# sealing concrete slab prior to laying tile



## druishprince

I have a client who wanted some ceramic tile installed to his slab. While prepping the floor we took up some old vinyl flooring and it had mold or mildewed at some time over the past seven years. The homeowner contacted the builder and the builder sent out a liscenecd crew to clean up the problem. The builder also wants to cover the whole slab with RedGard to seal it. Is this a good idea or should the slab be allowed to 'breath". And is RedGard an appropriate sealer for this application.


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## Bud Cline

*Absolutely. * This is an excellent idea. RedGard is a moisture barrier AND a crack prevention membrane. Let him do it. Just be sure he is going to apply the proper "mil thickness" recommended by Custom Building Products. I don't know off hand what that thickness is but you can call a Custom Techie to find out.

Typically you wouldn't "seal" a slab prior to tiling but in this case and using this product it would be recommended.:thumbsup:


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## MattCoops

I much rather use Red Guard than that Drylock stuff.


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## Mike Costello

How smooth does that stuff dry?

I get to replace about 2700 ft of metrfloor solidity (with another product) that someone put on a slab that is reading about 6 on the moisture scale.

I need a moisture retarder and Im trying to find one that dries smooth enough for reslient


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## Bud Cline

It can be rolled or sprayed with a gel sprayer and it would easily be flat enough for resilient flooring.

I don't know about the 6# of moisture though. Might want to check that out with Custom for that much area. Sealing in that moisture could be double trouble.


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## Floordude

I could be wrong, but I don't think RedGard is recommended to be a moisture control membrane for resilient flooring.

It is a funny thing, liquid moisture vs. moisture vapor.

WF Taylor has something and so does ARDEX.

Dependable has Vaporseal.


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## Bud Cline

That's why I'm saying talk to Custom. Sealing moisture OUT is one thing, sealing moisture IN is a very different animal.

Sealing a slab with active moisture migration and then adding resilient on top of that would be double trouble.

Ceramic tile would be more likely to work.

I think we have two different threads going here for some reason.


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## Mike Costello

My fault, I kinda hijacked it, I apologize.

I have some research to do


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## druishprince

Bud Cline said:


> *Absolutely. * This is an excellent idea. RedGard is a moisture barrier AND a crack prevention membrane. Let him do it. Just be sure he is going to apply the proper "mil thickness" recommended by Custom Building Products. I don't know off hand what that thickness is but you can call a Custom Techie to find out.
> 
> Typically you wouldn't "seal" a slab prior to tiling but in this case and using this product it would be recommended.:thumbsup:


Thanks for the reply, I saw that later in the thread you recommended to another guy that sealing in moisture wasn't a good idea, which is what I also thought. I believe the homebuilder is just doing it to show the homeowner that they responded to the problem. I don't believe there is a excessive moisture problem with the slab. I also checked with a old timer who installs tile and he said that there should be no sealer on the slab if you want the tile to have good adhesion, but then again he said that to address the narrow cracks in the slab you could just tape over them with duct tape (that kind of makes sense some how )


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## Bill_Vincent

I'll tell ya-- I've got a problem with this whole thing of using a membrane where moisture coming thru the slab is a problem. I have a feelling that at best, they'll end up with the same problem they had under the vinyl with the mold and mildew. The thing I'd most worry about would be hydrostatic pressure building up and lifting the floor in spots. I've always thought (wouldn't be the first time I'm wrong) that if a slab is allowing moisture through, the best bet is to let it breathe thru the grout joints, That way, the moisture doesn't collect under the floor and cause problems.


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## Bud Cline

True enough!!!

The problem is....Products like RedGard are being marketed to DIY's as crack isolation membranes. DIY'S don't usually think about slab moisture or care about it unless there is a real obvious problem that has somehow come to their attention. Moisture in a slab and it's possible effects on subsequent floor coverings is an issue that is basically only widely known to professional installers. This is another case of putting some products into the hands of DIY's and not offering all the facts of potential dangers and failures if inadequate conditions aren't recognized and addressed.

Liquid waterproofing membranes can be great and easy products to use but is their use going to be the lessor of the evils in all cases? No, it's not!

The duct tape procedure doesn't even deserve the time it takes to comment about that technique. Just think about it. When crack isolation membranes are used to address only cracks and not entire planes it is recommended that the product be placed at least two tiles distance on all sides of the crack being addressed. Duct tape at only two inches wide doesn't qualify for that recommendation. Duct tape place in strips two or four feet wide.............well that's just plane stupid.

You just as well be using two-faced carpet tape to install your tiles if you are going to use duct tape. Some so-called pros just don't have a clue.


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