Help- Need Advice for Laying Porcelain Tile on Concrete Slab [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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07-08-2010, 06:56 PM
Advice would be so appreciated here. I want to tile a large area consisting of living/dining/kitchen/halls etc. with 12 X 24 inch porcelain tiles. We have a 50 plus year-old concrete slab with radiant heat.

Our slab has several cracks spanning the length of the rooms, going different ways. In some places the crack width is approx. 1/8 inch. The floor isn't level, but it's not too bad. I don't see any places where one side is higher or lower along the cracks.

There is residue from the previous flooring which was old cork tiles and old linoleum. Mostly just old glue, I guess. We did the water test, and it doesn't soak in. This is trouble right? How easily should the water soak in before I know I have enough old adhesive removed? What should be used to remove it?

Also, I am hearing different ideas on how to address the cracks. Is there a strip-type membrane to stick on? How well does it work? I think Ditra might be the way to go. I need to make sure it will stick down well to the slab. I need modified Thinset, right? Do I need to remove old glue for the Ditra to adhere properly?

Wow, a lot of questions. Also wondering about floating out the floor. If I put down a layer of concrete would it solve many of my problems, or would it also not stick where there is old adhesive and crack?

Thank you soooo much and I look forward to hearing from you all.


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07-08-2010, 09:15 PM
Hi Monica, welcome. I would use a crack isolation membrane over the whole floor, myself. Something like Redgard, Hydroban or Semco is what i would use, although there are others on the market. I think these would be cheaper than Ditra and probably work better for a crack membrane.

With any paint on membrane, get the floor scraped clean with a razor blade scraper first, Home Depot has 4 inch scrapers in the paint department. I like to wash the floor with a bucket of water and sponge to get up all the dust. Then spread a thin layer of membrane over an area with a flat trowel and then add another layer with a paint roller. The second coat will add the thickness you need. After doing a small area, back up and do another.

Brad Denny
07-08-2010, 09:16 PM
Welcome to the forum, Monica. :)
Ditra calls for unmodified thinset over concrete, but there are a good number of folks that like to use a lightly modified thinset, namely Versabond.
You will need to get the slab to a point where it can absorb water to create the proper bond for any type of tile setting material. You can rent a floor grinder to do the job, or call someone who specializes in concrete grinding.
Keep in mind that Ditra does not advertise to bridge cracked floors, only to provide an uncoupling action to the assembly in the event of movement, but it sounds like you have horizontal movement only and it might do well under those conditions.
A mortar bed ( would be the best solution, followed with a good crack isolation membrane, but you need at least 1 1/4" of depth from the highest point in the floor to have a solid bed. That can be difficult for transitions to other rooms.
We love pichers 'round here, so if you can post a few more times to get past spammer's rules, you are welcome to give us a view of what you have.
Hope that helps. :yo:

07-09-2010, 09:09 AM
Welcome, Monica. :)

To clarify a bit of what Brad said, any visitor can post photos using the Manage Attachments feature below the Reply dialog box at any time. It's only links to other sites that are restricted by our anti-spam measure until you have a few posts.

And we like pichers. :)

07-09-2010, 07:56 PM
Thanks for the advice. I'll try to take some "pichers" of the cracks and the adhesive soon.

Oh, also wondering if an acid wash substance will make a difference with the adhesive, or do I really, really just have to scrape it off if I want to do it right?


Brad Denny
07-09-2010, 08:35 PM
Sorry, Monica, acid won't touch those adhesives, and scraping is a necessity for it to last. :shrug:

07-12-2010, 02:38 PM
Looks like I have to face the facts and remove the adhesive. My husband and I both have bad backs, probably because we do so much ourselves already.

I'm only 5' 1" tall so don't know if I can wrangle a grinder by myself. My husband's back is super messed up at the moment so he can't either. About how much would I expect to spend if I hired someone to grind down about 800 square feet? FYI- the layer of adhesive is pretty thin and spotty.

For scraping, what exactly do I need? I am guessing there is some type of specific tool that the blades work with?



Scottish Tile and Stone
07-12-2010, 02:55 PM
The scrapers are 4" hand held scrapers.. In the paint section of The box stores. Sounds like there isnt much down.. You should have no problem scraping it up..

BTW Im only 5'3" :)

07-13-2010, 03:39 PM
Thanks Scott, I should be able to handle that with a few trip to the chiropractor afterwards-LOL!

It seems like the Ditra might be overdoing it for the floor I have. My plan is to scrape up the residue, roll on Redguard, then use an good quality acrylic-based thinset for the porcelain tile. Anyone think this is NOT the way to go before I dive it? If so, what would your suggestions be?

Thanks again. Loving all this knowledge for the forum.


07-14-2010, 04:26 AM
Monica, those scrapers come in a long handled version, almost as long as you or Scott are tall. That should make scraping a bit easier on your back.

You could call over to the high school and ask the football coach if he has any players who could use a bit more "conditioning" before practice starts. ;) A few bucks and a pizza or two, and you could be tiling!

07-14-2010, 10:50 PM
Bob, you are a genius.

Dave Taylor
07-14-2010, 11:10 PM
You wrote: My plan is to scrape up the residue, roll on Redguard, then use an good quality acrylic-based thinset

What do you mean by "acrylic based thinset"?

If you're talking about a ready mixed mastic.... don't use it... mastic won't last.

If you meant to say.........
"a good quality latex modified portland cement based thinset"
your product selection will be more correct for laying floor tiles.

I hope this helps

07-15-2010, 05:13 AM
Bob, you are a genius.Aw, shucks, Ma'am! 'Tweren't nuthin'. :yo: