There are three grading methods for sorting Southern Pine dimension lumber and assigning design values:
Visually graded lumber
Machine Stress Rated (MSR) lumber
Machine Evaluated Lumber (MEL)
Visually graded lumber is the oldest and most common of the three methods. Visual grading is performed by qualified graders in the mill. These graders sort each piece of lumber into various grades based on visual characteristics known to affect lumber strength and stiffness, such as knot size and slope-of-grain. Consistent visual grading is achieved through proper training, education and supervision of the lumber graders. Visually graded lumber will adequately meet the structural requirements for most traditional applications.
Machine grading, which categorizes both MSR and MEL, reduces the variability associated with assigning stress grades to lumber. MSR and MEL can be advantageous, therefore, in more demanding engineered applications, such as trusses or long-span joists and rafters.
Machine Stress Rated (MSR) lumber is evaluated by mechanical stress rating equipment. MSR lumber is distinguished from visually graded lumber in that each piece is nondestructively tested and then sorted into bending strength and stiffness classes. In addition, each piece must meet certain visual requirements before it can be assigned design values. MSR also requires daily quality control tests for bending strength and stiffness.
Machine Evaluated Lumber (MEL) is similar to MSR in that each piece is evaluated by nondestructive grading equipment, checked for visual requirements and then sorted into various strength classifications. MEL requires daily quality control tests for tension strength in addition to the daily bending strength and stiffness tests required for MSR.
Grade marks for MSR and MEL can include “1W” or “2W” if visually graded to the wane restrictions for No.1 or No.2 visual grades, respectively.
The Standard Grading Rules for Southern Pine Lumber provide for numerous visual, MSR and MEL grades. However, not all of those possible grade/size combinations are produced or used in the marketplace. Available grades and sizes are subject to change, so check sources of supply at the time of your project.