Pay Attention to the Grade Stamp

Question

Our company recently received a shipment of lumber in which one of the units contained pieces marked with the grade stamp shown (see Figure 1). This is the first time I’ve seen a grade stamp with the size of the lumber included in it. Can we use this lumber in our trusses?

Answer

This grade stamp identifies the dimensions to which the lumber was dressed (i.e., surfaced) and the moisture content classification at the time the surfacing was performed. The dimensions are included because the dressed size is less than the minimum size requirements established by the grading rules for 2x4 dimension lumber.

The vast majority of sawn lumber used for structural applications in the United States is produced in accordance with the U.S. Department of Commerce Voluntary Product Standard PS 20 (2010), American Softwood Lumber Standard (ALSC). PS 20 establishes standard sizes and requirements for lumber grades of the various species, the assignment of design values, and the preparation of grading rules applicable to each species.

Table 3 of PS 20-10 lists the nominal and minimum-dressed sizes for boards, dimension lumber and timbers.  The portion of Table 3 that includes the sizes for dimension lumber is reproduced in Table 1 below.

PS 20 defines dry lumber as:

Likewise, PS 20 defines green lumber as:

Table 1 indicates that the dressed sizes for dimension lumber that is surfaced when green are greater than the dressed sizes for lumber surfaced when dry. This is due to the fact that wood shrinks as it dries (i.e., moisture content decreases). The greater the reduction in moisture content, the greater the expected shrinkage. 

Section 7.3.1 of PS 20 includes the following provision for grade-marked lumber:

A portion of the 2x4 lumber you received was apparently surfaced to 1-1/2" by 3-1/2" while still green, as opposed to being surfaced to 1-9/16" by 3-9/16", as required in PS 20. Since this dressed size is less than the minimum required dressed size, the grading agency included the size in the grade stamp as required in Section 7.3.1 of the standard. It is incumbent on the consumer to decide whether or not it is acceptable to use this lumber. In other words, the buyer must beware if there is a downstream design issue and this grade stamp was not accounted for in the design.

The thickness and width of this specially marked lumber is 1/16" less than the minimum dressed-size permitted in PS 20. In addition to possible plate pressing problems, if this material is used with standard size lumber, the potential effect on structural properties could also be significant. The reduced dimensions of this material result in an approximate 6 percent reduction in cross-sectional area, an 11 percent reduction in flatwise section modulus, and a 16 percent reduction in flatwise moment of inertia. Further, Section 6.3 of TPI 1-07 requires that:

Therefore, if this lumber is used in the manufacture of metal plate connected wood trusses, it will need to be added to the lumber directory in the design program as a specialty product with reduced dimensions.