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Old 02-10-2009, 08:44 AM   #16
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Purpose of gaps between sheets of plywood?


Let me clarify my earlier statement; we leave a 2-3 mm gap between sheets of plywood in the field, more around the room...Gosh, if all my plywood expands and contracts in a typical residential setting - in both length and width - by as much as it is claimed to do, and if all my tiles do the same - I'm sunk. I don't think many tilesetters allow for expansion or contraction of their tiles. This leads me to wonder if these aren't extreme statements - and are being taken out of context.

We see lots of water damage and we have on occasion seen plywood tent; but after a flood perhaps, but not after an average tile install in a residence. When we install Ditra as per manufacturers instructions we use modified over plywood or OSB and unmodified under the tile...

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Old 02-10-2009, 08:54 AM   #17
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Purpose of gaps between sheets of plywood?


why not just use thin masking tape over the gap?

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Old 02-10-2009, 09:05 AM   #18
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Purpose of gaps between sheets of plywood?


Quite a few contractors on other forums have said they routinely ignore DITRA's recommendations, and this is what steered me away from DITRA months ago. WEDI says to use modified below their panel on exterior plywood and they even have a video showing that.

But I've never yet been given advice about what to do with the gaps, if anything, that wasn't immediately contradicted. That's why I asked what the gaps were for. There seems to be a consensus that they're for expansion.

So as long as I don't do anything to interfere with the expansion, something that might cause tenting, things should be OK, it seems to me, with my zero years of experience.

A funny story: I thought maybe using Great Stuff (tm) for Windows in the gaps and then trimming away any excess might be a good way to fill the gaps without interfering with the expansion since that foamy material compresses easily. So I phoned DOW to ask whether it was alkali-resistant. When I told them what I was thinking of using it for, the DOW tech info rep at first REFUSED to give me any factual information about the product because the use I had in mind was not a recommended use. You wouldn't believe the song-and-dance I had to go through to find out that Great Stuff for Windows was alkali-resistant under mortar if allowed to cure.

Last edited by diy888; 02-10-2009 at 09:08 AM.
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Old 02-10-2009, 09:12 AM   #19
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Purpose of gaps between sheets of plywood?


i too thought about the GS foam, but for the flooring in all the new rooms going in here.
any reason why i shouldn't? stops minute air gaps and allows plenty for expansion.
but would i need fire-rated? orrrrr.....?

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Old 02-10-2009, 09:14 AM   #20
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Purpose of gaps between sheets of plywood?


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why not just use thin masking tape over the gap?

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Or PVC electricians tape, which is alkali resistant.
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Old 02-10-2009, 09:22 AM   #21
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Purpose of gaps between sheets of plywood?


Schluter doesn't say to use modified thinset on substrates everywhere, in fact go to lengths to say that each substrate is different. You use non-modifed over concrete for example, modifed over plywood or OSB. That's because of the method of drying of the thinset in both cases. So some contractors go against the manufacturer but you might ask yourself who has the most experience using their own products and who will uphold a warranty if you don't do it their way?

We end up filling the gaps with modified thinset because we're not used to having extreme humidity under Ditra - it is almost waterproof...so I really doubt that plywood would expand to the limit they say it would; perhaps in the extreme - but not in every-day situations.

So don't lose too much sleep over it; and certainly don't lose any sleep about your tiles expanding or contracting while you snooze.
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Old 02-10-2009, 09:25 AM   #22
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Purpose of gaps between sheets of plywood?


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Originally Posted by ccarlisle View Post
Lroom...Gosh, if all my plywood expands and contracts in a typical residential setting - in both length and width - by as much as it is claimed to do, and if all my tiles do the same - I'm sunk.
But the expansion, it was said, is limited by the fasteners (nails/screws). Those expansion numbers were for unfastened sheets.

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Originally Posted by ccarlisle View Post
I don't think many tilesetters allow for expansion or contraction of their tiles. This leads me to wonder if these aren't extreme statements - and are being taken out of context.
Does a really porous tile (e.g. water absorption > 7%) expand when the bisque gets wet? Michael Byrne in his Setting Tile book warns about using such tile in a shower stall, for example, because of bacteria issues since the bisque never really dries out (and I presume there could be mold/mildew issues too) but then he goes on to say that tilesetters use this non-vitreous tile all the time in showers, and if a latex-modified mortar is used, the potential problems are somewhat reduced.

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Originally Posted by ccarlisle View Post
We see lots of water damage and we have on occasion seen plywood tent; but after a flood perhaps, but not after an average tile install in a residence.
I'm going to use Sikaflex 201 US to seal the WEDI floor panels to each other, and at the wall-floor change-of-plane. I was going to install a floor drain but changed my mind. I probably should have done so -- then we could just hose the bathroom down.

Last edited by diy888; 02-10-2009 at 09:27 AM.
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Old 02-10-2009, 09:59 AM   #23
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Purpose of gaps between sheets of plywood?


Boy, lots of different subjects here...look, as a minimum system for using Ditra in a bathroom, and depending on what size room you have and on what deflection numbers you come up with, you can get by with a 3/4" T&G plywood or OSB, thinset, Ditra, thinset, ceramic or porcelain tiles. In total that's about 1 3/8" thickness approx. Now that's over 16"oc joists and to give a degree of waterproofing and uncoupling of the tiles from the substrate, In theory, this should prevent any cracks from appearing.

Non-vitreous tiles don't expand and contract when they get wet; the problem is mold. It has to dry out very quickly and hence needs more daily maintenance. They can be used in a shower but would have to be properly treated beforehand.

So, I gather you are about to use WEDI CBU on the floor with a polyurethane adhesive? are you now talking about a shower stall? And why the drain in the bathroom floor. Just where do you live?
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Old 02-10-2009, 11:51 AM   #24
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Purpose of gaps between sheets of plywood?


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Originally Posted by ccarlisle View Post
Non-vitreous tiles don't expand and contract when they get wet; the problem is mold. It has to dry out very quickly and hence needs more daily maintenance. They can be used in a shower but would have to be properly treated beforehand.

So, I gather you are about to use WEDI CBU on the floor with a polyurethane adhesive? are you now talking about a shower stall? And why the drain in the bathroom floor. Just where do you live?
The floor drain was something of a joke. My idea of a maintenance-free bathroom is one you can just hose down.

I'm in southeastern PA, but the humidity here in the summer is terrible and the room is in almost constant tree-shade and it has no cross-ventilation. We had a bad mildew problem before. I also have teenage sons who take two showers a day because they play sports. The room doesn't have much chance to dry out. So for the remodel, I've installed a 230CFM exhaust fan in the attic with one vent in the shower ceiling and one above the toilet. I also plan on using a trowel-on clay product from americanclay.com on the sections of wall in the room that are untiled; it absorbs moisture from the air (if left unsealed).

Daltile colorbody porcelain mosaics (Keystones) are going onto the floor in the shower, but field tile from their Semi-Gloss collection is going on the walls. Laticrete SpectraLock grout. The water-absorption rate of the Semi-Gloss tiles is high according to the website, "< 20%". Daltile assured me that the glaze protects the bisque from absorbing too much water, and the advice on various forums has been pretty much the same...not to worry; but I was concerned about absorption along the grout joints and mildew/mold formation there, given the room's constant dampness. What treatment of these tiles would be recommended? Or were you thinking I meant unglazed as well as non-vitreous?

The WEDI panels are on the walls in the shower but there's also a thinner 1/4" panel for the floor. The butt joints of the floor panels, if I understand correctly, can either be glued with polyurethane sealant or taped with alkali-resistant fiberglass tape in a bed of thinset like a standard CBU. The boards are then screwed in place with special dimpled washers. Since I have a lot of the sealant tubes left, I might as well use it up since it has a shelf-life.

Last edited by diy888; 02-10-2009 at 12:12 PM.
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Old 02-10-2009, 03:17 PM   #25
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Purpose of gaps between sheets of plywood?


Bob,

The only time you need modified thinset in a Ditra installation is to bond it to a plywood floor. You cannot bond anything to plywood using unmodified thinset as you should know. This point is not an option. I hope you've never installed Ditra on plywood with unmod.

Some do use a cheap modified to set tiles over Ditra especially if installing porcelain tiles. They just can't understand that an unmodified is recommended and works fine over Ditra.

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Old 02-10-2009, 05:21 PM   #26
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Purpose of gaps between sheets of plywood?


I didn't mean to create a storm of opinions from my statements. All I was trying to do is say that the use of caulk in the gaps is by far better than thinset. I don't know about you guys but something that staying flexible is better than thinset that gets hard. The expansion numbers I stated for the plywood were from a test performed by a lab (no, I don't remember which one!) and have been accepted by the industry. I picked up the caulk in the joint trick because after talking with people higher up in the food chain than me, they thought it was a good idea. Remember the figures are for a unrestrained sheet of plywood.
Everything I said can be verified. It's great when people think outside the box, it becomes dangerous when someone says something that is either wrong or misleading and that advice is taken as the gospel and that reader goes ahead and performs it only to find out at a later date that they have a problem. This is why Jaz and others ask about the size and span of your joists. It's not only the length of the span and the size of the joists, but the distance between the joists and the thickness of the flooring. The distance between the joists is much more of a deflection issue that also has to be addressed. I ramble on too much, so I'll go back into my corner and behave.
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Old 02-15-2009, 10:48 AM   #27
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Purpose of gaps between sheets of plywood?


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So don't lose too much sleep over it; and certainly don't lose any sleep about your tiles expanding or contracting while you snooze.
Right, there's plenty of other things to lose sleep over

I was concerned about the non-vitreous wall tile absorbing water along the grout joints and never drying out with nearly with constant use. In his book, Michael Byrne advises against using tile with greater than 7% water-absorption in a shower stall (p. 7) and the Daltile wall-tile is rated at water-absorption not exceeding 20%. That's highly absorbent tile.

Local tile distributor says don't worry, just be sure to seal the grout or use an epoxy grout.

Daltile phone support says this tile is "used all the time" in showers.

Still, there are enough products on the market for removing mold and mildew from showers that I wonder if absorbent tile contributes to the problem.
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Old 02-15-2009, 10:52 AM   #28
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Purpose of gaps between sheets of plywood?


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Originally Posted by DangerMouse View Post
i too thought about the GS foam, but for the flooring in all the new rooms going in here.
any reason why i shouldn't? stops minute air gaps and allows plenty for expansion.
but would i need fire-rated? orrrrr.....?

DM
If the foam insulation is used between fire-proof or fire-retardant layers that satisfy local code (e.g. 5/8" drywall, ceramic tile) I would presume it would be okay to use. But it never hurts to check with local inspector.
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Old 02-15-2009, 01:34 PM   #29
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Purpose of gaps between sheets of plywood?


The choice of tiles for the inside of a shower stall walls is less of a concern to me than the waterproofing that is done on it. In your case, you have an extreme case where let's say four showers are taken every day and here it is not surprising that you had a mildew issue - one that you would probably have kept on having regardless of the fan size.

Besides the fan is to remove excess humidity, not dry the room.

Tiles of any and all sorts get wet behind them because the grout that keeps them stuck on the wall is porous. The grout gets wet and so does the thinset behind the tile. Now in a lot of casdes, that water just sits there and eventually goes moldy because it doesn't drain away.

Modern installs now have a waterproof membrane underneath the thinset that allows the water to drin downwards towards the drain much faster that if the membrane wasn't there in the first place. Has nowhere else to go but downwards. So, with this membrane in place it really doesn't matter what tiles you have on the wall becasue there will be little water behind them to worry about.

So, your non-vitreous tiles with the high water absorption % are OK - but with a waterproof membrane behind them. You may still have to worry about mold occasionally - but not every day.

Sealing the grout is no cure either. Sealing just limits the amount of water passing throught the grout and prevents grout staining with oils etc.; behind the tiles has to breathe some and proper sealants allow this. You could always try to get away with putting something I saw this week: polyurethane on the grout, making it orange in colour - and then have temporarily waterproof grout joints!

And yes, you may still have "absolutely waterproof grout joints" in a years time -but by then the shower walls will have crumbled around your ankles.
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Old 02-15-2009, 07:47 PM   #30
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Purpose of gaps between sheets of plywood?


diy888, you want to use Wedi to waterproof a floor?, not a great idea, all those holes from the fasteners have to be sealed.

You want a waterproof room, USE Ditra, best system out there if used correctly.

With proper maintenance & prep work ANY tile or stone can be used in a shower, just because MB says not to doesn't mean it can't be done.

If I can't mud the floor, it's Ditra or I walk, will never set a CBU on a floor again.

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