Can I use modified mortar on waterproofed shower walls?

Question

I was told by a tile installer that I should not have used waterproofer on the walls now the thin set will not cure. He will have to use mastic for the tiles to adhere. But my research says never to use mastic in a shower. What is the correct product for installing the tile?

Answer 1

I have built dozens of tile showers, using that exact procedure. Cement board, Aqua Defense the entire walls, then modified thin set for tiles. I have never had any issue. This over about 20 years or so.

Mastic however should never be used in an area that is subject to water.

This from Mapei Aqua Defense Installation instructions : "... For tile and stone installations, use a MAPEI polymer-modified mortar meeting ANSI A118.4, ANSI A118.11 or ANSI A118.15, or an epoxy mortar meeting ANSI A118.3."

My advice is to find another tile installer.

(The 24 X 48 tiles are a bit big for most showers, unless the shower is extra large. The big tiles may result in just having the majority of them needing to be cut. However that is my editorial and has no bearing on the issue with Aqua Defense.)

Answer 2

Hire a different tile installer, this one's nuts.

First of all, you should never use mastic in a shower or other wet area - as you already found out.

Some adhesives need to be exposed to air in order to cure. If you stick them between two non-porous surfaces, they'll never cure properly. But thinset mortar is definitely not in this category. It works by bonding chemically with the water, creating a hard material. It would harden even if you sealed it in an airtight bag.