Wood Building Floor Sheathing

[Guastavino]

All,

Forgive me as this question isn't 100% structural, but in some ways it is...

So, I'm doing a 3-story wood building (medical office). The architect wants to use 3/4" gypsum concrete over #15 asphalt felt over (2) layers of 5/8" of T&G plywood. Floor system is TJI's at 16" o/c max. The gypsum concrete and extra plywood layer is for noise control. But in thinking through this, I'm wondering where their detail came from. I asked them about it and they didn't really give a reason, just said it was for noise control. I pointed out that the APA recommends the below pictured system. They didn't really comment back. I'm curious if others have used a system like they want with the (2) layers of sheathing. It seems to me like the APA detail is better, and possibly cheaper (more gyp-crete, less sheathing). And if it was me, and an independent trade organization recommended something, I'd almost always go with it. That way, you have ground to stand on if any complaints arise. Just point out that you went with the industry standard. (This of course assumes the industry standard makes sense and works in the particular application).

Any thoughts on their detail of 3/4" gypsum concrete over #15 asphalt felt over (2) layers of 5/8" of T&G plywood as opposed to the below? I don't want to confront the architect on this without good reason. As an aside, structurally I'm only going to count one layer of sheathing for the diaphragm.

 

[mike20793]

On my multistory wood buildings I usually see the sheathing and then a layer of sound mat and the poured gypcrete on top. This is pretty much a standard detail in my area. They probably pulled it from a listed UL assembly.

 

[Guastavino]

Thanks Mike. So you see (1) layer of sheathing? The arch. I'm working with has (2), and I'm curious what inspired it.

 

[Triangled]

Agree that this was pulled fro a UL Listing or similar as the STC and IIC are identified which is generally from testing as I understand it.
The way I would compare these to alternative systems is by comparing the STC and the IIC, STC for sound transmission and IIC for impact insulation noise. The STC and especially the IIC seem fairly high.
1. What is the STC and IIC requirement for the building?
2. What alternative systems provide a sufficient IIC and STC rating?
I used to have a dark green 8.5x11 160+ pages or so manual identifying IIC and STC ratings for various assemblies.....Catalog of STC and IIC Ratings for Wall and Floor/ceiling Assemblies, I'll see if I can find it.
something like this but larger, but I am sure there are others...Link

 

[jayrod12]

I've never heard of 2 layers of sheathing, seems like a waste of sheathing. I don't see the additional layer providing any real quantifiable sound dampening.

Around here the typical is 3/4" ply (single layer) with 3/4" gypcrete topping. I believe it has a UL/ULC rating.

 

[XR250]

I have seen this with 1/2" sheathing and the gypcrete cracked to pieces. I would use a minimum of 3/4" to prevent localized subfloor deflection from cracking the gypcrete.

 

[Tmoose]

Does the architect plan to use resilient channel / furring to attach the ceiling below to the floor joists ?

That is a pretty important part of the system.

 

[Guastavino]

I don't think this is a UL system. Here is architect response:

"This is a Type V-B building with 0 hour required for floor construction. The ¾” gyp. Crete is for noise control."

Thus I'm puzzled

 

[mike20793]

I agree with XR250. You should ask him to use 3/4" tongue and groove sheathing. It's pretty standard in my area because of the localized deflection. I guess you could always spec a higher span rating if you were worried about it, though.

 

[Triangled]

This is typically handled by selecting tested systems with certain level STC and IIC ratings appropriate to the final usage, although certainly no doubt, many assemblies are designed based upon experience. But this is his to determine, I would suspect, while you simply need to make the structure work.

Double plywood floor is an element in one assembly in the link above, the intent, as I believe is stated there, being to achieve a net thickness of wood, which likely was what the test apparatus had. I can readily understand an additional thickness of wood abating sound, particularly sounds from impacts like dropping something on the floor, which is considered in the IIC rating, maybe little to very little effect on the STC rating, but, hey, I'm not an acoustics guy. And, I like you, would use just a single layer structurally as you suggested, and consider the other layer as just an additional sound insulation blanket, like carpet...

RE 3/4" gypcrete, I am sure everyone has their own experience and anecdotal evidence...in my neck of the woods, 3/4" gypcrete got run out of town years ago in favor of 1.5" light weight concrete, but, whatever, I'd likely let the architect have 3/4" gyp crete if that's what he wants....and, if there was a specific noise rated assembly the owner required, I'd follow that exactly.. 3/4" GC or 1.5" LWC - whatever.

By the way, the APA system you showed seems to indicate solid sawn joists, whereas you indicate your system will utilize I-joists. In the world of finger-pointing when the floor is too loud, the testers of these assemblies may not back you if your argument is that I-Joists and solid sawn joists are interchangeable.... perhaps they are, but I'd get that in writing if I was making decisions for a floor assembly that was required to meet a specific IIC and STC rating.

 

[Guastavino]

@Triangled,

The reason I'm puzzled is because, as I said in the OP, I'm wondering where the detail came from. It is NOT a UL assembly, or if it is, it is by coincidence. I think it's possible the architect arbitrarily picked this system because they thought it might be a good system. I don't think they have any backup for it, because when I have asked about it, they didn't really give an answer. I am going to ask them again, and then if I don't get a response, might just let it slide realizing there is a paper trail for me advocating for researching the system.

I don't get the impression from this architect that they get into the details as much as I would like. Meanwhile, I am a detail person. I also know who's going to get the call if the floor system doesn't work out in terms of noise, etc., and frankly, I don't want that call. And if I get that call, I want a reason that I was OK with the system. Telling someone that (2) layers of 5/8" plywood and 3/4" gyp-crete sounded like it would work is NOT a good reason. If I tell them, it's what the architect wanted, and I recommended they look at other stuff, they chose not to, then I have at least a reason.

I know you may say, well it was the architect that picked it. Well, first, that's a cop-out to me. Second, everyone gets dragged into it, and I want to at least be able to say I questioned it.

 

[boo1]

I have used double layers plywood for stone and large tiles
- first layer is 3/4 Sturdifloor T&G plywood, glued and screwed to joists
- second layer is 1/2" CC grade plywood, not T&G, screwed to subfloor, not joists, with 1/8" gaps.
- third layer (we use backerboard or Ditra).

http://www.schluter.com/5138.aspx

With the lightweight concrete; single layer plywood with thicker topping would perform well

 

[Guastavino]

Thanks again.

And just to cap this thread off. Architect said that they have successfully used this on several recent projects. Not sure what inspired it originally, but at least I know it's been done before. And, if for some reason there is an issue at a later date, I can at least say I've raised a question. I definitely get into the weeds too much some times...

 

[Guastavino]

UPDATE!

Gypsum folks said NO ASPHALT PAPER unless you have 1.5" of gypsum concrete because imagine a 3/4" floating gypsum floor. They want to bond directly to the sheathing. More research to come, but I'm pushing back on the architect...hopefully I survive this...luckily my contract isn't with the architect in this case...

 

[MileyDiley]

Standard architect. No reason for what he's doing and it comes down to the manufacturer/installer to make him change the detail. Nothing better than coordinating some slightly odd framing scheme to accommodate his assembly and then everything going "back to normal" once the contractors get involved, despite how much you question him before it ever gets submitted.