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Unread 02-07-2014, 06:00 PM   #1
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Slate floor underlayment help

Hello all,

I've posted here before, and you were very kind to give me advice regarding my botched shower job. Someday it will be fixed, I hope.

Until then, I've decided to work on a different room. I want to install slate in a teeny powder room. Myself. It's a 5' x 6' room. I've chipped away the marble mosaic that was there to find the original porcelain mosaic peeking through a very thin layer mortar/thinset. The tile itself is visible in large sections of the small floor.

The original porcelain mosaic is 100+ years old and set in what looks to be a very sand-heavy mud (?). You can see the mud by looking up through the floor boards of my basement. It's a very coarse sand mix.

Can I tile over this? The floor is out of level only to the extent that the thinset from the marble remains. Won't the thicker mortar bed I need to use for slate "compensate" for the slight roughness of the old thinset? I don't see any cracks in the floor or tile, though there are holes from where the radiator used to be.

Would you recommend a decoupling membrane in this situation? The powder room is on the first floor of a wood framed house, above the basement. The POWDER ROOM hangs a bit lower than the floor joists because it's under the stairs (there's a step down).

Thank you

Last edited by Stefanie3434; 02-07-2014 at 06:02 PM. Reason: I used the wrong word and didn't catch pre-post
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Unread 02-07-2014, 08:58 PM   #2
Brad Denny
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Howdy Stef,
If the floor before didn't have cracks and there aren't any cracks visible in the original tile floor, then yes, you can go right over it.

I would personally try to scrape clean all the thinset off of the old tile and apply a liquid applied crack isolation/waterproofing product. A little insurance doesn't hurt. You'll need a decent latex modified thinset to install the slate. If you can't get off the old thinset you can "float" a smooth coat of new thinset over it, allow it dry, smooth out any rough spots, then apply the crack iso.

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Unread 03-10-2014, 06:07 PM   #3
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Predictable, you say

So the floor was installed. I did a skimcoat of medium bed mortar (the HD stuff), Redgard, and then the slate tile.

What I didn't mention with my subfloor question was that my newbie installation was a 2"x 8" slate in a herringbone pattern. With an 1/8 grout line.

I now see that this was not the ideal job for a non-professional. Huge struggle. We didn't do that bad, most of it looks pretty great. But we have three or four tiles that really should come out -- the lippage is a problem.

We haven't grouted yet. What's the best way to remove these tiles and replace them? Can it be done?
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Unread 03-10-2014, 06:25 PM   #4
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Yes it can be done. Very carefully dig the grout out from around the offending tiles and chip away until the tile is out. Remove the thinset so the new tile will set correctly and not be higher. (Thats what you are fixing!) Be prepared for collateral damage to the neighbouring tile. Good luck.

Just noticed you havent grouted yet...my bad!
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Last edited by evan1968; 03-10-2014 at 06:30 PM. Reason: im an idiot...
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