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OSB Span for Ceiling + Drywall

9K views 27 replies 6 participants last post by  DeeBunker 
#1 ·
I know a lot of people are using OSB as a first layer in room construction, then drywall on that. It gives you double layers for soundproofing plus a continuous screw/nail surface for applying the drywall and hanging anything else from the surfaces later on.

I'm wondering about the span rating on OSB when used for the ceiling. Let's assume 5/8 drywall will go up after the OSB. Right now I'm working in a 12x16' basement room and I've got 7/8" hat channel on clips running the 12' dimension, 9 in total about 21" spacing. I'm set on using OSB for my first layer as it'll solve a number of design issues but unsure if I should go 5/8 or step up to 3/4" OSB. I'd hate to ever have any noticeable sag years down the road. It's a 7ft ceiling in a long room so a 6ft tall person would easily notice if the ceiling wasn't flat (being able to see down the plane of the ceiling).

Are you guys using standard drywall screws to hang the OSB on steel channel? In that case do you countersink first? Drywall screw heads often don't want to sink flush with the OSB surface - either the head could snap or I imagine the hat channel would strip out.

Thanks for thoughts!
 
#2 ·
I would use 5/8 OSB. 3/4" is heavy and i'm not sure if the you have calculated the clips/channel spacing for the extra weight.

Although the OSB gives you a good base to screw anywhere, I made sure I hit the channel for the second layer drywall too. Can't imagine not doing it this way as the ceiling would only be as strong as the first layer screws that gripped the channel. I used the fine thread drywall screws and paced my impact drill to ensure the screws are flush enough. I had few screws that snapped or stripped the channel.

once the second layer DW went in there was no obvious unevenness that resulted from the screw heads. also, the green glue between is dense
 
#3 · (Edited)
Find the Grabber brand drywall screws in your area intended for metal framing. I would use the 5/8 OSB. But I only use OSB if I'm convinced that I need to. Making it easier to install drywall is not a legitimate reason to bother with it in my book. I will use only one full sheet in the projector location

What else are you planning to hang that requires the OSB? One word of warning there is a clear and present danger of following what other people do in this forum if you don't know the reason and have evaluated the alternatives. Vender instructions should supersede someones blog of their project.

Putting the Chanel's every 21 inches was a mistake. If you did them according to directions they would be every 24 or 16 inches. The later if for three layers. It makes hanging 8 ft long sheets less complicated to hit the edges.

You want or screw into the Channel on both layers.
 
#4 ·
I used 3/4" tongue & grove OSB for my first layer (actually 32/32") The OSB will make it a lot easier to attach soffits. I wanted the extra mass compared to 5/8" OSB. I had to countersink holes for the screws that went in on the first layer, otherwise they would not drive flush. It was quick and easy. I had one drill with a countersink bit and another for driving the screws. I would do a quick "touch" of the countersink bit along a row where I wanted screws, it only took a second or so per hole. I'd put down that drill, grab the other one and then drive the screws in to the existing countersunk holes.


For screwing into the channel, you want fine thread screws that are designed for screwing drywall into metal studs. Most of them have standard pointed ends, and it takes a little pressure to get them to poke through the steel hat channel. I found some with self drilling ends work much better, they go in quickly and easily. They are a little harder to find, but my local Lowes had them.


I did screw the second layer of drywall into the channel as well, they sink in to the proper depth with no problem.


https://www.lowes.com/pd/Pro-Twist-...e-Head-Black-Phosphate-Drywall-Screws/3295194
 
#5 · (Edited)
The reasons I want osb:

1) I'll be installing ~13 4" can lights in my 12x16 room. I hate the little legs that hold the cans in and they suck even more on a 1-1/4" thick substrate. Also the lip of the can that rests on the drywall surface is bumping out my LED retrofit bulb trims, leaving a little gap between trim and ceiling surface. I'm going to cut the lips off the cans, throw out the retaining legs, and mount them in my ceiling with 3 screws from the inside of the can to the edge of the osb. Much better.

2) Little bit nicer mounting hvac register in the ceiling, screwing from inside the boot to the osb edge, then the register itself screws right into the osb through the drywall.

3) I need access in several spots for utilities. I plan to band the entire perimeter of the room with some type of wide crown molding (maybe a 7.5" baseboard) to cover my access holes and still look decent AND I won't have to tape/finish the inside corners. The osb backing will give me something to screw these to. If at any point I need to pull more wires I can cut all the holes I want under the molding and put it back without having to do a lot of patching work.

4) Easier drywall install - don't have to land edges perfectly and I can put more screws along the seems.

5) Easier repair - if something ever does have to be cut open, you cut a bigger hole of drywall and a smaller hole of osb - then you've already got something to screw your patch piece into.


The reason for the ~21" spacing: I'm really pressed on ceiling height so I put my channels between the joists and slightly below. ~21" is the furthest I could space them out without exceeding 24". My room is slightly under 16ft long so I'm cutting something to length to get it to land on the channel no matter what. At least the drywall layer doesn't need that consideration and yes I do plan on screwing the drywall into the channel as well. I guess it's worth mentioning that I did 48" spacing on the clips but I did not stagger from one channel to the next.

I guess if there's no danger of long-term sagging then I'd be fine with 5/8 osb. I don't know if there's much to be gained in terms of soundproofing with 3/4"
 
#11 ·
I would not trust the ones in the first two pictures that are only attached on one side. They already moved with no weight hanging from them. Drywall is heavy, and with the blocking installed the way it is, the weight will act on them like a lever and they will sag over time, possibly to the point of failing and breaking off completely. It is quite likely that the wood may split in half where the screws go through. Another thing that might happen is that the joists way twist over time, with the bracket side moving downward.


It is a mistake not to use clips at the ends of the channels. You do not want the channel to be "firmly attached", you want the ceiling structure to "float" inside the clips. They will not be effective unless they are used as they were designed to be used.
 
#17 ·
I agree with all of your points about the wood blocks I made to clear the ducting. As I said I wasn't too pleased with them after the fact. I plan to go back and glue them. I could also glue some 1/2" ply to the bottom face of the block and move it up 1/2" which would fix the splitting issue. Or I'll just make all new blocks out of double or triple layered/glued 3/4" plywood. I don't think joist twist is a big issue since I don't think joists will twist so severely that the flex in the hat channel couldn't deal with it.

I don't agree on the soundproofing point. I suspect a firm attachment to the wall (which doesn't touch the joists or foundation wall) is better then a clip attachment to the joists. Room within a room. As I mentioned these wall blocks also end up being held up by the drywall that gets attached to the walls (I always drywall with the 'walls hold up the ceiling' philosophy). What do you think of this?

In my basement I have 2x8 joists, so 7.25". 1.25" is eaten up by the double drywall against the subfloor. That leaves 6" for ducting within the bay and if I put solid blocking across it really squashes my flex duct. My joists average 85" from the subfloor so I'm already challenged to maintain a 7ft finishing ceiling. I'm fighting for every quarter inch.

Further comments appreciated.
 
#13 ·
Interesting you added blocking there. In the installation guide for the RSIC-1 clips I'm using (link to guide), it says to hang them perpendicular to the joists, which is how I plan to do it in my build. Any reason you decided not to do it this way? You mention having them at the end for height, but you could just make a mark on the wood instead for reference instead. It seems like the blocking would definitely be less safe/structurally sound than the existing joists.
 
#16 ·
If you have tall ceilings, at least 8' or 9', an extra inch or two won't be noticeable. If you have lower ceilings, which can be 7-1/2', 7' or even lower in some cases, every inch is important! Speaking from my experience, almost everything I have done in my theater has been "extra work". I really only have once chance to build it right and the extra work is worth it!
 
#19 · (Edited)
This is what I would do for clip hangers where I could only secure them to a joist on one side.



You say that your walls are fully decoupled from the rest of the structure, and only in contact with the slab floor. Are they secured to the rest of the structure in some way, such as IB-3 brackets? http://www.soundproofingcompany.com/product/ib-3-bracket/ What holds them in place? If your plan is truly for the walls to be secured to the ceiling via the drywall, and visa-versa, I would come up with a different plan. Drywall and taped joints do not make for structurally strong connections.
 

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#23 ·
This is what I would do for clip hangers where I could only secure them to a joist on one side.

You say that your walls are fully decoupled from the rest of the structure, and only in contact with the slab floor. Are they secured to the rest of the structure in some way, such as IB-3 brackets? http://www.soundproofingcompany.com/product/ib-3-bracket/ What holds them in place? If your plan is truly for the walls to be secured to the ceiling via the drywall, and visa-versa, I would come up with a different plan. Drywall and taped joints do not make for structurally strong connections.
Thanks Dave. Yes, your blocking is better, I should have done it more like that. Keep in mind though that one 2x won't work as the clips are too wide. You'll need double 2x. I may switch out all of those duct bay blocks completely with something more robust.

I'm hesitant to redo all of my top plate blocks. I think gluing gussets to the sides of what I have and to the top plate could make them really strong and it'd be a lot less work. What I'm trying to describe to you is that it's impossible for a ceiling to sag or move down at the edges if the wallboard was installed last - it can't crush the wallboard downward. Also, the wall/ceiling is NOT connected via the drywall, it's connected via the metal channel.

I used a few IB-3 brackets to secure the walls to the joists. Fact is, after I put in the hat channel like I did, the opposing walls hold each other in place brilliantly. Any bow in the tops plates got taken out just like if I'd spanned with 2x6's. I'm not even going to tape/finish the ceiling wall joints - there will be molding there. I also wouldn't trust IB-3's to hold the walls still enough in place that the wall/ceiling joint wouldn't eventually crack. Wood bow over time is too strong for the IB-3's to hold back.
 
#22 ·
Heh, joists are not even close to straight. The house is 100 years old and they're plenty bowed. But 1/8" is worst case at the bow in the middle, and only on one side of the room. On the other side of the room the gap gets over an inch (house isn't level). I setup the whole ceiling with a laser level.
 
#24 ·
Clips are typically 1-1/2" x 3" when viewed from the top/bottom. That should fit on a the end of a single 2x4, which is 1-1/2 x 3-1/2". Attach the 2x4 to the joist with the narrow side against the joist. If you need the clips spaced further away from the joist, use a 2x6 or larger. Again, keeping the narrow side against the joist.


Your initial concern in this thread was having the drywall sag over a 24" span. I would be much more concerned about sagging due to the structural rigidity of the supports holding your clips. Assuming the maximum design load of 36 lbs per clip, I would make certain that whatever you use to support your channel, it can easily hold at least that much weight directly on every hanger over the long term with no deflection or movement.


I understand what you are saying about the standard practice of installing the ceiling drywall first, then the wall drywall. I'm just having a hard time understanding the system that you've developed. It certainly isn't what the manufacturer recommends. If you are confident that it will meet your needs, go for it!
 
#25 ·
I calculate my ceiling assembly will weight about 870lb. I have my 9 channels spaced ~21" apart and 4 clips/fasteners per channel (48" spacing). That gives me 36 connections total so 24lb per clip. Actually, I think the clips in the field bear more weight than those at the edges. I figure those could bear up to 31lb each. My assemblies can easily handle that weight but the issue is more about what the wood blocks will do over time. They're smallish blocks and as you noted they could crack after a while.

2x4 on edge is exactly what I did and I posted photos above. You said you wouldn't trust it but now you're advising exactly that.

I think you're bothered by my hard block fastener method on the ends of my hat channels. Right, the sound clip people advise no such thing, but the hat channel people clearly advise that the channel can be attached directly to the framing with screws (http://www.clarkdietrich.com/products/drywall-framing/furring-channel-hat-channel). I'll I've done is implement a hybrid of isolation techniques and I see no reason it won't work well. I just should have designed more robust blocking.

Jeff's knowledge of a lot of people having using 5/8" osb 24"oc and no reports of sagging was what I needed to satisfy that concern.
 
#26 ·
I was suggesting putting a 2x4 on edge, but oriented vertically. If your joist is 10 inches tall, cut a section of 2x4 about 8" long. Now, mount it so that the 8" dimension is pointing up and down. The end of the 2x4 (which will be 1-1/2" x 3-1/2") will be at the bottom. This will provide a lot more strength. Yours are placed horizontally, with the 1-1/2" x 3-1/2" dimensions on the sides.


Keep in mind that for the link that you are quoting, the application is not listed as having acoustical isolation benefits. Rather, it is simply a structural support method to use it as furring channel to allow attachment of drywall to masonry walls and ceilings. For proper acoustical isolation, I go back to recommending the use of the entire system as a system.


Food for thought, "Resilient Channel" is designed to be used for sound isolation and does NOT require the use of clips. There are two main types, RC-1 and RC-2. RC-1 has one "leg", RC-2 has two "legs". If you look at the resilient channel, you will see that the "legs" have holes in them that are designed to minimize the transmission of vibration. Different manufacturers have different hole patterns, and some manufacturers have more than one pattern, depending on where and when the channel is manufactured.





Resilient channel can be effective when used properly, although not nearly as effective as clips and hat channel. Resilient channel has received a bad rap over the years, much of it due to improper installation. This link will shed additional light on the subject. http://www.soundivide.com/uploads/content_file/resilientchannel_aprovenliability-58.pdf There is more out there if you search for it.
 
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