Leveling floor under shower base

[KillerCharlie]

I'm going to install a new acrylic shower pan, in an alcove, which specifies being set in mortar for stability. This particular model sits on a bunch of built-in rings

On the long end of the OSB subfloor it's about 0.25-0.50 inches low. The other half of the floor is very level.

Should I:

1) Level the subfloor with self-leveling compound, then set it with mortar

2) Roughly level the floor "by hand" using a cheaper non-self leveling compound, then set in mortar

3) Level and set in mortar in the same step - first put down mortar thickly to level it out, then set the tub in the mortar, taking more care to keep it level. This seems the cheapest and easiest, but I'm worried it'll be hard to level in the mortar.

 

[Greenman]

3.

 

[jmagg]

Any of those methods will work, but I generally want the substrate level extending past the leading edge of the pan so the finished floor meets correctly. Use full strength bonding agent (and let dry) under the flash to keep moisture away from the OSB, then tar paper under the mortar bed. I also use a level pencil line on the studs as a reference to level, from the leading edge of the pan. Remember the mortar will displace when you set the pan in.

 

[KillerCharlie]

What do you mean by flash?
My plan was OSB -> primer -> leveler -> tar paper or poly sheeting (not sure how to attach to leveler) -> mortar. If I use self-leveler the primer will be specifically designed for that brand of leveler.

My subfloor situation is a bit odd. The floor under the tub alcove is not level. The rest of bathroom subfloor is very level. There is a 5" wide transition strip of plywood between the two. The new shower will sit on part of that, and the remaining showing part of the strip will be sanded down to meet the rest of the bathroom.

For that reason it sounds like the best thing to do completely level the subfloor so the edge of the shower base isn't floating off the ground. If I make the leveled base too big (past the edge of the shower base), I'll have to grind it down to meet the bathroom floor.

 

[jmagg]

Flash patch would be by hand leveling under the paper.
Sounds like you have it worked out. Work hard to finish easy.