dead-load sanity check, please!

Ok, so I'm trying to figure out how much drywall I can add to my subfloor before building out the studio room below. I plan to build the ceiling of the studio to be supported by the studio walls themselves, so completely decoupled from the upper floors - so this is strictly to add mass to the upper leaf. Pictures of the joists in question can be seen here: viewtopic.php?f=1&t=11551 The floor joists used in my house are these http://www.gp.com/build/product.aspx?pid=1390 There seems to be a mix of GPI-40/14 and some GPI-60/14 (which seem to have been replaced with the current GPI-65) depending on where the trusses are. They are installed 19.2"OC. Based on Georgia-Pacific's Product Literature (here: http://www.gp.com/build/DocumentViewer. ... entid=4372) - the smaller of the 2 sizes, the GPI40, seems to be rated at up to a 19'10" span with a dead load of 20psf (and over 21' with a 10psf dead load). So, did I read that right? My span is almost exactly 19' (I'll triple check) - and I have hardwood floor above (should be somewhere around 7.5psf including subfloor, right?).. so if I'm reading this all correctly, I should be FINE with 2 layers of drywall added to the subfloor. Just looking for a sanity check here to make sure I didn't miss something (and don't want my floor to collapse). "-)

One other thing.. If I'm right - and I can easily put 2 layers of drywall under my subfloor (possibly 3) - the next question is.. how exactly can I do it? The diagrams and description of how to add mass to a subfloor from Rod's book are perfectly clear - but they describe it in the context of a standard 2x10 floor joist. My house has wood I-beam style joists, which seem to support more weight than a normal joist, but they have a stringer at the top and bottom of them that will make it a little trickier to cleat in layers of subfloor drywall. Picture attached below. THoughts?

By the design, these engineered joists should not be loaded in an effort to mass the underside of a subfloor. The OSB webbing, spacing, top and bottom flange, it's all part of the design to spec out the loads the company recommends. On page 16 of the brochure you linked above it shows a typical acoustic ceiling using the type/kind of truss you have. It is also a common example of what has been recommended around here on occasion. As with most construction related components, everything matters. Depending on if your area is just a simple span(as explained in the pdf file) or uses multiple spans, these things have to be taken into consideration. The deflection of the floor system is a high consideration and is just another part of the concern. It is not recommended to have lower level walls abutt this kind of engineered joist. So the brochure reads that at least, RC and 5/8 inch sheetrock can be used on the underside. Outside of that, maybe it would be better to get in touch with GP. Brien

Thanks, xSpace.. I saw that reference in the GP literature as well - but really was thinking that completely decoupling the studio rooms from the house above would be ideal. I honestly hadn't thought that it would matter whether loading the joists completely from the top vs. top/bottom. I'll put in a call to GP this week to verify - but I guess I may also have to re-think it. Take a look at my other thread, if you don't mind viewtopic.php?f=1&t=11741 See how the front of the room slopes down? How would I do that if I'm hanging the ceiling from RC on the joists? "-) PS - which exit 4? 20 or 65?

By the design, these engineered joists should not be loaded in an effort to mass the underside of a subfloor. The OSB webbing, spacing, top and bottom flange, it's all part of the design to spec out the loads the company recommends.

OK - I just got off the phone with GP, and they said that adding a couple of layers to the bottom of the subfloor is 'absolutely no problem'.. he actually said 'not only will it help with sound, it'll make your floor upstairs feel more solid'. I explained what I was planning - and he basically said that I can either add mass to the subfloor, or use RC and hang drywall from the bottom of the joists - either is fine, and would likely be well under spec. So... the next question - how on earth do I cleat in drywall around those stringers?

Did he say either, or... or did you confirm both? If it is a matter of one option, then the ceiling on the bottom side of the wood I joist would be better overall with the air provided by that install. If both places are acceptable then you would want to do the math on what the distance leftover is from the bottom of the new material you will be using, to the bottomside of the top 2X4 cord. Once you know that measurement, then you will have to get a piece of lumber of sufficient size and cut a portion out using a table saw, to build an "L" shaped board(running the length of the entire new board) that you can screw into the bottom-most side of this top cord. Or, as an alternative, using the same measurement you could notch both ends of a 2X4, cut to fit the distance from 1/8" shy of the plywood webbing, and allow it to press up against the new mass, to be screwed off to the underside of the 2X4 top cord. I can't draw pictures conveniently, this is about as good as it gets short of me driving over there and doing it for ya ;) Brien

Sorry if I wasn't real clear, Brien.. appreciate the patience. My plan (ideally) is to NOT hang any drywall from the bottom of the joists.. but to have the ceiling of the studio be completely decoupled (supported from the studio walls). That would make the existing subfloor one leaf, and the new ceiling the other leaf (acting like a normal double-wall construction). This whole exercise was to see if there was an issue adding mass to the subfloor, rather than RC/drywall on the bottom of the joists. The guy from GP said that the joists can handle a few more layers of drywall, and it doesn't matter if I put them up against the subfloor or on the bottom (as far as they're concerned - load is load, doesn't matter if it's top or bottom-loaded)... so I guess that would be an OR. "-)

Sounds good, Was my description clear enough to make a notched cleat or have you designed something far easier already :)

Your description was perfectly clear - and pretty much what I was already thinking. Only issue is - I'd prefer to make it so that it screws sideways into the i-beam, rather than up into the stringer.. I need to give this some thought.. the guy from GP didn't have a good answer either. "-)

The only thing that will prevent you from using something like a 1X4 ripped down is going to be, how much of the material you want to screw(or nail with a finish gun) into will be available after installing the added mass. That would be your only stopper. Oh, that's I-10, Exit 4

xSpace wrote:

Oh, that's I-10, Exit 4

aah, so you're in LA.. I made it down that way a few times. "-)

Hate to resurrect a thread from the dead.. but there was another thread talking about filling gaps between subfloor joists with closed-cell foam (to cover up nails/screws/staples sticking through) - then adding the drywall to it. That got me to thinking that I could use that tactic to kill 2 birds - it would make it easier to cleat in the drywall, and cover up some random screws/nails/staples sticking through (I looked, and I have a bunch). SO... here's 2 drawings, with 2 options.. only real difference for me is the additional cost of the closed-cell foam (which would total about $400 to do this whole area -so not a budget-breaker, but not insignificant either). Let me know what you think. -mb

I'd go for option 2. Direct attachment of GB on the existing floor/ceiling.

Thats what I was originally planning.. but after seeing the thread on stray nails & screws poking through, I looked and noticed that I have the same problem. Here's the thread I'm referring to: viewtopic.php?f=2&t=8399&hilit=foam