Originally Posted by Stoney

Talk to a company called Silpro. they make one of the best self leveling tile underlayment and a range of repair materials that work with their system.
Ardex also makes similar products but I don't recommend them as their after use product support is crap.

If you want a truly water proof system you need to look into Ditra and Kirdy. Ditra is a waterproof membrane that goes down in top of the wood subfloor and the thinset goes on top and then the tile and Kirdy is a membrane system that goes on the concrete wall board behind the tile under the tile thinset. Installed as per manufacturer specs thay will render a totally waterproof bathroom. think about this-I installed Ditra/Kirdy when I redid my MOTHER IN LAW'S bathroom! If that ain't proof, then I'm crazy!

I also strongly recommend using the dense foam shower pan and curb system as it will not absorb water and is easy to install and comes pre beveled so no field adjustment is needed.

It sounds like you need to rip out all the particle board and install marine grade tounge and groove plywood using stainless or rhodium screws and Silpro system as a underlayment then flashpatch to do final leveling. Any flake board, particle board or luan will always absorb water as they are all just sawdust and glue pressed together under high heat. They have NO structural rigidity. Plywood is designed with contrasting grain to act as a self supporting material and marine grade is treated to resist absorpition.

The choice of fastener is also important as rust/corrosion is a path for leakage. The 3x expansion of rust will force open/up anything and become the conduit for water intrusion.

IMHO the entire subfloor should be done as a patch is the weakest part of the system.