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MikeDobbs 06-23-2012 10:42 PM

How smooth does the surface under Ceramic Tile need to be?
I am remodeling a bathroom that is on a poured concrete slab. When I pulled up the old tile some thinset mortar stayed behind and is very difficult to get off. I am wondering how big of a deal that is?

If the surface is slightly bumpy can I just trowel on a bit more thinset when laying the new tile so long as I make sure the tile is sitting nice and level?

If not, what are my options for making the floor "smooth enough?"

I was thinking of perhaps laying down a layer of thin set and putting concrete board down on top of that. I figure the thinset will bond the concrete board to the slab, and I can level the concrete board as it goes in.

Thoughts? Thanks in advance for any comments :thumbsup:

CopperClad 06-23-2012 10:46 PM

If its just a small bathroom just chisel the thinset out. No need for concrete board.. You're already on concrete. Smooth like a babys bottom sir.

oh'mike 06-24-2012 05:14 AM

There is a product called 'self leveling compound' --SLC can be used to achieve a flat floor --

It is not cheap, but will give you the smooth flat floor you need for tile.

allfloorremoval 06-24-2012 06:29 AM

Scrape or grind "SMOOTH"

MikeDobbs 06-24-2012 09:56 AM

Thanks for the feedback everyone. I'm aware of self leveling compound, but am very nervous to try it due to the size of the space. This bathroom is pretty large (roughly 10x13) and from what I understand SLC sets very fast so I'm concerned about having to remix additional buckets while the first one or two are setting up already and ending up with a bigger mess than I currently have.

Would my idea to use a cement board work? I understand that for a pro SLC might be the easier or faster route- but as I am just a homeowner, I don't mind spending some more time or money laying concrete board if it will work. With thinset and concrete board I can work slowly and not have to be under the gun as it sets up. I can trowel on, lay out, and tap level, then move on. What do everyone think?

Thefloorman 06-24-2012 09:59 AM

You do need to get up all the thin set. If you float over it you may have a problem resetting the toilet back after changing the elevation with the float. Not to mention base boards, doors, etc...... If it is really tough rent a 30# chipping hammer. Lots of fun. :0)

CopperClad 06-24-2012 10:02 AM

Thinset comes up fairly easy on concrete. Just get about a 3'' cold chisel and tap tap tap and away! You can also grind it , but that creates a huge mess! It will take some time with a chisel but man you will feel so proud when complete ! and if your adding concrete board your creating a step up from your bathroom to whatever is on the other side of the doorway. An uneven transition. Good luck

Bonzai 06-24-2012 10:11 AM

If you don't have the skill set to lay self leveling cement then you're unlikely to have the skills to install cement board without any air pockets and all perfectly uniform ... Not that this would be a good idea any way. Ideally get the remaining thinset off ... Yes it can take a little while but there are no short cuts to doing a job properly.

yummy mummy 06-24-2012 10:20 AM

I had the same problem when I tiled my basement. There were areas that I needed to chisel away.

If yummy mummy can do it, then you can do it too, trust me, I don't have that much strength but I can kick butt with a
Yes, it will take time, but will be worth the effort.

Good Luck

MikeDobbs 06-24-2012 10:22 AM

Thanks again everyone!

OK, so I'm getting the message loud and clear- the thinset needs to come up.

I'll pick up a chisel or rent a power chisel to get that done. Do I need to be concerned about pits in the surface? In some areas there are little spots where perhaps I was a little overzealous with the power chisel when taking up the floor and there are slight indentations in the concrete surface. Will this just be filled in by the thinset mortar when I tile or do I need to go back in with SLC to hit those tiny areas indivudally before tiling?

Bonzai 06-24-2012 10:27 AM

If you have pits left in the concrete you can fill them with a fast setting concrete patch compound ... Often comes in a small bucket that you just mix with water and apply with a trowel.
Are you putting down a crack prevention membrane before tiling?

MikeDobbs 06-24-2012 10:29 AM

Thanks for asking Bonzai- I'm putting down an electric radiant heating system under the tile, so I plan on laying down a 1/4 inch cork underlayment as per the manufacturer's specs to provide some degree of insulation from the slab.

Thefloorman 06-24-2012 10:30 AM

No worries on the chug holes. You can fill them with the thin set as you install. But if you set your chisel at the right angle you should not go into the slab at all. Good luck.

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