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Unread 11-20-2021, 06:04 AM   #1
Bruce H
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New tile in kitchen issue

I'm going to be installing new ceramic tile in kitchen this winter. But I think I have a potential problem. A couple weeks ago I installed a new dishwasher. I discovered that the existing flooring consisted of 1x8 diagonal shiplap board subfloor and 5/8" plywood underlayment GLUED to the subfloor. The old ceramic is laid directly on the plywood.

My problem is that the dishwasher just barely went in under the countertop. I can take the leveling legs off the dishwasher and gain about 1/2". That MIGHT be enough for 1/4" Durock set on thinset, but I'm nervous about that. Do I have any options that might be thinner than the cement board and thinset?

Also, when using 12 x 12 tiles, is back-buttering the correct way to set them? I've laid a lot of tile over the years, but never tile that large before.

If it matters, joists are 2x10 @ 16" o.c., spanning just 9'. So joist deflection should be good.
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Unread 11-20-2021, 07:43 AM   #2
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Welcome back, Bruce.

Lots of thinner substrate options available.

You are planning to remove the existing tile and try to re-use the existing plywood?
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Unread 11-20-2021, 07:50 AM   #3
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Yes, remove the tile and leave the plywood. It's going to be time consuming and a mess for sure, but I can grind down the old thinset with my angle grinder with a diamond cup.

One of the possibilities I thought of was two coats of Redgard, but I don't know if that would be appropriate
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Unread 11-20-2021, 08:17 AM   #4
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Custom appears to say that would be appropriate, Bruce. But those must be two very heavy coats in order to meet ANSI A118.12 as a crack isolation product. You'll want to read their installation instructions, but an application rate of 50 square feet per gallon is gonna be a very thick wet thickness application. Twice.

You might wanna consider a sheet-type crack isolation membrane.

My opinion; worth price charged.
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Unread 11-20-2021, 08:36 AM   #5
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Thank you for the help cx! I did read Custom's info on Redgard and my thinking is that both applications are going to be by trowel to get 50 sf/gallon.

Do you have a link to the proper application of 12 x 12 tile? Or is that no different than 6 x 6 or smaller that I'm used to doing?
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Unread 11-20-2021, 09:33 AM   #6
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Larger tile almost always involves a larger notch trowel to obtain the proper thinset mortar coverage between the tiles and substrate. Ideally, you obtain as close to 100% as possible, but with a minimum 80%. Most folks would start experimenting with a 3/8” notch trowel to install a few tiles and then pull some tile off the floor to check how much coverage they’re getting…and adjust to a different sized notched trowel to get the coverage they wanted.

But, about your glued plywood: can you see how it’s glued? Like, is it random ribbons of adhesive from a caulk tube? Or is it a full spread of troweled adhesive? I ask because the random-ribbons-of-caulked-adhesive-method may be leading to less (not more) support that the person gluing intended. I’m concerned that it may be flexing too much. If your existing tiles are cracked or loose, I’d be concerned about building atop the same weakness within your plywood. If you’re not sure on the tile integrity, you can tap them with a large screwdriver hammer to listen for sounds of looseness.

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Unread 11-20-2021, 09:50 AM   #7
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I was trying to hide this fact, but I was the idiot that glued the plywood to the subfloor boards. I don't recall for sure, but it seems to me I ran 3 strips of a PL adhesive the length of each diagonal board. Then I screwed the plywood down with a LOT of screws using my drywall gun. This was done 21 years ago and I'm just in the last 3-4 years starting to see some cracked grout joints. Seemed like a good idea at the time...
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Unread 11-20-2021, 12:32 PM   #8
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You’ve got nothing to be bashful about. You took on a tough tiling job and did what you thought was best at the time.

There are multiple things going on. I’ll start with talking about bonding tile to plywood: bonding tile directly to plywood is rarely recommended on this forum because it requires you to “walk a relatively narrow path” in your methods of building a tile assembly. And bonding mortar didn’t used to have the ability to stay adhered to plywood for more than about 7 years (stats taken from a long forgotten report from the 1990’s. FYI: mortars today are significantly better and well adept at things they couldn’t previously). These days, if you wanted to tile directly to plywood, one of the requirements is to have a double layer of ply. This drastically cuts down on the “scissoring” movement you’d have at the “over joist” seams of a single plywood layer. Think about how the single layer of plywood seam would microscopically open and bend when you stepped down to one side of the joist or the other. The ply would bend down under your foot and cause the end of the ply to both pull away from the adjoining ply and (this part is important) bend up. This bending is too much and can cause tiles to crack right there…right over the joist seams. But a double layer of plywood smooths out the seams by cutting down on the bending. This is just one part of tiling over ply, but the part I want to consider because…

Now…you didn’t build your double layer of subfloor the ‘way it’s approved’. But if your tiled floor lasted 17 years before cracking, you might have enough support that your well-meaning adhesive isn’t causing you too much trouble. When you get to demo-ing the old tile, I would pay close attention to where the tiles and grout has cracked. If the cracking is random, I might just proceed with your plan to install a crack isolation membrane (though I’d choose a higher performance and more costly sheet membrane like NobleSeal CIS using their proprietary suuuuuuuper sticky adhesive). But if the tile cracks are mostly over the plywood seams that are parallel to the joists, I’d consider an alternate plan to what you’ve got in mind because a crack isolation membrane only helps alleviate lateral movement stress. When a tile assembly bends under both live and dead loads, the tile is stressed in a bending direction and the tile shifts laterally a little. The crack isolation membrane only helps with the later. That’s why I’d pay close attention to WHERE your existing tile failed.

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Last edited by Tool Guy - Kg; 11-21-2021 at 12:12 AM. Reason: Spelling
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Unread 12-03-2021, 05:18 AM   #9
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Another quick question. I'm thinking I'll use my angle grinder with a diamond cup wheel to take down the old thinset. I've never used a cup wheel on my angle grinder before; I have about 180 sf to do. Do you think this will work or is there a better way? I'm retired, so I have lots of time...other than my wife asking how much longer.

The local rental place has a big walk-behind floor grinder with a dust collection system for leveling concrete floors, but that seems a little like overkill; especially at almost $300 for the day after it's all said and done. But if I have to bite the bullet, that's what I'll do.

I just had another thought. I have an air hammer and could buy a thinset chisel for it. Do they work as easy as the manufacturer's "propaganda" videos show?
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Unread 12-03-2021, 09:23 AM   #10
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The grinder/cup will work, Bruce, it'll just take a while to do 180 SF. It'll be dusty, too, so you might look into the accessory hoods they make so you can connect it to a shop vac.

The air chisel will probably work to get the bulk of the mortar off, just be sure to keep the blade angle low to avoid gouging the floor. You still might need to hit some spots with the grinder/cup.

Regardless of what you use be sure to where a mask.
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Unread 12-06-2021, 12:22 AM   #11
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Air chisels are faster at popping up tiles than removing the mortar. I’d use it to remove the tiles and then switch directly to the diamond cup wheel to grind the thinset mortar away. Definitely get yourself a dust shroud and hook it up to a decent vacuum. If you can install a $45 “dust deputy” dust separator between the tool and vacuum, you’ll save on the many, many filter cleanings you’d otherwise need to do.

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Unread 12-06-2021, 09:23 AM   #12
Bruce H
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Thank you everyone for the responses! I now have a clear path for removing the old tile and installing the new with crack isolation. I'm leaning toward a trowel application of two coats of Redgard to meet the ANSI specs, but the jury is still out on that. The best news is that I can justify some new tools/accessories! I'll let you know how it goes and if (when?) I run into Murphy's Law.
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Unread 12-26-2021, 07:59 PM   #13
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What is your opinion on grout sealers for my kitchen floor?

The grout I'm planning on using is Polyblend sanded grout and 1/4" grout joints. Home Depot carries two, both from Custom: Tilelab Grout Sealer and Aquamix Sealers Choice Gold. Or is there a better sealer I can get elsewhere?

As an aside, I ended up buying 12 x 12 porcelain tile, rather than ceramic like I originally planned on
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Unread 12-26-2021, 08:05 PM   #14
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What is it that you'll be expecting of your chosen grout sealer, Bruce?

Your porcelain tiles are ceramic.
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Unread 12-27-2021, 04:47 AM   #15
Bruce H
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I'm hoping the sealer will slow down absorption of kitchen "stuff" and make cleaning the grout easier. Something that did occur to me later after I posted this is that in all the tile specs I wrote for our commercial buildings we designed, I don't ever recall spec'ing a grout sealer. Maybe there was a reason for that I'm not remembering?

My understanding has always been that there's a slight difference between porcelain and ceramic tile. You're testing my old memory, but I think porcelain is denser (harder?) and has less water absorption?? And I ended up with porcelain tile only because HD had a color choice we liked better in a Daltile selection.

PLEASE correct me if I'm wrong or misguided, I have a thick skin. In writing specs, etc., I was never bashful about calling manufacturer's reps, contractors, etc.
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