laying slate over tar-sealed concrete? [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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02-19-2010, 08:43 PM
Howdy helpful folks,

I'm going to install slate in our newly-rebuilt laundry room. Part of the floor is a concrete slab, and part of it is new wood framing (yes, it meets L/720 deflection). The concrete is about 50 years old, and had some sort of rubber tile adhered to it. The adhesive under the old flooring isn't coming off any time soon, and it seems to be an asphalt-based material. Am I going to have any problems thinsetting slate to that surface? I'm using Laticrete's Multipurpose Pro if that makes any difference. On a related note, I had to cut into the slab to install some new conduit & plumbing, so there's a section of new concrete down the middle. Should I worry about putting some sort of vapor barrier between that and the new flooring? Is blooming an issue?

Finally, can anyone give me a specific recommendation on a sealant to use as an expansion / flex joint between the concrete-supported and wood-supported sections of the floor? I'll plan for a grout line at that spot, and figured there was something that wouldn't look entirely unlike a grout line, only would be a rubbery material. I don't think I want to use Schluter's joints, as nice as they are.

Thanks in advance,

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The Kid
02-19-2010, 09:14 PM
Hi John,

I would definatly machanically remove what ever it is thats on the old concrete. A good test is add water to the slab, if it soaks right in, then you should be good, but if it dont, well then you got to remove whatever is sealing that concrete in order for the new to bond properly.

What is blooming? I would use a membrane to address the cold joints.

Lotsa companies make a color match accrylic sanded caulking to match your particualar grout color for that control joint.

02-19-2010, 09:14 PM
Hey John,

I'm sure someone will be along to get to most of your questions soon. I just wanted to ask why you are opposed to the Schluter movement joints to transition between the wood and conctete substrate? That's exactly what they are designed for and they come in many different finishes and colors. To me it just seems so much better and cleaner that squeezing some tube of pookey into a gap. Anyway, just curious. Good luck!

Oh, and I think all of that tar stuff is going to have to come off the concrete. But let's hear it from someone who knows better than me.

02-19-2010, 09:49 PM
As far as not wanting to use the Schluter joints, well... I'd have to order it, which means hassle.
It's very straight and flat and even, which is not really a good match for the organic look of the slate I'm using nor a good match for the other grout lines.
The floors don't match perfectly, so the flanges on either side wouldn't lay in the same plane with eachother, putting a permanent distortion in some parts of the rubber.
If (when) something happens to the rubber, I'll just have to repair it with caulking anyway.
Most importantly... The slate is on my back porch and I want to install it in the morning!


The Kid
02-19-2010, 09:53 PM
John, if your floor is so far out that a expansion profile wont sit flat, you got some problems that need to be addressed prior to tile instalation. What about the bond break on the concrete, how are you going to handle that?

02-19-2010, 10:48 PM
The unevenness at the concrete-wood junction is on the order of 1/16", which is easy to fix with thinset while setting the tiles level across the gap.
And it's not as though the floors are out of plane, it's just that the edge of the concrete isn't perfectly flat.

The concrete floor does seem to be water permeable in about 20% of it's surface area, where the adhesive is thin and you can see the concrete. It looks like it was applied with a notched trowel like you'd use for mastic, but what's left is no thicker than a layer of paint.

I'm not sure I understand why it would be a problem. I mean, how different would it be if I just put a layer of Redgard or similar on a concrete floor? You lay tile right onto those barrier products, right?

Blooming refers to the migraton of dissolved minerals up through some substrate, being deposited on the surface as a white powder or stain.


02-19-2010, 11:16 PM
Redguard is engineered to adhere to thinset. Tar (or whatever that stuff is) was probably not. That would be a gamble since we don't really know what it is. Thinset adheres to concrete which is under the unknown black substance. Hence the recommendation to get rid of the black stuff down to a substrate we know adheres to thinset. Your project and your call.

The Kid
02-20-2010, 12:15 AM

only 20% permeable seems like a major gamble to me, how big of an area are you talking about?

IMHO, grind it down. Whatever you have there, its definatly not going to work like a membrane, In fact, it will more then likely do the oppisite and become your bond break. Dont belive me, tile it and then call me when it fails.

What you are refering to as blooming is what we call efflorecence, its typically salts/minerals migration to the surface.

02-20-2010, 12:48 AM
It's only about 35 sq. ft. Hope the neighbors don't mind the grinding at this hour!

Just kidding. I'll hit it in the am. Thanks for being more persuasive than th little voice in my head that told me the same thing.

So, academically, what's the deal with these liquid-applied decoupling membranes then? Cracks in the substrate don't telegraph through, but the tile and thinset are still well-bonded?

I've obviously never worked with any of them. I figured they were sort of like elastomeric roof sealant, leaving a thin rubbery layer.


The Kid
02-20-2010, 01:12 AM
your on the right track, just alot more science behind it. Snets is correct however, anti-fractures and uncoupling membranes are designed to have a bond coat directly over top of them, where other products are not.