# STC Ratings



## anne t (Nov 7, 2016)

I need to maintain the 1 hour rated wall and an STC of 50 between living units (a party wall).  I have searched and the UL systems and GA systems do not indicate the plywood needed for shear in their systems. I found that UL allows for the plywood to be added into the tested assembly and maintain the rating, BUT I am looking for something that allows for the added plywood and how it affects the STC Rating? Is it allowed or is there some testing agency that clearly states that it helps or it reduces to the STC Rating? It is a plan check correction.


----------



## cda (Nov 7, 2016)

Well just welcome!!!

Which state are you in??

Sometimes it drives the answer


----------



## anne t (Nov 7, 2016)

Thank You. I am in California.


----------



## cda (Nov 7, 2016)

The land of CBC!!

There are a few Califorians on the site. Give it  day or two for replies


----------



## steveray (Nov 7, 2016)

Like this?

http://lpcorp.com/media/3579/15-oflb-0060-m1rb-flameblock-bldr-assembly-brochure-web.pdf

The important question is "Where is the plywood located?" Or what assembly did you modify?


----------



## steveray (Nov 7, 2016)

http://www.woodworks.org/wp-content/uploads/Acoustics_Solutions_Paper.pdf

That is a good one too....It matters where you put stuff...


----------



## north star (Nov 7, 2016)

*@ = @*


anne t,

Do you have to use plywood ?......Some of the approved
Shear Assemblies can also use gypsum board, or other
methods and components to brace a wall line.

I have found the folks at Simpson-Strongtie to be most
helpful in analyzing various applications, and then
[ naturally  ] offering one or some of their products to
meet the prescriptive design.


@ = @


----------



## mark handler (Nov 7, 2016)

In the gypsum association manual it states you can add it to any assembly without effecting the ratings.
They have changed the vebage a bit but see number 22in the GA 800 MANUAL


----------



## tmurray (Nov 8, 2016)

You can usually add stuff to wall and floors without affecting the STC rating as long as the required components are still present. Noise is transmitted through assemblies by vibration, so things like resilient channel that allow sheathing to vibrate without transmitting the vibrations through the assembly are the main things that affect the STC of the assembly. You just have to make sure the gypsum guy doenst screw through the channel into the framing member it's attached to.


----------



## anne t (Nov 9, 2016)

steveray said:


> Like this?
> 
> http://lpcorp.com/media/3579/15-oflb-0060-m1rb-flameblock-bldr-assembly-brochure-web.pdf
> 
> The important question is "Where is the plywood located?" Or what assembly did you modify?





steveray said:


> Like this?
> 
> http://lpcorp.com/media/3579/15-oflb-0060-m1rb-flameblock-bldr-assembly-brochure-web.pdf
> 
> The important question is "Where is the plywood located?" Or what assembly did you modify?


Thanks for the link. Maybe next time I will try this. The plywood is attached to the wood studs then the gyp. is attached over.


----------



## anne t (Nov 9, 2016)

steveray said:


> http://www.woodworks.org/wp-content/uploads/Acoustics_Solutions_Paper.pdf
> 
> That is a good one too....It matters where you put stuff...


Thanks.. I have this one. The wall figure 3 is what I am using. People are saying it should be okay to add the plywood  because I am adding mass to the wall but I was hoping for some more definitive answer. I can not be the only person to come across this. Many walls are shear walls.


----------



## RLGA (Dec 1, 2016)

Adding layers increases mass, thus, reducing sound transmission. Therefore, adding plywood to an assembly (permitted by both UL and GA 600) will increase the mass of the wall and thereby increase the STC rating, albeit marginally.


----------



## Yikes (Dec 1, 2016)

A few years ago, I reviewed this issue with a well-known acoustical engineer in Southern California.  For a typical STC 50 assembly, layer he recommended as follows:
1.  5/8" gyp board
2.  1/2" plywood shear
3.  wood studs and batt insulation
4.  1/2" resilient channel
5.  5/8" gyp board​The reason the plywood is placed on the _opposite_ of the resilient channel side is so that the 1/2" air cavity doesn't function like a drum head with the gyp board, amplifying the sound.  The building codes are silent on this issue, but if you have the choice of which side of the stud to put the plywood on, put it on the non-RC side when you can


----------



## anne t (Dec 2, 2016)

Thank everyone for the information. We will be meeting with the Plan Check Engineer next week. We have a lot of information and hope that they will be reasonable as to the theory of how sound works. There is no approved test for our exact situation and that was the initial request.


----------

