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Unread 07-29-2019, 11:12 AM   #1
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Tiling a painted concrete slab floor with efflorescence issues

Hello Everyone,

I am in an area near the coast with a high water table. The bottom level of our home contains a studio/workshop area (leads out to pool) of about 450-500 square feet that has a concrete slab floor. We did a partial renovation several years ago of the space and tried to do a concrete stain on the floor. That failed, so we had it painted soon after with an industrial paint.

Fast forward years and the paint has peeled up. In addition, there are what look like salt deposits which I have to come to learn is likely efflorescence in several areas.

Long story short, the floor looks terrible with paint peeling and constant “dustiness” from the efflorescence and I would like to tile it to solve the problem. I don’t want tiles to pop or have it come up through the grout later on.

I am thinking of DIY, or if not at least know what the latest products are available to help get the job done right.

Can anyone let me know what are good products and in what order to lay them down? What do I need to look out for?
  1. What is the best way to get rid of the existing paint that is still there covering about 70% of the floor? I know I can scrape up a bunch but what about the parts that don’t come up easily? Sand it?
  2. Do you think the old concrete stain presents any issues?
  3. In my brief research, it looks like I could use Ditra as a vapor diffuser. Is that right? What type/brand of thinset should be used to put the Ditra down onto the subfloor?
  4. Do I need to seal the seams of the Ditra? What about the edges where it meets the wall?
  5. Is there a special thinset I need to use to lay the tile onto the Ditra?
  6. Is there a suggested tile, tile brand, tile size to use to help with the issue? (Same with grout)?
Thanks in advance! I'm a newb.
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Unread 07-29-2019, 02:00 PM   #2
Tool Guy - Kg
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Welcome to the forum, Regan!

The very first thing to consider is asking where the moisture is coming from. Okay, you've got a high water table. Is there anything else that could be contributing? Like rain gutter down spouts that discharge close to the house? Or a back-pitch to the grade around the house?

1) Shot blasting would be quick. Look up online who offers the service near you. Or, you could scarify the floor. This is a specialty tool that removes coatings and paint and anything else in a big hurry...picture a tool that has 500 tiny hammers at the business end that beat the snot out of the surface of the slab, causing little micro chipping to occur. You can rent this tool. Or you can check with that same rental center to see if they have any floor polishing equipment with a specialty head to scrape away the paint. Or....the least desireable option is to grind the floor with a diamond cup. This last option is usually the least desireable as the paint may put up a good fight trying to clog the diamond cup...and it'll take awhile. And while this option can be outrageously dusty, you can rent a grinder like a Bosch AK1771 that has a built in dust shroud that makes it considerably cleaner than you might think if you can hook it up to a good, well-filtered shop vac (assuming you clean the filter quite often during the process).

2) Any of the above processes will take care of any concrete stain that remains. The whole object is to remove the coatings so that if you drip some water on the concrete, that it soaks into the pores. This part is critical to getting a good bond later on.

3) Ditra is your best and only friend in this situation. You'll be using a high quality mortar meeting ANSI 118.1 like: Ditraset or Kerabond.

4) Yes, seal the seams between the pieces of Ditra with KerdiBand. No, do not seal anything against the wall. This area is a place where moisture will evaporate from a liquid under the Ditra to a gas. You don't need to do anything special at the wall. Just stop the Ditra short of the wall by 1/4".

5) Follow the manufacturer's instructions. A high quality unmodified mortar meeting ANSI 118.1 is fantastic. It works better than ok. But realize that practically the entire rest of the industry thinks that Schluter is crazy for allowing this. The generally feeling from most folks is: Well, if that cheap $19 bag of unmodified works ok and it only has a bond strength of "X".....then this other $29 bag of modified mortar that has a bond strength of 2X.....or 3X....or 4X...or whatever *must* be better. Well, yes and no. The fact of the matter is that unmodified mortars have been used for literally thousands of years and while the bond shear strength might be 100 pounds per square inch and that doesn't sound like a lot....you could literally park a car on a single tile hanging on a wall if you could somehow figure out how to balance the car on the ledge of a tile! But, here's the part few people talk about: Unmodified mortars require some skill in reading directions. This part passes over many folks that lay tile for one reason or other. You need to follow the directions on mixing, slaking, keying mortar onto the backs of tiles, and troweling the ribbons of thinset onto the floor in a single direction. In other words, you need to follow best practices. And lots of folks would rather use a modified mortar that was automatically a little stickier and stronger thinking that they have some built-in margin of error...that they can be a little sloppier in working with it and still have okay results. That's partially true. And is probably a key reason that the rest of the industry really, really 'requires' you to use a modified mortar with their other products. While an unmodified mortar would work fine, these manufacturers are run by marketing folks who understand the consumer's basic need to not read directions, but still have a successful install. Part of the problem with modified mortars is that in a few cases, modified mortars have to physically dry in order for their chemistry to develop the bond strength. And that's a problem with Schluter's basically impervious membranes when used with other substrates and/or floor coverings that are also basically impervious. So, Schluter has wanted to avoid the occassional problem of using certain mortars in certain situations with potentially disastrous results. So, why doesn't Schluter just compile a list of mortars that are compatible and those that aren't? Well, just think for a second....how on earth could Schluter keep track of all the manufacturers and all their mortar formulation changes to know what to recommend and what to avoid? So, they've told us to stick to unmodified mortars. After many years of complaining, they've teamed up with a manufacturer to product DitraSet...basically to put people's minds at ease for using an unmodified mortar. After many more years they've develped a modified mortar that is compatible with their product for those folks that still really, really want to use modified mortars. So...that's a semi-brief and incomplete history of mortars relating to Schluter membranes.

6) Pick whichever size you like. The Ditra is handling the moisture problem, not the tile grout lines. I personally would look for a porcelain tile that was flat-flat-flat. There are tons of high quality porcelain that look great these days. However, many of them are bowed where the middle of the tile is higher than the ends. While this doesn't cause a problem when you install square tiles in a regular grid pattern, it does make for a sloppy install if you attempt to put them in a running bond pattern (1/2 tile offset). Especially with loooooong skinny tiles. If you find a tile you like, put two tiles together "good-face-to-good-face" to see if they rock and roll on top of each other. Hold them up to the light like a sandwich to see if you can see light between the layers. You want flat tiles...not slightly rounded tiles. It's for the reason of non-flat tiles that the industry recommends offsetting large format tiles no more than 1/3rd their overall length. As far as grout, I love Laticretre Permacolor as it dries to the expected color...which can't be said for all grouts all the time.

P.S. How come that support pole is on top of the slab? The bottom of that thing belongs on a footing under the slab.

Tonto Goldstein... but my friends call me Bubba

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Last edited by Tool Guy - Kg; 07-30-2019 at 05:15 PM. Reason: Correct the ANSI standard to 118.1
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Unread 07-29-2019, 07:28 PM   #3
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Hi and thanks so much for the welcome and the information. You are awesome for responding so thoroughly! Much appreciated.
  1. I’ll definitely look into shot blasting. All other advice here is well taken. I want to try to avoid the dust if at all possible.
  2. Good to know.
  3. Ditra does sound the best. Glad to learn about this product.
  4. Okay, I’ll do that. Reminds me, I imagine I’ll have to remove the existing baseboard before tiling.
  5. Wow – thorough info. A bit intimidating for the newbie like me but nevertheless this information is GOLD. I’ll refer to this whether I take the DIY route or not.
  6. Okay good! I was looking at the faux wood grained look, which typically are more long and skinny. I’ll take the tips.
(I have no idea re: the support pole. I should look into that.)

I appreciate your suggestions!
So glad I stumbled upon this discussion forum and site this morning.
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