My house is a tri-level. The 3rd level down is 30" below grade and on a concrete slab covered with 1x4 sleepers with solid foam panels between them and then a 1/2" plywood subfloor on top of them installed in the 1980s. (I have a snapshot of the construction in progress.) There's some cracking in the sheet vinyl floor in the bathroom, but I can't tell if it's from a leak or just a breakdown of the vinyl. When I remove the vinyl to replace it, what should I replace the plywood and maybe the sleepers with if there is rot? I want to have the result the same height as the existing floor to prevent creating an uneven floor at the doorway. I don't know how the sleepers were installed either (masonry nails or adhesive). The products I've seen aren't thick enough and/or create a lot of seams that might be noticeable when the new vinyl flooring is installed even with leveler of some kind. I'll also need to remove and reinstall the toilet. Another bathroom can be used during the repair. The bathroom is 7' x 7' at the longest measurements and there is a shower stall and jog near the doorway and toilet to deal with. Any suggestions on how to approach this would be appreciated.

Pressure treated 1X4 and pressure treated plywood would be my choice. Glue with construction adhesive or screw them down using TAPCO fasteners. Make sure there is plenty of blocking around a toilet or the rocking will cause wax ring failure and leakage.

The pressure treated lumber should dry out for a month or so before installing, right? Should something be applied to the seams where the plywood sections meet before installing the sheet vinyl?