Our existing tile shower stall in the upstairs Master bathroom was leaking and we are finally replacing it. We decided to use cultured marble for reason of ease of maintenance. There was considerable rot after the demo, so the contractor fixed the subfloor and put some additional layers of ply on top.

The cultured marble shower pan sits right up against the studs on the exterior wall, and there is apparently an almost � inch drop between this edge and the unsupported opposite edge. This may be attributable to the age of the home (25 years), but at any rate it�s a real problem since the pan needs to sit flat.

The cultured marble installer (different company) put some shims on the floor to level the pan, then slathered drywall mud under the pan to support it. When I saw what they did I called the owner and said that I was not comfortable with drywall mud as a load-bearing base for the pan. He told me that they have never had a problem with it, and we agreed that they would pull the pan and remove the existing mud, but now I am on the hook for leveling the floor.

My hope was to simply use some self-leveling compound since the area is outlined with 2x4s and the compound should settle nicely within that area. However, I called one company to ask them about the use of their self-leveling products for that application and they said that was not the proper solution. They instead recommended making a base using Sand/Topping mix.

My concern is that everyone I talk to seems to only be familiar with tile shower installations. While I understand that the Sand/Topping mix would be correct for a tile base (with extra sand mixed apparently), is it the best way to build a flat surface for my already waterproof cultured marble base? I want to do the right thing but I am very doubtful that I can trowel something exactly level.