Flooring Tile - floor design for tile shower and floor

I have a question about the design of my floor and how it applies to installing a tile shower and floor in a master bath and closet. The subfloor is constructed from 2x10 joist 16" OC. The flooring is 1/2 plywood (non tongue and grove) with 3/4 particle board on top. None of the seams overlap. The foot print of the bath and closet is 13' by 8' and the unsupported joist length is 13' (i.e. iner most point is against load bearing wall and outer most point is exterior wall). I'm planning a 3.5 by 4' tile shower with a pre-pan, pvc liner, and pan made of deck mud. I have doubled the joists in the area under the shower and added cross bracking between the joists to add stregth. In addition, the entire floor of the bath and closet will be tiled with porcelin tile (1/4 Hardiebacker underneath). I'm a little concerned about all of the added weight (old bath just had a fiberglass shower) and the durability of the tile on the floor. Is the floor design sufficient for what I have planned? Thanks for any input.

The joists are spanned to the max, do you know their species and grade? Can you sister them?

Are you sure the sub is 1/2"? How did that happen? How old is the house? Are you sure the top layer is particleboard? If it is, you should be removing it. Maybe it's OSB? You say the seams do not overlap? They are stacked? Not good. You need to correct that when you install the new plywood.

Jaz

Would the species and grade be stamped on them somewhere? Otherwise I wouldn't know. The house was built in '84. The first layer of subfloor is definately 1/2" ply and the second is 3/4" particle board and not OSB. What I meant about the seams not overlapping is that the plywood and particle board are staggered so that the edges don't break on the same joists. i have sistered under the shower and most of the bathroom area and added blocking where the seams are.

You are at bare minimum with your joists. Sister all of the joists, not just some of them.

As to your subfloor, the particle board has to go. 1/2" plywood is unusual for a subfloor, and when it comes to tile, 1/2" plywood is like having no subfloor at all. Remove the particle board and replace with 3/4" exterior glue plywood, cc plugged or better. Make sure that the plywood seems are staggered. Screw the new plywood into the joists as well. It will not be adequate to just screw the new plywood to the existing 1/2" plywood.

Will you be using ceramic/porcelain tile or natural stone tile? I should have asked this first.

I will be using porcelin tile on the floor.

What is it about the particle board that makes it bad enough to replace? I'm just trying to educate myself. The floor as I described it had a ceramic floor installed directly on the particle board for ~ 15 years without any issues except for a water issue near the toilet which has been repaired. I figured that adding 1/4" Hardibacker to the particle board would just make it better.

Particle board sucks up moisture like a sponge and expands and deteriates.