Do I need to waterproof my concrete foundation before applying thinset?

[KARENDEV27]

Hi all! I'm wondering, do I need to waterproof my concrete foundation before applying thinset, when installing slate tile in a basement (it's fairly dry)? What about a bathroom floor? Also, is anti-fracture membrane always required?
Thanks in advance!
-Karen

[Lazarus]

Karen....the LAST thing you want to do is to "waterproof" a slab before tiling. The sealer or waterproofer is a "bond breaker" when thinsetting tile.

Sprinkle some water on the slab. Does it absorb within a few minutes? If so, you're good to go for tiling.

Anti-fracture membrane is only needed if you have cracks in the floor. If they are only small horizontal cracks, then an anti-fracture membrane is in order....HOWEVER, if there is VERTICAL displacement......this is a floor you DON'T want to tile over.

[KARENDEV27]

Thanks, Laz, that is REALLY good to know!
-Karen

[MAPEI - Technical Service]

Does the basement see regular water intrusion? Some varieties of natural slate aren't a great choice for environments that are regularly wet, the minerals from the slate can (and do!) leach out of the stone into the grout and make it look rusty. I'd recommend a slate-look tile. They're lower maintenance too.

What is under your bathroom floor? If it's well cured on-grade concrete with no cracks, a crack isolation membrane isn't required. If it's above-grade and there is a chance of the house settling or deflection, a crack isolation membrane would be cheap insurance to protect your installation.