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royboytn 11-14-2011 03:12 PM

Is thinset waterproof?
I have slight dampness on my concrete slab. I want to ceramic tile over it using thinset. Should I Drylok it first? Other posts say to REMOVE Drylok. Will the thinset alone keep moisture out from underneath the slab?

oh'mike 11-14-2011 03:41 PM

Thinset is unaffected by water and will not let go of the tile or concrete if subjected to water.

It is not 'waterproof' --water will pass through it---so if you have moisture coming through the slab it will eventually work it way up to the back of the tile---grout will allow water to pass---

The thinset might not bond well to Drylock---

JazMan 11-14-2011 03:44 PM

Hi Roy,

Thinset mortar is waterproof in that; it can be used in wet areas such as a shower or outdoors.


Will the thinset alone keep moisture out from underneath the slab?
Impossible, but maybe you didn't meant to say "from underneath the slab".

Thinset will not prevent water from going through it from either the top or from below. It is not a waterproofing.

In addition all surfaces must be in proper condition and dry. It sounds like you have a moisture problem, thinset nor anything else will cure that. Furthermore you shouldn't install anything until it's corrected. Trapping moisture under tiles or any other floor covering will cause mold and rot.

This is what one of the thinset manufactures has to say on the subject.


Concrete must be free of efflorescence and not subject to hydrostatic pressure.

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