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Old 08-14-2018, 02:47 PM   #1
mrberryman
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Tiling direct to plywood.

So a question/hypothesis I have is though tiling direct to ply is seemingly taboo nowadays, almost all thinset manufacturers warranty their higher end thinsets for a direct to ply install as long as you have 3/4" or 5/8" ply and 16" joist spacing. But, as I understand it, this is not TCNA approved until you get to 1.25" of plywood? Same with the "medium bed mortar" thing, some manufacturers warranty their products for 3/4", TCNA says about 1/4" max?

My question with direct to ply installations, and people who've done them/have a job kicking around in their house, etc. It seems more the problem with direct to ply installs is because it's cheap, usually other things go wrong too, and perhaps this is why it has a bad reputation? As in, people use the cheapest unmodified thinset and not an A118.11 or 15, people do swirl trowel lines and/or spot bonding, and use too small of a trowel, don't dampen the plywood before, then of course there's JMJ where seemingly the issue there is the metal lath cuts the thinset up from movement.

So, my question is, is it really that bad of an idea, or is it more that 99% of the time other things go wrong on direct to ply installs and that's why it gets the bad reputation? I know even from jobs I've done painting, being in people's houses, one kitchen had a tile job with cracked tiles and loose tiles, and it seemed to be direct to ply, and picking up a tile piece and looking at it showed it was all swirls, 1/4" notch trowel, and the tile itself had very little thinset coverage, and of course the corners were grouted to the baseboards, too.

Just want to open this can of worms and ask the question.
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Old 08-14-2018, 03:10 PM   #2
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Which tile installation product manufacturers have you found that recommend bonding tile directly to a single layer plywood subfloor, Joe?

Links would be helpful.

The ceramic tile industry does recognize the method of bonding tile directly to a plywood underlayment (F150 in the TCNA Handbood) that is properly installed over a properly installed plywood subfloor. And there's no question that it can work if done exactly right. We don't recommend it for our DIY visitors, or our pro members, for that matter, because it is very unforgiving and also because most of the installations we deal with here on the forums are not done over new construction and there's frequently no way to know if the subfloor has been correctly installed and it's very difficult to properly install a plywood underlayment over the ones that are.

It's just so much easier in most cases to use a recognized tiling substrate that is more compatible with the ceramic tile and thinset mortar.
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Old 08-14-2018, 04:42 PM   #3
mrberryman
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So reading TDSs, most of Custom's say "refer to TCNA F150" which is the 1.25" total plywood. Laticrete as well specs an underlayment layer as well. Mapei on the other hand, seemingly specs nothing as long as it's exterior grade plywood, at least in the thinset TDS. So perhaps I'm wrong. I think Mapei in another doc does say for floor prep to use an underlayment, but it's talking about vinyl too and says at least 1/4" and 1/4" ply is forbidden for tile since basically no exterior grade 1/4" ply exists.

One interesting thing is Mapei's UK products do spec direct to plywood installs in what would be in US terms, very forbidden situations.
http://www.mapei.com/public/COM/prod...2_quick_gb.pdf

Quote:
Laying on flexible substrates or substrates subject to
vibration: marine plywood, wooden agglomerates
(if sufficiently stable in the presence of water), old
wooden floors, etc.
I know it's not apples to apples, as the US products are obviously different, again I'm just throwing questions out. I obviously don't know for certain, and industry standards are industry standards, I just personally wonder if with most direct to ply installs if their failure point is something else going wrong, whereas with a CBU it just allows more slack for otherwise wrong installations to happen. Even in "real life" with CBUs I'm guessing maybe 5-10% of people even thinset them down, and the guy at Lowes working in the tile department who bothered me when I was looking for cheap thinset to throw under CBUs seemed mind boggled that I'd thinset and screw them down and thought I was absolutely insane for even suggesting that.
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Old 08-14-2018, 05:42 PM   #4
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When I lived in Virginia, I did numerous floors over plywood,,,,but it was with two layers ...either 1/2" times two or 1/2" plus 3/8" exterior and was well bonded and screwed together... This was 20 plus years ago and I didn't know better, but SURPRISE...never had a callback or a failure, but this was in the days of "Color Tile" and I used a three part epoxy mix and worked great.

I imagine that if you use the Platinum 254...you should have no problems...
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Old 08-17-2018, 11:59 AM   #5
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Long ago the industry found that you needed two layers of plywood to reduce deflection. Deflection is the reason for most failures with plywood floors. If sufficient mortar is applied a mortar meeting A118.11 will have a strong bond to the plywood and the tile. Another problem is that many subfloor are OSB, that does not have the stability of plywood.
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Old 08-17-2018, 01:31 PM   #6
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Quote:
Originally Posted by Joe
So reading TDSs, most of Custom's say "refer to TCNA F150" which is the 1.25" total plywood.
Joe, the requirement is not for "1.25" total plywood," it's for two layers of plywood, the first layer of which must be a minimum of nominal 5/8ths" and the second a minimum of nominal 1/2" plywood. That's a total of 1.125" of plywood, but the real point is that it must be two layers of plywood, each properly and differently installed, not just a particular total thickness.
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Old 08-17-2018, 01:46 PM   #7
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With products like ditra so readily available and so easy to use it's just crazy not too do them and bond directly to plywood. Minimal thickness and fast install.
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Old 09-23-2018, 01:20 AM   #8
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On a paint job I was on that was part of a gut of a condo, I came across a peculiar tile setup. I thought it was tiled direct to ply, then I noticed a height difference, and thought they layered 3/8" or 1/2" ply over it as it was about 1/2" high of a difference. Nope. It was actually double layered 1/4" or luan? underlayment stapled on. Two separate layers. Kitchen had blobs of liquid nails under the underlayment layer randomly placed.

Only two cracked tiles and little to no cracked grout in the whole place (lots of stained grout, though....) One cracked tile was under the fridge area, which after the underlayment was pulled the subfloor under it was rotten enough to poke your finger through from some sort of leak I guess from the adjacent condo. So can't really blame the install (unless they tiled over the rot...) The other was a cracked tile in the corner of one bathroom. Overall the subfloor in this place wasn't good either, really creaky and nasty feeling. Tile install seemed to be old, box of tiles in the garage had a .com address on them and were ceramic, I'm gonna guess early 2000s? Guy did use thinset on the tiles from what I could tell from what I saw in the vents, so that probably helped compared to mastic.
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Old 10-15-2018, 09:20 PM   #9
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Another one that's probably been completed already. But for the benefit of anyone who ight do a search and come across this page, this is copied and pasted from my website's FAQ page:

For those who still want to tile directly over plywood, though, Here are the specs. Shortcut anyone of the specs I'm about to give you, and you can kiss that floor good-bye:

There needs to be two layers of plywood, the top layer being spruce or fir (preferably fir), exterior grade BCX or better. The layers need to be screwed (spec calls for every 6" along the edges, and 8" in the field-- I go every 6" throughout, ONLY into the bottom layer-- NOT into the joists) BUT NOT GLUED. When laying the second layer of plywood in, make sure the joints of the top layer fall at the 1/4 and 3/4 mark from the layer underneath. You don't want the joints in the two layers to be any closer than necessary. Also, when laying them in, leave about a strong 1/16" between the sheets for expansion, and make sure you're laying it in with the grain going across the joists. Make sure, when screwing down the top layer, that you're going no further than the bottom layer of plywood. DO NOT drive the screws into the joists. This completely negates the effect of double layering the floor by transmitting the movement from the joists right to the top layer of plywood. Once it's all screwed down, take any cheap latex caulk you can find, and caulk the joints between the sheets of plywood. The reason for this is those joints are for expansion, as I said before. Now you're going to go over the plywood with thinset. Sorta kinda defeats the purpose of gapping the plywood if you fill that joint up with thinset. That's ALL the caulking is there for-- to fill the joints with something that will remain pliable and at the same time, keep the thinset out. Last prep spec is that just like with cement board, you need to use a fiberglass mesh tape to bridge those joints . Easiest to use is the self sticking blue plasterboard tape, and then just go over it with thinset when you set the tile. Last thing that's different and this is paramount-- the thinset. Just about ANY bag of modified thinset will tell you it can be used to go over plywood. DON'T BELIEVE EM!!!!! The ONLY thinset I'll trust is an UNmodified thinset, mixed with a liquid latex additive, full strength, such as Laticrete's 317 thinset mixed with their 333 additive, or Mapei's Kerabond thinset mixed with their Keralastic additive. The reason for this is it'll give you the highest latex content possible in a thinset, which does a couple of things for you. It's the strongest stuff you can find, and it's also the most pliable, so that it'll take the extra expansion and contraction that plywood goes through, as compared to cement board.

The only thing I'll change here is the "supermodifieds" will work as well as the liquid latex and unmodified thinset. By "supermodifieds", I'm talking like Laticrete's 254 Platinum and the like.
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