How to Read a U.S. Softwood Lumber Grade Stamp
The softwood lumber used in the manufacture of structural building components are graded under rules approved by the American Lumber Standard Committee’s (ALSC) Board of Review (BOR) and certified for conformance with the American Softwood Lumber Standard PS 20. Under this system, a grade stamp made of indelible ink is applied to the surface of every piece of lumber. This means that every stick in a bunk of lumber must have a grade stamp on it when it leaves the manufacturing mill.
A grade stamp contains specific information that allows a lumber user to reference specific published design values ascribed to that piece of lumber, as well as allow the lumber to be traced back to its source in the event of an issue. Under the ALSC system, each stamp must be legible and contain the following five elements:
While all lumber species are graded at the same four levels of strength and appearance (Select Structural, No. 1, No. 2, and No. 3), they do not have identical properties from a structural standpoint. A joist made of Douglas Fir - Larch, for instance, will have a greater structural capacity than an Eastern Hemlock joist of the same size and grade.
The species of a piece of lumber will be indicated on the grade stamp with the name or abbreviation of individual species or species combination. Following are examples of commonly used abbreviations.
- DF-L (or D. FIR): Douglas fir or western larch
- SYP: Southern yellow pine
- HF: Western hemlock or fir
- SPF: Spruce, pine, and fir
2. Agency Trademark
For acceptance in the U.S., six grading agencies have jurisdiction over establishing softwood lumber design values and have oversight of lumber manufacturing mills:
- National Lumber Grades Authority (NLGA)
- Northeastern Lumber Manufacturers Association (NeLMA)
- Redwood Inspection Services (RIS)
- Southern Pine Inspection Bureau (SPIB)
- Pacific Lumber Inspection Bureau (PLIB/WCLIB)
- Western Wood Products Association (WWPA)
There are also non-rule writing agencies that use the grade rules above. Each of these rule writing agencies has their own trademark, and a mill will use the trademark which corresponds with the agency that monitors its grading procedures.
3. Mill Identification
Each lumber manufacturing mill will also include their own unique trademark, which is typically a number or set of letters.
There are various ways to determine the grade of a board of lumber under the ALSC system, but the most common is referred to as visual grading. For structural light framing, which is what is used in today’s structural building components, visual grades typically include (from highest strength to lowest):
- Select Structural
- No. 1 and Better
- No. 1
- No. 2
- No. 3
Another common way for structural lumber to be graded is with a machine. There are several technologies today that are employed by machines to produce machine stress rated (MSR) or machine evaluated lumber (MEL), where a machine nondestructively tests each board during the manufacturing process to determine its physical properties. Common MSR grades include (from lowest strength to highest):
5. Moisture Content
Finally, every stamp must identify the lumber’s moisture content at the time of surfacing (e.g., when the lumber surface is planed of charring after it is removed from a kiln). Green, or unseasoned lumber, is labeled as surface-green (S-GRN), and has a moisture content of 19 percent or more. Air dried lumber has been dried without a kiln and has a maximum moisture content of 19 percent and is labeled S-DRY.
Kiln-dried (KD) or surface-dry (S-DRY) lumber has a maximum moisture content of 19 percent. HT is used when the lumber is also heat treated to kill pests, which is typically required for exported lumber. KD15 or MC15 indicates the lumber has a maximum moisture content of 15 percent.
While every board is stamped before it leaves the mill, it’s important to note that cross-cutting graded lumber into pieces that are shorter than the original piece is a very common practice in the light-frame construction industry. The ALSC system recognizes that whether the lumber is cross-cut on a jobsite or in a manufacturing facility, the shortened pieces are assumed to maintain the same grade and design properties as the original piece.
Since every board typically only has one grade stamp on it, the process of cross-cutting often produces one or more board sections that do not have a stamp. As a consequence, component manufacturers are encouraged to have appropriate quality control procedures that will allow the lumber grade used in each structural component to be traced and verified.
The next article in this series will explore the species groups under the ALSC system and how differences between those groups may impact design values and structural component design.