I've taken my shower stall down to the studs, and removed the shower base. I'm on the first floor of a condo, and the base sits on a slab (see pic). It appears that the original installer (20+ years) used some type of mortar under the base, as it took on the imprint of the shower base bottom. If the slab is level, do I need to put any underlayment down for the new shower base, or can it sit directly on concrete? It will be a one-piece acryllic shower base, with a 3 piece direct-to-stud shower surround unit.

[drive.google.com]



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You need to follow the shower base manufacturer's instructions. It should state if mortar is required. If it's optional, use the mortar.

You definitely want to use mortar and make sure your shower drain is solid and not flexible so the silicone will stick around the ABS/PVC. If you have cast iron it is not usually an issue. I have had to add silicone over the rubber seal that is used to seal the drain to the pipe.

Thanks for the reply. Typical of instructions, they are vague. "If the subfloor is flat and level, apply adhesive on floor
and spread using the notched side of the trowel." They don't indicate what adhesive to use. What would be appropriate for a concrete floor?



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you absolutely need support around the center of the new base where the big square hole is.
fill it with cement leaving a 5 inch hole for clearance for the shower drain fitting.
then just make a few pyramids of mortar not mixed very wet. smush the base gently into the mounds of mortar making sure it is level.
do dry fit the base to make sure the height of the trap riser is correct.

I'm doing a new acrylic shower base install over a new, 98% level OSB subfloor. Mortar seems to be the way to go as also indicated in this thread.

Soooooo, with 500 different types of mortar out there and me being a non-professional DIY-er, WHAT KIND OF MORTAR?????

Will this work: MAPEI Ultraflex White Powder Polymer-Modified Thinset Mortar ( [www.lowes.com] ) ????

I see that a thick mixture and several pyramids so it squishes in is also an appropriate installation technique.