and loses it when the air is dry. When lumber is delivered to a site, it will eventually reach equilibrium with the ambient humidity level. Although it’s impossible to control the final moisture content of wood, architects will often specify maximum allowable moisture content for framing lumber.
Dimensional lumber is typically specified to be no more than 19 percent at the time of surfacing. Interior trim products are usually specified at 15 percent. This is the third consideration when ordering framing lumber. Make sure you know the species, the grade and the moisture content required for your project.
Reading stamps of lumber grades
Information from plans and specifications will tell you what grade lumber you need, the grade stamp on the actual lumber will tell you what you have. Grade stamps for common dimensional framing lumber contain five pieces of information.
(a) The trademark of the ALSC accredited grading agency. In this example, the agency is the WWPA (Western Wood Products Association).
(b) The lumber mill identification number. Each mill is assigned a specific number. If there is a problem with a product, this number allows the lumber to be traced back to its mill of origin.
(c) The lumber grade. In this example, the grade is “Standard.” Standard grade is the most common grade of framing lumber produced. Dimensional lumber is broken down into the following classes and grades.
- Structural light framing, intended for applications where high strength is required includes: Select Structural, #1, #2 and #3 grades.
- Light framing, for use in light-frame construction such as wall framing, sills and blocking includes: construction, standard and utility grades.
- Stud, intended for vertical use in load bearing walls is Stud grade. Structural joists & planks, intended for use as joist, rafters and beams 5-inches and wider includes: select structural, #1, #2 and #3 grades.
(d) Species of lumber. In this example the grade is “Douglas Fir.”
(e) Moisture content at the time of surfacing. “S-DRY” means the lumber was kiln-dried to a maximum moisture content of 19 percent before it was surfaced. Other recognized levels of seasoning (drying) are:
- MC-15 or KD-15: Lumber dried to maximum 15 percent moisture content.
- S-DRY, KD or KD-HT: Lumber dried to maximum 19 percent moisture content.
- S-GRN or HT: Unseasoned lumber with moisture content in excess of 19 percent.
To learn more about lumber species and grades, download the Western Lumber Product Use Manual, Western Wood Species Book, Vol. 1, Dimension Lumber, or species specific guides: Douglas Fir and Western Larch, Hem-Fir Species Facts, Ponderosa Pine Species Facts, Redwood Lumber Grades and Uses, and many more useful free PDF titles under the Resources Tab at the Western Wood Products Association website.
—By Michael Davis, Framing Square Construction Co., Conifer, Colorado