Stud Sizes and Spacing in Rated Wall Assemblies
I recently worked on a project that utilized a number of different rated wall assemblies to separate sleeping rooms in an assisted living facility. Most of the walls were going to use 2×4 wood studs, but some locations needed to use 2×6 studs to fit plumbing. The wall assembly that I chose to meet the separation requirement was UL U305 which basically sandwiches 2×4 wall studs at 16” o.c. with layers of 5/8” gypsum board.
Several weeks after submitting the permit set, I received the following comment: “Provide documentation to verify where the assemblies show 2X6 studs can be used in lieu of 2X4 studs.” I had never seen this comment before, but the UL U305 assembly (and most other assemblies that I could find that would provide the separation) used 2×4 studs. I wasn’t sure how I was going to address the comment until I found the following information buried in the “Q&A” section of the UL website:
“Can a larger wood or metal stud be used than what is specified for a specific UL wall assembly?
Wood and/or metal studs specified in UL Rated Designs are minimum sizes. Larger dimensional studs can be substituted.”
So the answer to this question is “yes,” using larger wood or metal studs than what is shown in an assembly is permissible and the sizes shown are a minimum. It should be noted that this doesn’t mean that you can exchange wood studs for metal studs as that would change the materials in the assembly.
The next question that sometimes comes up (and did on this project) is in regard to stud spacing in the rated assemblies. Often an assembly such as UL U305 will call out something similar to “studs spaced at 16” o.c. max.” This means that the studs can be placed closer than 16 inches on center within the assembly, but they can’t be spaced any further apart than 16 inches on center. I received an RFI from the contractor asking if the studs could be placed at 24” o.c. which would not have been permissible for the assembly.
If you would like to learn more about stud sizes and spacing, please contact us and we will happily assist you with your commercial or residential project.