In my part of the country we normally have slab on grade. This is my mothers home which is one of a few pier an beam homes in this area.
Original floor was:
Sub-SubFloor - 3/4 inch T&G pine layed at right angles to beam
SubFloor - MDF with 1/8 inch plywood laid on top.
Finished Floor - tile.
Where I'm at.
Cut flooring down to Sub-SubFloor at all wall plates (FWIW used an oscillating saw) and removed all damaged flooring.
Sub-SubFloor is bowed and cupped in places and appears to have 15 or 30 lb tar paper
sticking to to a number of boards.
Sub-SubFloor boards appear to be structurally OK, as well as the joints, but does appear to be water stained.
1) If water stains are from toilet (likely) can the Sub-SubFloor be saved or does it need to be replaced?
2) If saved, what is the best course of action?
(a) Wash boards with a bleach solution and what about the piers?
(b) Saw those boards that are cupped / bowed and renail?
(c). Get a floor sander and sand the boards as best as possible flat -- What about the sander sanding off nail tops?)
3) Replace Sub-SubFloor with CDX or ADX back to the joists closest to the walls?