What should I use to raise the height of floor tiles in a shower?

Question

Initially, the wall tiles were installed too high off the shower floor so if floor tiles were installed, there would be a gap at the bottom of the wall tiles to top of the floor tiles.

I hired a company that installed a second mud pan on top of the existing mud pan in order to thin set the floor tiles to cover the gap below the wall tiles. They had scored the mud pan with a grinder to install the second mud pan on top of the first. Then the porcelain tiles were laid on top of the second mud pan with thin set.

Years later the grout washed out from the shower tiles at one end of the shower, lifting several tiles up from the shower floor. I elevated the raised shower floor tiles from that section of the shower floor and was able to remove them. Then I scooped up and scraped up the second mud pan off the first mud pan. The first mud pan was completely hard and not disturbed.

Now I need to fix the shower floor so we can use the shower again. I am wondering what product to use to reinstall those shower floor tiles. I need to establish a bed of about 1 to 2 inches to get them at the proper elevation, so they will be level at the height of the 2/3 remaining shower floor.

Thin set could be used as a second step to adhere the tiles, but I can't build up to that thickness using just thin set. What should I be using to raise the mud pan about 1-2 inches?

How can I achieve a strong bond to the first mud pan besides relying on just the scoring that was done with a grinder, which I have now cleaned out?

Answer 1

if you are making up 2" of space, standard premix concrete is fine. you dont need anything else. its mass and mechanical locking expansion will hold everything together just fine. if its 1", you can still do it with premix, bud you need to add polymer modifier and i would put in a sheet of steel rebar mesh (3 x 3)

Answer 2

If you want it flat without having to screed, then a pre-made flat surface that is water resistant is a good choice, for example cement board. If you want 2" you'll need several layers, or you can use aerated concrete panels. I've used that with success, it's pretty easy. Old building, only drain available was 10cm above the floor. Fourth floor, no elevator, so the low weight was really a plus in the stairs... Don't use anything that isn't fine with being permanently wet, like gypsum.

This can make things much easier if you have a diamond-shaped shower pan and don't want to screed the thing. If it's a conical shower pan with a drain in the middle, then that won't work of course, you'll have to screed it.

How can I achieve a strong bond to the first mud pan besides relying on just the scoring that was done with a grinder

Concrete doesn't adhere to tiles. However there are excellent thinset mortars that are specified to adhere to old tiles. We just did a shower like that. So after cleaning and scoring the existing tiles we applied a bit of thinset on top with the 3mm notched trowel, that makes a cement-based surface with a good texture for the concrete to stick on. Then we poured concrete, ie sand and cement... and then screed. You might want to add some fibers or mesh in the concrete to prevent cracking.

Why not use just thinset? Well, it's meant to stick to things like walls. So if you try to screed it flat, all you're going to get is a big blob of thinset stuck on your screed.

Also I recommend epoxy grout for your tile joints. It's harder to apply, but much easier to clean, doesn't get moldy, doesn't crack... much better.