I accidentally laid Mapai aqua defense crack isolation membrane instead of concrete primer prior to applying a self leveling compound. How much trouble am I in?

I’m preparing my bathroom floor to lay ceramic tile. Removed the vinyl flooring to reveal cement which needed leveling. I was planning on first priming the concrete, pouring concrete leveler, pouring Mapai aqua defense crack isolation membrane, then tiling.

However I apparently forgot my plan (doing this late at night) and poured the crack isolation membrane directly to the concrete subfloor prior to concrete leveling instead of the dedicated concrete leveler.

Will this cause any issues? Can I continue to move forward with the leveler?

Might be helpful to know my house is on a concrete slab and in Florida (humid).


  • 2
    Have you read the instructions for the self levelling compound? They should be the best to find out what it can go on. Without knowing those products, I have a feeling it wants plain concrete.
    – crip659
    Commented Nov 7, 2023 at 14:32
  • If the instructions don't specify (they may not, this isn't an expected order of operation), contact the mfgr of the SLC and talk to them about it. If anyone knows, they'll be the ones.
    – FreeMan
    Commented Nov 7, 2023 at 15:53

2 Answers 2


According to the spec sheet of your liquid membrane product, once it's dry you can apply thinset over it and tile. So it is designed for thinset to stick to it.

It's not too far fetched to conclude that your self leveling compound will also adhere to it.

Basically, this membrane is liquid rubber, and primers are also a similar kind of liquid rubber, just much thinner and not designed to be watertight.

If your self leveling compound is designed to adhere to primer... it should stick to that membrane just fine.

Most self leveling compounds require a primer over bare concrete. The reason is not adhesion (it'll stick to concrete just fine) but to prevent the concrete from sucking the water out of the self leveling compound, which makes it harden in place too fast before it has time to smooth itself.


The AquaDefense is designed as a substrate for modified thinset, so put down a skim coat of modified thinset and apply your leveler on top of that.

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