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Unread 03-22-2013, 01:39 AM   #46
Lump
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Maybe Steve Rausch (tiger pilot) with USG can shed some light on this subject. USG must have tested or run into this question over the years with the millions and millions of square feet of durock and fiberock installed.
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Unread 03-22-2013, 01:51 AM   #47
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Here is a link to why OSB should be gapped.

http://osbguide.tecotested.com/pdfs/en/el814.pdf
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Unread 03-22-2013, 02:37 AM   #48
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Brad, it only gives the reason for roofs, not floors. It says about roofs "to allow for movement", but leaves that sentence out completely of the floor section. In fact, if you look close, all floor edges should be lightly butted except just the short ends are gapped when a second layer of underlayment is added (long sides always butted).
It's notable that every other surface there besides floors: both roofs & wall sheathing get gaps all the way around. Only floors are lightly butted.
Dana, I understand; the thinking could be 'it can't hurt to gap them', and I have no problem with that. It's only when guys start trying to enforce this silly tape thing, or caulking subfloor joints that I have an issue with. I don't want the already-pervasive myth that expansion causes peaking to become entrenched in the hive mind to the point it is driving national policy.
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Unread 03-22-2013, 05:25 AM   #49
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Tom it is confusing. Here is that clip from the link:

Lightly butt tongue and groove panel sides together and leave 1⁄8" (3 mm) gap at panel ends. Panel ends of single layer combination subfloor/underlayment should be lightly butted.

Went to an 8 year old build on Monday where some of the tile was cracked directly above the 3/4" subfloor joints .....not sure of the cause of it, luckily it was not our install.
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Unread 03-22-2013, 06:40 AM   #50
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If you measure modern plywood panels you will find them a bit shy of the 4X8 nominal size. That provides for the 1/8 in. gap. Flooring panels are the same size as roofing panels. If you are covering a large framing section you will either gap the sheets or come up short.

I think all plywood panels should have a gap . . . just because we can do it.
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Unread 03-22-2013, 07:25 AM   #51
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And filling the gap with thin-set should be ok as well .

Improper installation or deflection or etc. will create problems , but filling the gap I think is of no concern .

Remember guys , all the literature are clearly worded -- as recommendations -- , but -- not as a must do step(s) -- , so everything spins .
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Unread 03-22-2013, 07:28 AM   #52
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I hadn't realized this was a thread in the pro's hangout searching new posts, so pardon my past and current incursion into this area.

Having worked with a good amount of ply during my lifetime, I wouldn't hang my hat on non-T&G ply being cut shy of the 96" length. If you putting down 1/2" ACX underlayment I think you'll find the factory cut to be 96".

But if your installing gaps and going with the premise that the ply will expand, what is the proper way to deal with these gaps? Even caulking like pliable silicone would bulge up is response to gap reduction since it has to go somewhere and in theory become a fulcrum to produce a stress crack in the tile above. So tape over only and tape with what?
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Unread 03-22-2013, 07:44 AM   #53
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Jack,

I think we have to understand that if plywood is not installed -- cumulative expansion will happen -- vs. properly fastened where -- expansion will be limited and locally ditributed at each fastening point -- , hence negligible expansion at perimeters of the sheet.

And this will happen -- dramatically -- if flooding conditions or long periods of humid environement ( 90% & more ) are present .
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Unread 03-22-2013, 08:22 AM   #54
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After being an R&D test manager for 30 years I question all answers to be point of being a PITA. And being currently unemployed I got a lot of inquisitive energy built up.

I've worked with ply for many years building cabinets for my wife's endeavors, and her brother owned a cabinet shop so I've heard other opinions. I don't believe in the normal home environments of today that plywood is going to expand into these gaps unless as you state there is excessive humidity or a flooded kitchen. (What about the need to build for the possible dishwasher or sink failure?)

As we know plywood is it's own resistance to moisture expansion. Being that wood cells, like us, expand in width to excessive moisture or nutrients and not in so much in height. With perpendicular plys solidly bonded together, plywood restricts itself in expansion, more so with increasing ply count. Plywood without moisture resistive adhesive, non-exterior, not so much as the glue's water softening characteristics can let the individual plys move in expansion. Truly flooded conditions, all bets are off.

All IMO.
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Unread 03-22-2013, 09:27 AM   #55
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Important to remember though Jack that the 'expansion vs. not-expansion' question is not quite as relevant to this discussion. The key 'question is will it peak from expansion', and the answer is definitely no. Edge peaking is really edge swelling from liquid running water encountered during construction, and once it is covered with tile it is virtually impossible to exceed that much water, even in a home-wrecking flood. Regular expansion causes the whole board to compress, not edge peak.
Btw, on a side note, not directly related, I had a customer call me back a year after install to tell me that their house burst a pipe while on vacation and the whole house had 6" of water on it for more than a week. They said every single thing in the house was destroyed except for my tile (Ditra underlayment). Looked like brand new, the water never phased it.
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Unread 03-22-2013, 09:33 AM   #56
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Quote:
Originally Posted by Jack
I hadn't realized this was a thread in the pro's hangout searching new posts, so pardon my past and current incursion into this area.
Jack, the content of the Hangout is supposed to be more on the professional level (emphasis on "supposed to"), but is not at all limited to use by any particular group. You, and any other visitor, are more than welcome to participate in the discussions here.

Caution: The gloves worn in here are not always as thick as in other of our forums.
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Unread 03-22-2013, 09:58 AM   #57
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Quote:
Originally Posted by CX
Caution: The gloves worn in here are not always as thick as in other of our forums.
We are friendly fellows , right ?
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Unread 03-22-2013, 10:04 AM   #58
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Thanks, I'm OK with bare knuckles. I didn't get the self imposed 'Curmudgeon' handle because of being nice guy on automotive forums when brake opinions and mis-information gets tossed around.

Tom, thanks for the info on your kitchen job. I'll sleep better, and so will my wife when I redo our kitchen that had a dishwater leak we did not see. One reason for going to joists in that room.

I've beat this horse to the point of good French hamburger so I'll leave it with the only remaining question being what do you use to tape over seams to prevent thinset from entering if one wants to go that path?
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Unread 03-22-2013, 12:43 PM   #59
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Use anything you like, Jack, now that we've pretty well decided it doesn't matter.
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Unread 03-22-2013, 01:57 PM   #60
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Scotch Tape it is! Now just have to figure out shiny clear or transparent.



j/k

Good discussion though.
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