u/Dh4pres

Say you have a 20’x14’ den (plus mudroom and laundry room) that’s eventually getting tile put down, and the subfloor is 1 1/4” out of level in 14’ (the lowest point in the room) while the rest of the room is only out about 3/8“. Also, the subfloor isn’t screwed down and it’s all pieced together (smaller than normal pieces and sometimes seams that have 4 sheets intersecting 🤦‍♂️) and there’s soft spots in areas due to no blocking/an absence of tongue and groove plywood throughout. Do you A: do the right thing and remove all the subfloor, shim everything to level, and then glue and install 3/4” tongue and groove subfloor before installing hardiboard and an uncoupling membrane, or B: go the quicker cheap route and screw down the existing plywood (that isn’t glued OR tongue and groove, and has soft spots), then glue and screw down 3/4” tongue and groove plywood on top (still out of level), then install hardiboard to lock everything together, and then pour self leveler (up to 1 1/4”) and then uncoupling membrane before tile. If option B is even a potential, what type of glue should be troweled down to bond the existing and the new sheets together?