Leveling Kohler cast iron shower base

[Scarbo]

I'm preparing to install a 60" x 36" Kohler cast iron shower base. The base rests on the subfloor along the front (curb) and at four well-inset feet - really five, if you count the tab with a hole in it, most likely used during manufacturing.

I did a trial fit on a perfectly level table top using an engine hoist to lower the pan, which weighs 220 lbs. With the curb touching the floor and all four sides showing dead level, none of the feet touch. They all will need to be shimmed between 1/8" and 1/4". This looks like it will be a nightmare. How do I reach in 8" or 9" to shim under each foot once the pan is set on the subfloor, with framing on three sides? The instructions are worthless - "add shims as needed to level."

I'd appreciate any tips from anyone who has installed one of these beasts. Also, are metal shims needed, or could I use composite or UHMW - definitely not wood?

 

[oldVermonter]

Last year, I installed a Swan bathtub (<1/3 the weight of your beast), but I think my process would work for you:

1. Using clear plastic and a marking pen, create a map of your shower base, showing the exact location of each foot.
2. Flip the plastic over, and place it in the enclosure at the required position of the base.
3. Mark the location of each foot on the floor. I drove a nail through the plastic.
4. Now use a level to determine the relative height of each footpad.
5. Construct shims to create a level base.

This was time-consuming but it paid off—the tub was level on the first try.

 

[Sponsor]

 

[Scarbo]

That's an excellent system to prepare an out-of-level floor. Unfortunately, that's not my issue. My subfloor, just like my trial tabletop, is dead level. My problem is that even on a dead-level surface, when the front touches and the edges are level, none of the feet touch without shims, and all the shims are slightly different thicknesses. The shims/footpads will not be level across their tops, because all the feet are different, so step 4 doesn't apply. I wish it did.

If I had access around the sides and back, I could raise the edges with a pry bar and shim from the outside. But once the base is in the alcove, I have no way of doing that. And lifting it in and out multiple times at that weight is a non-starter.

Your suggestion of creating a map got me thinking. I could repeat my table-top trial using a sheet of 1/4" plywood cut to 60" x 36" and mark the feet location. Then shim each foot - easy on a table-top, measure each shim thickness, and create shims. Transfer the marks and the shims to the actual subfloor and cross my fingers. If both the table-top and subfloor are flat and level, it should work.