Can you make substitutions of different lumber grades and species? The short answer is: yes. Even so, you have to take several factors into consideration when making a substitution.
To narrow the discussion, let’s focus on southern pine. The substitution must have design values equal to or greater than those of the specified lumber for each of the seven components that make up a species’ design values. These seven components are:
- Extreme Fiber in Bending Fb
- Tension Parallel to Grain Ft
- Horizontal Shear Fv
- Compression Perpendicular to Grain FcꞱ
- Compression Parallel to Grain Fc||
- Modulus of Elasticity E and Emin
- Specific Gravity G
The Southern Forest Products Association (SFPA) publishes the southern pine design values in the Southern Pine User Guide available in PDF on its website as a free download (see excerpt below).
What about using Stud Grade?
Stud grade is a composite of No.3 strength and No.1 edge characteristics for a better nailing surface. Note the highlighted item in the excerpt from the SFPA design values table [for image See PDF or View in Full Issue]. Because No.3 and Stud grade have the same design values in southern pine, they can be substituted as long as the gradestamp does not indicate “Stud Use Only” or “Vertical Use Only.” (This is permissible per ANSI/TPI 1–2014 Section 3.4.6.)
What about Construction Grade?
Can I substitute Construction when No.3 southern pine has been specified? Yes, because all of the Construction grade design value components are greater than or equal to those of No.3 including the specific gravity for this species.
The best way for a truss designer to determine this would be to input the lumber into the truss design software and run it through the engineering program. These values are available from your plate supplier and are usually enabled by selecting what inventory is available for design.
ANSI/TPI 1–2014 doesn’t publish lumber design values, but it recognizes other standards that provide this information. An important reference for this discussion is Section 6.3.1. Design Values for Solid Sawn Lumber and its commentary. Published in a technical bulletin originally issued in May 1994, it has been revised and approved by the board in June 2008. It states the following:
The TPI Technical Advisory Committee (TAC) recommends that the minimum grade of visually stress-rated lumber used for truss chords should be No. 2 grade. This recommendation does not apply to small trusses up to eight feet in span (such as jacks and mansard frames), trusses and truss frames that have support at intervals of 4 ft or less such as valley fills and gable end frames, or trusses used in manufactured housing (where the truss structure is sheathed prior to site erection). This is a minimum recommendation, and it may be necessary to supply higher grade material on any type of truss to meet the truss structural requirements or the end user’s quality requirements.
So, in other words, as long as the lumber is not being used as a chord in a structural truss, other grades of lumber can be substituted provided they have the necessary design values.
An ANSI/TPI 1 3rd Party Quality Assurance Authorized Agent covering the Southeastern United States, Glenn Traylor is an independent consultant with almost four decades of experience in the structural building components industry. Glenn serves as a trainer-evaluator-auditor covering sales, design, PM, QA, customer service, and production elements of the truss industry. He also provides project management specifically pertaining to structural building components, including on-site inspections and ANSI/TPI 1 compliance assessments. Glenn provides new plant and retrofit designs, equipment evaluations, ROI, capacity analysis, and CPM analysis