Screw size & length for osb/gg/dw - AVS Forum
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post #1 of 24 Old 10-23-2011, 04:35 PM - Thread Starter
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Please help with advice about screw size and length.

My ceiling has whisper clips recessed in the joist cavities, and will have 3/4" osb, gg, 5/8" drywall up in a few days. I have size 6 x 1 1/4" self drilling drywall screws for the osb layer, and 6 x 1 5/8" self drilling drywall screws for the drywall layer. Since these were the only length self drilling drywall screws I was able to find, I hope they're OK.

I have two walls with 1/2" osb furring strips, which will have 3/4" osb, gg, 5/8" drywall applied. What size/length screws would be best for these walls? And just standard wood screws? I'm an accountant and know next to nothing about construction.

Also, I've read here that with drywall over osb it isn't necessary to have seams centered on the hat channel for the second layer. Even though it might not be necessary, is it still best to try and do it that way?
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post #2 of 24 Old 10-23-2011, 08:16 PM
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Quote:
Originally Posted by AFA View Post

Also, I've read here that with drywall over osb it isn't necessary to have seams centered on the hat channel for the second layer.

You don't need to bother with 5/8 drywall over 5/8 drywall either. In fact with two layers of anything you don't need either layer to line up on the channel. Drywall and your OSB should be hung perpendicular to the channel.

That is the beauty of using two layers, just put it up overlapping the seams.
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post #3 of 24 Old 10-24-2011, 09:28 AM - Thread Starter
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That will certainly makes things easier. Thanks for the info.
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post #4 of 24 Old 10-24-2011, 10:08 AM
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Please hold...Both layers need to hit the channel and here's why.

#1 these drywall screws are weak, and only work because there are so many of them.

#2 Let's say you install 100 screws on the first layer. If you install the second layer to the first layer of plywood, that first 100 screws is now holding up both layers. Not good

AFA, did the source you bought these items from not give you any instructions?

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post #5 of 24 Old 10-24-2011, 10:18 AM
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On my build the OSB layer lined up with the studs (on the walls) and the channel (on the ceiling) The layers of drywall did not line up perfectly on the studs and channel but I still put the screws into the studs and channel through each layer.

The OSB is great in that you do not have to terminate each peice or drywall at the stud, but you should still try and make sure the drywall is attached to the framing.
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post #6 of 24 Old 10-24-2011, 10:24 AM
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Yes hit the channel (or framing) with the screws on both layers, but the the edges of the sheets in a double layer system don't have to line up with the channel.
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post #7 of 24 Old 10-24-2011, 10:42 AM
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Sounds good!

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post #8 of 24 Old 10-24-2011, 11:26 AM
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Ted, While it is wasted effort and material couldn't you could just double up the screws on the first layer (OSB) and then just attach the second layer to first.
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post #9 of 24 Old 10-24-2011, 11:49 AM
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Hmmmm.... Hmmmm...

If the second layer were attached with enough screws to meet code, maybe. So if local code required screws every 8" along the framing, and you used 2x as many screws to attach the initial OSB layer (screws every 4") then it would seem to be OK to me. Performance-wise, you're good. There's just some nagging in the back of my mind about building code.

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post #10 of 24 Old 10-24-2011, 12:34 PM
 
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I used a lot of extra screws in the first layer of OSB and then tried to hit the channel with the second layer. I know I was not successful many times, but I am confident there are enough screws in the first to hold up both. If I DID manage to hit all the screws properly on the second layer, so much the better.
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post #11 of 24 Old 09-24-2012, 01:08 PM
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Fine thread drywall screws are fine for the first osb layer?

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post #12 of 24 Old 09-24-2012, 01:17 PM
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Yes, if you are going into metal channel.
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post #13 of 24 Old 09-24-2012, 01:18 PM
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Yes, because you're anchoring into steel. Drywall or plywood anchored to steel all takes a fine thread screw

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post #14 of 24 Old 09-24-2012, 01:37 PM
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Quote:
Originally Posted by Ted White View Post

Yes, because you're anchoring into steel. Drywall or plywood anchored to steel all takes a fine thread screw

What about for soffit construction, what should a person use when attaching the 2x2 into the ceiling channel? Would need to be at least 3" or more for size too

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post #15 of 24 Old 09-24-2012, 01:53 PM
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Just add up everything you're screwing through. Then add 1" for the screw length. Still a fine thread screw. I would also suggest pre-drilling through any 2x2s to avoid splitting.

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post #16 of 24 Old 09-28-2012, 05:47 PM
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Quote:
Originally Posted by Ted White View Post

Just add up everything you're screwing through. Then add 1" for the screw length. Still a fine thread screw. I would also suggest pre-drilling through any 2x2s to avoid splitting.

Could I use drywall screws? Having a hard time finding fine thread wood screws locally.

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post #17 of 24 Old 09-28-2012, 07:37 PM
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When I attached the 2x2s for my soffits I used fine threaded drywall screws. I think I did one about every 6 inches since they are notoriously weak and I was paranoid.

Then again, the weight that they are holding is pulling down so they aren't likely to shear off. I'd still use some extra just to be safe though.

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post #18 of 24 Old 09-28-2012, 10:15 PM
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post #19 of 24 Old 09-29-2012, 06:51 AM
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Quote:
Originally Posted by BIGmouthinDC View Post

Deck screws work.

This is going into metal channel though. Shouldn't it be a fine thread screw?

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post #20 of 24 Old 09-29-2012, 06:55 AM
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It's all about what the screw is getting finally anchored into. If the final attachment is channel, use fine drywall screws. If the final attachment if 2x2, use coarse thread screws and pre-drill

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post #21 of 24 Old 09-29-2012, 07:09 AM
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I need a lab study to prove to me that fine thread screws into two layers of 5/8 and channel out performs a larger ridge screw. Hanging the drywall I use fine threads intended for metal channel, Hanging soffits I like a more aggressive screw.
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post #22 of 24 Old 09-29-2012, 08:49 AM
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The fine thread grabs the steel more tightly. The thin 25 ga metal swims in the space between the coarse threads. The thin steel fits nicely in between the fine threads. I think if you sunk a coarse thread screw (that could pierce the steel) and a fine thread screw both into thin 25 ga channel, the coarse thread screw would be looser.

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post #23 of 24 Old 09-29-2012, 09:06 AM
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I'm talking about the total pullout strength of a taller thread screw in the composite assembly of 1 1/4 inch of drywall and that thin channel. Of course screws by themselves in the thin channel, the fine thread wins. In the composite assembly the drywall keeps the screws from rocking back and forth and you have a larger ridge of the screw resting on the top side of channel. Bigger grip.
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post #24 of 24 Old 09-29-2012, 09:12 AM
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I'm gonna say that in no time, given the brittleness of drywall, the screw threads will render the drywall around the screw threads to powder. At least you have to assume the worst. I really think this boils down to pullout of the bare screw from the thin channel.

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