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Unread 07-15-2021, 05:01 PM   #1
Kschlach
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Mortar help

We’re redoing our bathroom and trying to finish up the wall tiling. We have Kerdi board in the shower and then green board on the walls outside the shower. We did the back shower with large tiles and used Mapei rapid LFT mortar we were told to use by Floor and Decor employee. Not realizing we weren’t suppose to use modified thinset. It has been curing for about three weeks now, so I’m not too concerned about it. We were running out of time so hired a tile setter to finish the floor and other walls. He said he was going to use mastic on the wall - which doesn’t seem to work with Kerdi or in a shower! We will make sure he uses mortar. However, The subway tile will be set with the built in grout seam so there won’t be a whole lot of area for the mortar to dry out which made me feel the need to get Unmodified thinset. Can’t find the schluter All set anywhere. So I went to Lowe’s and bought Mapei uncoupling membrane mortar which says it can’t be used on wall board. Any suggestions on a mortar that can be used with Kerdi board and green board as the tiles will go over both??
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Unread 07-15-2021, 05:15 PM   #2
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Common wisdom on these boards has it that most modified mortars are completely ok to use with Schluter products from a practical standpoint. It seems that all other sheet membrane manufacturers allow or even require the use of modified thinset mortar.

Problem is that most DIY people are intimidated by the Schluter prescription and then end up with a low quality unmodified (such as Customblend). I seriously think that Schluter with that requirement inadvertently causes a lot of problems on that end.

As a DIYer I have done two Schluter showers. For the first I used unmodified, for the second I tried a high quality modified. In terms of workability, bond and performance, the latter was a much better product to deal with.

Seem like you put some sort of primer on that green board? I hate green board, as its waxy surface can be a bond breaker. I have seen that with properly primed paints peeling off. I'd probably rough that up with some emery paper before tiling. There is no reason why thinset mortar couldn't be used throughout the bathroom. Mastics are ok for areas that don't get wet, but using modified thinset mortar would be my choice.

I like Custom Flexbond as well as Versabond. Easily available at your local Home Depot and a good product. The Flexbond is probably overkill for most situations, but it's very creamy and sticky. Just a great product. The Laticrete mortars are also fantastic if they are available to you locally.
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Unread 07-15-2021, 05:32 PM   #3
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We haven’t put primer on the green board but have used joint compound at the seems and screw holes. Do we need to do anything to the green board? You can’t use mastic on Kerdi board, can you?
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Unread 07-15-2021, 06:06 PM   #4
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You can use Mapei's Eco Prim Grip on the green board, it's a primer made just for tile. I wouldn't use mastic, I would use a modified thinset on everything for the reasons Wolfgang mentioned.
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Unread 07-15-2021, 06:25 PM   #5
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Do we have to put a primer on the green board? Are you suggesting to use a modified thinset on Kerdi? Should we use the Mapei Ultraflex LFT rapid or regular? Am I too worried about this? We had planned on doing tile ourselves (did one wall) but the time was too much a burden so we got a setter to come finish it. I hadn’t given the thinset too much though as Floor and Decor told us to use the LFT, but when ruler said he was going to bring mastic to use on the wall (Kerdi and Green) I did some more research because I don’t want this bathroom to have problems down the line. I wish I had bought Schluter All set just to ease my mind if anything. My guess is if they allow their own modified, a high quality Mapei is sufficient. The smaller grout joint on the subway tile also concerns me, but their also only 4x16 and not big old 12x24 tiles. And then the green board started worrying me when I went out to get the Mapei uncoupling membrane mortar and it specifically says no wallboard with water mix only. But you can’t use the polymer mix with Kerdi and have to use water! Sigh. Thanks for all your insights.
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Unread 07-15-2021, 06:42 PM   #6
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We didn’t do anything to the joint compound, so should we try to prime bothered joint compound and the green board before the tile setter comes back tomorrow? I had read that thinset works well with new drywall that was not primed. Tile installer didn’t seem to have an issue when he did floor today.
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Last edited by Kschlach; 07-15-2021 at 06:55 PM. Reason: Update
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Unread 07-16-2021, 04:19 AM   #7
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Kyle, I go against Schluters recommendations and use modified mainly because it's hard to find a good unmodified thinset. I can get modified just about anywhere. I'm not going to tell you what to use but I haven't used unmodified in many years. If you question the thinset, stick a piece of scrap tile to a piece of Kerdiboard and let it set.

Modified will stick to the green board real well. I like using the Mapei primer over sheetrock and greenboard but it's not required. When I'm hanging new sheetrock that I know will be tiled, I bed the joints with thinset.

The main reason some installers like to use mastic is there's no mixing. You probably wouldn't have any problems using mastic on the greenboard but it will take longer to set behind those large tiles. No way do you want him to use mastic in your shower. Over the Kerdi, it could take a couple months for it to set. Plus, it may emulsify. I question your installer after wanting to use mastic in your shower.
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Unread 07-16-2021, 07:36 AM   #8
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Davy, thanks for the response. The rest of the wall will be 4x16 subway tiles. Do you think a modified thinset would cure properly behind those? The installer also suggested setting the tiles without spacers and just use the built in groove that leaves a 1/16 space (though it seems much smaller). I’m questioning if the tiles are physically touching if the thinset could actually cure properly. If we were to prime the green board, could Unmodified be used on it despite the package saying not to use it in wallboard?
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Unread 07-16-2021, 07:50 AM   #9
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use a modified thinset like the manufactures of the thinset say..(considering they make it that's who I would trust)
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Unread 07-16-2021, 07:57 AM   #10
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Do you mean the thinset manufacturer? Or Kerdi? The say Unmodified unless it is their brand of modified… The Mapei
Unmodified we have says it’s OK for Kerdi but not for wallboard. The modified Mapei we have says good on wallboard but Kerdi says not to use it
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Unread 07-16-2021, 08:09 AM   #11
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Follow the manufacturer of the thinset. Would you ask the paper company or the glue company what to use to stick.
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Unread 07-16-2021, 12:23 PM   #12
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Versabond for all of it.

And I'll echo Davy's concern about your installer wanting to use mastic over Kerdi, and under large format tile. Either of those would be cause for a do-over.

If you decide to keep him, I'd check his work often to make sure it's being done properly.
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Unread 07-16-2021, 12:39 PM   #13
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The built in lugs on the tiles will leave you joints about 1/16. The pic below shows 4x16 tiles with lugs but the homeowner wanted bigger joints so I used 1/8 wedges. We installed it with a 1/3 stagger.
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Unread 07-16-2021, 12:58 PM   #14
Kschlach
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I did prime the green board with Mapei Eco prime grip. After speaking with both Mapei and Schluter tech service and the feedback here, I feel good about using the LFT for all the walls (especially since that’s what I used and n the one wall and the Kerdi band!). I’ve confirmed that thinset will be used in the shower though he may want to use mastic on the areas outside of Kerdi (is using mastic that much of a time saver to have to use two different materials??).
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Unread 07-16-2021, 04:29 PM   #15
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No, in my opinion it doesn't save enough time to be worth it. Mastic costs more than even the higher priced thinsets.
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