Use mortar between OSB and old concrete subfloor, in shower?

Question

Replacing a shower pan in a 32x32 alcove. The prior acrylic base was bedded in mortar on top of concrete subfloor. The new pan I intend to buy has an OSB and foam underside, seemingly meant for a flat wood subfloor.

The old mortar is uneven, chunky, from prior install. I'll try to grind or chisel it down to be a little more level, but it's still won't be level enough.

So the question: which approach should I take?

A) self-leveling cement, allow to dry, then simply set base on it

B) same as above but add a waterproof barrier between OSB and cured, level floor

C) bed the OSB base into thick mortar without taking the extra step of leveling first

The instructions seem to say that the OSB can be set directly into mortar, but worried about that because my subfloor is concrete, so potential for moisture to come up from below.

Delta Classic 400 https://images.thdstatic.com/catalog/pdfImages/c6/c6ee3e0f-07bd-41ca-b00f-169f15d08ca2.pdf https://images.thdstatic.com/catalog/pdfImages/77/77861eb2-d9b8-41c3-ba4d-610fb28e576e.pdf

Answer 1

Most acrylic or fiberglass tubs and showers come on a base whether it be plywood or OSB. It is typical to set these in a mortar bed to support and distribute the weight of someone and water inside them.

Typically on new construction we do not do any waterproofing and the mortar bed goes directly on the slab (or the plywood subfloor if it is a raised floor).

If you have doubts or concerns about moisture coming up through your existing slab then you can simply lay some visqueen (plastic sheet goods- like a plastic drop cloth) down between your slab and the mortar bed- typically we use 6 mil thickness for this.

Just a tip- do not lay the mortar solid across the surface- rather lay down lines of piled mortar- maybe 2" tall and 3" to 4" between them. This allows the mortar a place to go as you set and level your fixture. Too much mortar and you will not be able to compress it all the way down and you will be fighting to get it in place and level.