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Unread 11-29-2019, 08:02 PM   #16
jadnashua
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Having threads (nearly) the whole length of the screw make panel jacking more likely. There isn't any advantage of having threads in the top layer.
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Unread 11-29-2019, 08:39 PM   #17
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Very interesting Jim you mean the the top panel pushing up off the panel below?
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Unread 11-30-2019, 01:07 AM   #18
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Yes. Screws oftentimes need a turn or two before they begin to penetrate into the material they’re being driven into. And when you’ve screwed down through the top layer and aren’t sufficiently pressing down hard enough, a revolution or two of the screw before penetrating into the bottom layer causes the top layer to separate (screw jacking). Now, if the screw has threads all the way up to the head, the screw head is likely to stop dead in its tracks when the head is driven flush. The screw feels tight, yet the separation between the layers remains.

Therefore, it’s better to use screws with some bare shaft near the head when your goal is to drive the layers tight to each other.

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Unread 11-30-2019, 08:25 AM   #19
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The picture of the $29.00 looked like than half of it is smooth, in that case I feel like the only thing holding the 2 plywoods together would be the head of the screw. This is what I use
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Unread 11-30-2019, 08:35 AM   #20
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You're definitely gonna want the 1 5/8" screws, Scott, at least. The 1 1/4"'s will strip out just as cx said. Some of the 1 5/8th's might also. I just checked and it looks like the last 1/4" or so of the 1 5/8" screws are tapered, leaving 1 3/8" ish of full shank diameter. The #8 1 5/8" have a smooth shaft length of just under 1/2", the #8 1 3/4"'s have just over 1/2". In the Grip Rite line the head diameter for #8's and #9's is the same.
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Unread 11-30-2019, 06:59 PM   #21
jadnashua
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The head is the primary clamping device when bonding two sheets of ply together. Having that area smooth of threads helps ensure you actually get the two panels tight together. WHen working with say hardwoods, you usually make a through hole in the top board the diameter of the OD of the screw shank, then the threads hold in the second, with the head holding it all together. It's important to not overtorque the screw, as you might snap off the head.
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Unread 11-30-2019, 07:06 PM   #22
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thanks for the clarification Jim.
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Unread 12-02-2019, 09:58 AM   #23
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Thanks all for the replies. I've skipped the $29 "Construction Screws". The threads didn't seem well formed. They probably would have been fine but I went with the "Exterior Screws". I'll pre-driil/countersink if needed. Sounds like a terrible job but it's my house so I'm shooting for perfection.
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Unread 12-16-2019, 08:45 PM   #24
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The second layer of plywood is screwed down! 2500+ screws and I pre-drilled for all of em.

I called Schluter to ask about the 1/8" gaps I so carefully left between all the panels. The customer service rep told me that Schluter's instruction was to fill these gaps with the thinset that I am going to use to adhere the Ditra mat.

So why did I bother to leave those gaps?

I ran across a thread from 2003. https://www.johnbridge.com/vbulletin...ead.php?t=5306
It doesn't seem that this question was definitively answered then and I have a feeling it is still an unknown. Do we leave the gaps to satisfy the plywood manufacturers recommendation and then fill them with a non-compressible material anyway? I'm okay with doing this if it is proven to have no ill effects but it doesn't make any sense.
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Unread 12-16-2019, 09:52 PM   #25
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The thinset that gets into the panel gaps will eventually pulverize...IOW, don't worry about it. SOme people go to the trouble to put caulk in there, but IMHO, and Schluter's (they have a lot of experience with this), say it's not necessary. The forces that cause the panels to expand and contract are enough to do that. If the panels are butted up together, when they expand with moisture content, they tend to bow up, and that's catastrophic to tile.

FWIW, if using underlayment on an enclosed structure (say in remodeling versus new build that may be exposed to the weather), call for butting the panels together, since they are not likely to see the huge affects of being rained on and radical variations in moisture content. The gaps are designed for new construction where it can see liquid water on the structure. They don't hurt if you install them during remodeling, though.

IOW, don't worry about it!
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