When was this possibly a good idea?
 

I've finished gutting the master bath. Shower pan is gone and everything. Under the shower pan, and also under where the vanity was, there was a sort of cutout from the plywood of the rest of the room, and particle board of the same thickness had been put there. Huh? Why take extra effort to cut out the shape from the plywood just to put particle board down there, where it could get wet? The wood under the shower pan is totally swelled up in some areas, and completely falling apart in other areas where it did get wet (water damage is the reason for this remodel). Under what circumstances was it a good idea to use particle board and not plywood under the shower pan and vanity?

Never was a good idea
Someone had no clue what they were doing

Here's how it happened.

see...the original pan leaked, they cutout the rotted wood, replaced it with crap and set new pan. That leaked and swelled the crapboard.

I could be wrong now, but i don't think so....

D(Monk)

They didn't cut out the plywood and install particle board. The whole thing was originally particle board and someone removed it and replaced the floor/subfloor during a remodel without disturbing the vanity and shower pan. Cheap original construction method to use particle board, not uncommon. :whistling2:

So, a variety of possibilities, and all finish with me tearing out the particle board and the plywood layer, replacing the entire thing with good, solid plywood, and concrete board (not under the shower pan, though) for tile, and then rebuilding the rest of the bathroom. It's kinda fun!

There you go, nothing like enjoying your remodeling job.:thumbup: