Both categories of timbers, Beams and Stringers, and Posts and Timbers, contain three stress grades: Select Structural, No.1, and No.2, and two non-stress grades (Standard and Utility). These grades and their uses are shown in the table below.
The stress grades are assigned design values for use as structural members. Non-stress grades have not been assigned design values.
No.1 or No.2 are the most common grades specified for structural purposes. No.1 may contain varying amounts of Select Structural, depending on the manufacturer. Unlike Canadian dimension lumber, there is a difference between design values for No.l and No.2 grades for timbers.
Select Structural is specified when the highest quality appearance and strength are desired.
The Standard and Utility grades have not been assigned design values. Timbers of these grades are permitted for use in specific applications of building codes where high strength is not important, as in the case of deck uses, blocking or short bracing.
Timbers are generally not grade marked, since they are surfaced rough and may be used in exposed locations. If needed, a mill certificate may be obtained to certify the grade.
Cross cutting can affect the grade of timber in the Beams and Stringers category because the allowable size of knot varies along the length of the piece (a larger knot is allowed near the ends than in the middle). Therefore, for this grade category, the timber must be regraded if it is cross cut.