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Old 02-09-2009, 06:51 AM   #1
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Purpose of gaps between sheets of plywood?


Two layers of 3/4" plywood on my bathroom floor which is going to get CBU and then tile. I've left 1/4" perimeter expansion gap between the plywood and the wall, and 1/8" gap between sheets.

What is the purpose of the between-sheet gaps? Are they for expansion too? Something else? Are they meant to function like the gaps between CBUs which let you "glue" the boards together with thinset and tape? I'm asking because the advice varies on what should be done with those between-sheet gaps:

-- fill the gaps with caulk
-- fill the gaps with thinset

If I knew their purpose, I could decide on what advice to follow.


Last edited by diy888; 02-09-2009 at 07:22 AM.
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Old 02-09-2009, 07:23 AM   #2
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Purpose of gaps between sheets of plywood?


if the gaps are for expansion, i would not fill them with anything. it could wreak havok on your tiles above!

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Old 02-09-2009, 07:53 AM   #3
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Purpose of gaps between sheets of plywood?


But if they're not for expansion....

I'm going to use CBUs on top of the plywood, and am not going to put tile directly onto plywood, so maybe this advice does not apply -- but I have read, and in a reputable book, that when setting tile directly onto plywood ("no longer considered ... ideal"), the sheets should be "edge-glued" by filling the gaps with "tilesetting adhesive".

So, the question remains, what is the purpose of the gaps between sheets?
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Old 02-09-2009, 08:18 AM   #4
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Purpose of gaps between sheets of plywood?


Expansion. Dont fill.
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Old 02-09-2009, 08:29 AM   #5
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Purpose of gaps between sheets of plywood?


From the construction of plywood, there isn't a lot of expansion possible and furthermore I doubt whether "plywood lateral expansion" would be given as the reason to leave any gap between the sheets. We put them close but not glued to each other...1/8" or less is fine. Don't fill it.

Two 3/4" sheets on top of each other? Why? height concerns?

What thickness CBU are you considering...would have to be 1/2" unless you have space to fill or you want to step up into your bathroom. Add thinset and tile to that - and you may indeed need new doors.

What about waterproofing and/or uncoupling membrane? thought of that? What kind of tile?
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Old 02-09-2009, 09:20 AM   #6
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Purpose of gaps between sheets of plywood?


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Originally Posted by ccarlisle View Post
From the construction of plywood, there isn't a lot of expansion possible and furthermore I doubt whether "plywood lateral expansion" would be given as the reason to leave any gap between the sheets. We put them close but not glued to each other...1/8" or less is fine. Don't fill it.

Two 3/4" sheets on top of each other? Why? height concerns?

What thickness CBU are you considering...would have to be 1/2" unless you have space to fill or you want to step up into your bathroom. Add thinset and tile to that - and you may indeed need new doors.

What about waterproofing and/or uncoupling membrane? thought of that? What kind of tile?
I'd read that two 3/4" layers were recommended. Yes, with the two layers of plywood, the threshold is an issue. But there was a fairly thick mud bed in the original room and the threshold was already high, so the door is pre-trimmed. The mud was ripped out so fixtures could be relocated/pipes rerouted. If I had this project to do over again, I'd sink the subfloor between the joists resting on sisters, so the surface would be level with the joists, and lower the threshold by 3/4". Live and learn.

The type of CBU was chosen mainly for its ease of cutting and very light weight (I hurt my neck last year and was told by the doc to minimize weight when lifting). 1/4" WEDI panels. Waterproof foam sandwiched between fiberglass-reinforced cementitious layers. It serves as an isolation membrane. I'm using thicker WEDI panels on the walls and the WEDI Fundo prefab shower base. Very easy to work with. I considered DITRA months ago when I bought this (I'm a real slowpoke) but was in some doubt about the type of thinset I could use between the DITRA and the plywood.

Floor tile is flamed granite. The floor is dimensionally better than L/720 deflection and I've also reinforced it with blocking and extensive sistering. The floor excluding shower is 8x6. I'm going to cut the 12" tiles down into 6" squares.
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Old 02-09-2009, 09:29 AM   #7
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Purpose of gaps between sheets of plywood?


Still need the gaps in the plywood. Also the sheets should not line up. Overlap the top sheet edge at least 4" past the first sheet. You only needed 1/2" plywood with exterior glue grade, not CDX. Ditra would be the way to go. Unmodified thinset under it and modified over it.
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Old 02-09-2009, 11:19 AM   #8
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Purpose of gaps between sheets of plywood?


Diy 8888,
Yes on the expansion and contraction of the plywood. I have used simple painters caulking. Don’t believe everything you read or hear. If you do, then I’ve got some ocean front property in Arizona that I’d like to sell you!!

ccarlisle,
There have been tests performed by reputable firms costing millions of dollars covering this one item. One of them is the American Plywood Association. This is a quote from the CTIOA after their research had been completed.


“Ply-wood: The several layers of wood that go to make up a sheet of plywood are placed so that the grain, of individual layers is opposite that of the preceding layer. However, the least amount of shrinkage will take place parallel with the face sheet grain. A typical sheet of Douglas Fir plywood unrestrained will expand and contract 7/32" over its length and over 3/32" on its width. The amount of expansion and contraction will also depend on how securely the sheet of plywood is restrained nailed or otherwise fastened.”

CDX plywood should never be used on any thing to do with tile and stone. The reason is the voids between the sheets. As a matter of fact I haven’t seen any of that stuff in years.

I’m just trying to keep you guys on the correct path of installation. I hate to say anything to someone who already did the wrong thing and wants to be congratulated for doing a good job.

Remember what I said in the beginning about the ocean front property, The way of getting around owning it is to go to the source of the info or standard. TCNA, NTCA, Plywood Association, thinset manufacturers all have web sites and they also have 800 numbers to their technical dept. These people will not give you bad info since they are there to see that the product works correctly. A happy customer will tell their friends etc.

Good Luck and think things out first,
Jim
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Old 02-09-2009, 02:21 PM   #9
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Purpose of gaps between sheets of plywood?


Thanks for the info.

JJC
I'm all for thinking things out first, so your detailed answer leads me to ask this: if the plywood can expand almost 1/4" along its length, could the expansion when it narrows the gap cause the painters caulk to extrude from the gap and push upwards against the mortar? Does some caulk compress better than others?
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Old 02-09-2009, 02:27 PM   #10
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Purpose of gaps between sheets of plywood?


some if not most caulks are very flexible, but there is no reason to use caulk in these joints.
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Old 02-09-2009, 03:14 PM   #11
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Purpose of gaps between sheets of plywood?


Quote:
Originally Posted by Bob Mariani View Post
some if not most caulks are very flexible, but there is no reason to use caulk in these joints.
If the gaps aren't filled (with something that can compress, like a caulk or foam) how do you keep the thinset from clogging the gap and getting in the way of the expansion?
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Old 02-09-2009, 04:15 PM   #12
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Purpose of gaps between sheets of plywood?


If it expands it will crush the thinset. But we use unmodified thinset over the plywood which is used so that it does not stick to the plywood allowing the wood to move and the thinset to not move. If you are concerned about the thinset moving into the gaps, cover them with a 1" strip of felt.
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Old 02-09-2009, 07:19 PM   #13
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Purpose of gaps between sheets of plywood?


Hi Bob,
The compression of the thinset is not an issue, it's the shear strength that one has to be concerned with. If the plywood is forced against each other it will cause what is called tenting and that is usually after the tile has sheared from the surface. This is very predominate when the tile shears from the floor and starts to lift off the floor as they are compressed against each other. Look at it this way the tile expands and contracts by temperature and can expand only by moisture and is not reversable. That's why we make sure we use the correct tile for the intended installation. That little tidbit should keep you awake for a few nights wondering which one has the wrong tile. This is another reason why the industry has spec. 1/4" expansion joint around the perimeter of every room including closets. It is also specified that expansion joints be spaced in the tile installation and this is based on EJ171-07 of the TCNA Handbook. This is taken from the ANSI A108, A118 I have a problem with your statement that you use and unmodified thinset on the plywood. Since it will not bond to the plywood, how do you bond the tile to it? I question your statements because of the amount of DIY'ers that read this forum and will take that as gospel. You can do the felt or tar paper over the joints but it still leaves the bonding issue of the thinset. Oh by the way, tar paper or as you call it felt is not an approved product. It has been known to react with the alkaline in the thinset and turns to mush.

Diy888,
No, remember that the numbers are the maximum and the expansion happens on both sides of the gap, not just on the one side of the plywood. I think you are trying to read too much into the answer. When in doubt, go to the source or ask someone at the source. I think it is great that you think things out, please continue! Some caulks do compress more than others, but if the gap is not filled to the “brim” it’s OK and the compression or movement of the caulk isn’t the same as the expansion of the plywood. Remember that everything in this world expands and contracts and to really make it complicated, it doesn’t happen at the same rate as something else.

Jim
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Old 02-09-2009, 09:55 PM   #14
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Purpose of gaps between sheets of plywood?


Yikes, so much wrong info in this thread. Thanks Jim for stepping in.

The gaps are for expansion, and most, (if not all), thinset manufacturers want you to fill with thinset as you install the tiles. This will not cause the floor to tent unless the floor floods and stays wet for some time, and the tiles were bonded direct to the plywood. Which of course is not a recommended method for floors that can get some water now and then.

Although it may be just fine, I don't know of any manufacturer that recommends caulk in the joints of plywood.

Mariani: You keep getting the thinset for Ditra wrong, have you ever installed any?

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Old 02-10-2009, 06:46 AM   #15
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Purpose of gaps between sheets of plywood?


I did use Ditra but use Noble more often. Schulter does recommend using unmodified above and below the ditra. Many professional installers also use modified as I do. The only reason they want unmodified is their claim in the drying of the latex additive. I usually allow two days and have never had a problem. And I feel that the latex adds a little more flexibility. But you are both correct, follow the manufacturer's directions is a better way to go. It's why I switched to Noble products. I never used felt either, I was responding to a concern of filling the gap which I feel was not an issue anyway. And neither of you clarified this. Do you caulk the gaps or not? I do not and never had any problems.

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