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Envelope #20: How to quickly determine the allowable height for interior, non-bearing, metal stud partition?

Happy Thursday! Welcome to Back of the Envelope. This is email #20! My goal is to get to at least #50, then decide if I should continue. For now, let’s talk about some metal studs!

Here is a scenario that happened recently.

A client called up and asked:

“Hey Andy, so we have a 16’ interior non-bearing partition. Studs are 3-5/8” x 20 ga @ 16”o.c. with gypboard on both sides. It doesn’t meet the STC rating (sound transmission class), so we need to increase the spacing to 24”o.c. How can we make that happen?”

Hmm good question.

Let’s dissect that and come up with a solution in 2 minutes and 15 seconds!

What do we know?

1/ First, we know there are gypboard on both sides, so studs are considered fully braced.

2/ Interior non-bearing wall means we must design it for at least 5 psf lateral load.

3/ 3-5/8” x 20ga non-bearing stud means the member designation is 362S125-33. (for more info, see SFIA documents)

4/ The finishes are brittle, so the deflection criteria is L/240.

What are the constraints?

1/ Floor plan is pretty set-in-stone, so changing to 4” or 6” studs is not ideal.

2/ Using a thicker gauge is also not an option since steel transmits sound waves – a heavier gauge actually lowers the STC rating.

In other words, we need to increase the spacing but can’t increase the depth nor the gauge. Interesting challenge – how do we do that?

Oh, by increasing the flange width, you say? Smart. Thanks batman.

Let’s see.

Stud Check

362S125-33 @ 16” non-composite

First, I was curious to see how close we are with 362S125-33 @ 16”.

Pulling up SFIA table (equivalent-ish of SSMA) for “Interior Non-Structural Non-Composite”:

The maximum height is only 15’-5”… not quite close to the intended 16’. Perhaps the architect had specified the proper assembly to allow for composite?

Let’s see if that works.

362S125-33 @ 16”o.c. composite

Pulling up SFIA table for “Interior Non-Structural Composite”:

Alright, that’s better (assuming the correct screw, spacing, etc., are specified).

Now, how close are we using the same stud but at 24”?

362S125-33 @ 24”o.c. composite

Short by 9”. Let’s try 2” flange instead of 1.25”.

362S200-33 @ 24”o.c.

Since the size is not typically used for non-structural applications, it’s not listed on any of the “non-structural” tables. We have to use the Curtain Wall (Single Span) table instead.

Strangely, SFIA doesn’t list allowable height for 5 psf, so we have to open up the SSMA technical guide (maybe we should’ve done that since the beginning…):

That worked for 16’. And the height is based on non-composite so we are all golden.


Well, that was fun – looking up tables without running numbers is nice sometimes.

And what if I tell you, there is even a quicker way?

ClarkDietrich has a suite of free online tools to help you specify their proprietary studs – but part of it also includes a lookup filter for generic studs.

Check it out here: (link)

First you enter all the good stuff that we just talked about:

And the online app automatically gives you the sections that work!

Alrighty that’s all for now – thanks for reading. Let me know if you have any questions and I can perhaps cover them in future emails.

Have a good one!

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