I've been needing to replace the floor in my daughter's bathroom for some time now, and I finally got around to it. We bought our mobile home as a repo, and agreed to do any needed repairs ourselves, in order to save several thousand dollars. Pretty much the only problem we had was that the bathroom floors had been soaked pretty bad, and, being made of particleboard, were naturally in bad shape.
The seal had gone out under the toilet, as the floor was sinking- you can see where I repaired that. The problem that followed was that the repair was thinner than the particle board, after it had absorbed water. The solution was to pull out all of the old particle board, and replace it with plywood.
Removing the old PB was a big job- it was glued, nailed, and stapled to the joists, and they used plenty of all three! Plus, the walls were built on top of the floor, so I had to cut the floor at the edges, then build up a frame for the new floor to rest on. It was sawzall and chisel work (I used a cheap harbor freight chisel- and I have to say that it held up quite well to being pounded on with a claw hammer).

Above, you can see where I've put in the cross braces- they are mostly cut from an old waterbed side, and screwed into the joists using pocket holes. I've also run some up under the wall, and screwed them into the next joist over ( no *that* was tough to do! laying on my face and doing it blind!). I've also put just under the wall, screwed into the frame I built, to support the edge of the bathroom floor, as well as the floor in the next room over. As you can see, there was a lot of frame building to be done here- the people who built the house didn't include any cross bracing, but simply relied on the particle board to hold everything together. Of course, they were working with full sheets- and I was working with random pieces that I pulled out of dumpsters :)



I have to admit that it was some work to get all of this fitted together- note that I also replaced my earlier repair. Total cost for the job? Maybe five bucks, including screws, and the tube of caulk I used to fill in the cracks. When the caulk dries, we'll cover the floor with some self-adhesive tiles that a friend gave my wife last week, and it should all look pretty decent, for now. When I've got time, I'll probably come back and pour a thin layer of thinset concrete to level everything out perfectly, then tile it with squares of Corian (that I'll be cutting from my scrap). In the meantime, though, my little girls's going to be happy to have her bathroom back, and that's the best part of the job.