How Lumber is Assigned a Grade in the U.S.

Industry News,

Originally Published by: SBCA Magazine — January 23, 2024
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In the U.S., six grading agencies have jurisdiction over establishing softwood lumber design values. Each of these grading agencies follow grading rules approved by the American Lumber Standard Committee’s (ALSC) Board of Review (BOR) and are certified for conformance with the American Softwood Lumber Standard PS 20. Within those grading rules are guidelines for determining the grade of each individual board of lumber prior to it leaving a lumber manufacturing mill. 

There are various ways to determine the grade of a board of lumber, but the most common is referred to as visual grading. Traditionally, this is performed by one or more qualified people in the mill who look at all four sides of a piece of lumber and apply grading rule criteria and evaluate the characteristics they see to determine to which grade the piece belongs. These are highly trained individuals who do this process very quickly. 

Because wood is an organic substance and no two trees grow exactly the same, nor are their trunks cut in exactly the same configuration, visual grading rules define the size and frequency of different characteristics that are permitted for each grade. The characteristics may affect the structural strength of the lumber, things like slope of grain, holes, and knots, while others may affect their appearance or ability to withstand long term structural use (sometimes called serviceability), such as wane, warp, or skipped dressing (rough surface). Below is an illustration of the most common appearance and strength-reducing characteristics that visual graders look for: 

Depending on the species group of lumber, there are different visual lumber grades. For structural light framing, which is what is used in today’s structural building components, visual grades typically include (but are not limited to): 

  • Select Structural 
  • No. 1 and Better 
  • No. 1 
  • No. 2 
  • No. 3 

This photo illustrates how these visual grading criteria will typically impact the grading of lumber into the different visual grades: 

In modern mills, it is not uncommon for companies to employ machines to conduct visual grading. Using a system of lasers and camera scanners, visual grading criteria can be applied through the use of software and these digital scanning techniques. This approach allows lumber manufacturing mills to assign visual grades to lumber without a human looking directly at a specific piece of lumber. This can significantly increase the accuracy and efficiency of applying visual grading criteria to manufactured lumber. 

Since the 1960’s, another common way for structural lumber to be graded is with a machine. The ALSC has developed criteria for how machines can evaluate one or more specific characteristics of lumber related to its strength and stiffness. There are several technologies today that are employed by these machines to produce machine stress rated (MSR) or machine evaluated lumber (MEL), but the most significant difference is that a machine nondestructively tests each board during the manufacturing process to determine its physical properties.  

MSR grades are designated by an “fb-E” rating. Some common MSR grades and corresponding allowable design values can be found in the table below, along with their corresponding design values. It should be noted that while the values in the table below remain the same regardless of the species or size, other properties, such as shear parallel to grain or compression perpendicular to grain, may vary by species: 

Grade Designation   Bending (F )(psi)   Tension Parallel to grain (F )(psi)   Compression parallel to grain (F )(psi)   Modulus of Elasticity (E)(psi) 
1650f-1.5E  1650  1020 1700  1,500,000 
1800f-1.6E  1800  1175  1750  1,600,000 
2100f-1.8E  2100  1575  1875  1,800,000 
2400f-2.0E  2400  1925  1975  2,000,000 

Similarly, each piece of MEL is subjected to nondestructive testing and analyzed by a machine and then separated into grades. The biggest difference is that MEL requires quality control tests for both bending strength and stiffness as well as tension strength.  

Once a piece of lumber is assigned a grade, a grade stamp is applied to its surface. The next article in this series will explore the information contained in the grade stamp and discuss how component manufacturers can apply that information in their manufacturing process.