Rotten floor near toilet

My small bathroom job just got bigger. I want to replace the tile and toilet in the kids bathroom. It's about a 3' by 5' room with a toilet and tub/shower. I pulled up the tile and saw that 2nd layer of plywood was black and moldy behind the toilet, in the corner by the bathtub. So, I decided to pull that up too, and I plan to put down hardi board instead.
Well, it appears that the water also got down to the main floorboards (subfloor?) in that corner. I went down to the basement and checked from above, and I'm happy to report it doesn't show any signs of rotting from the bottom. But, the top shows water damage; discolored wood and some of the layers of plywood has rotted away. I have a fan running in the room now to dry out the wood. When I step and press the floor in the worst spot, it does not sag.
I took some pics if that helps: floor:
http://mysite.verizon.net/paulaner/bathroom.jpg
basement:
http://mysite.verizon.net/paulaner/basement.jpg
Here's my dilemma. The house is a rancher, and the walls are not aligned with the beams below. The beams are 2x4 with vertical diagonal spacers supporting the whole house on the cement foundation perimeter. So, in order to tear up this rotten piece of wood and replace it, I'll have to move the bathtub, and the wall behind the toilet if I want to cut out a section that can be nailed back down to the beams below. This is looking to be a much bigger job that I want to get into.
I believe the wetness is a combination of things: 1. my boys had very bad toilet 'aim' when they were younger. 2. they didn't always close the shower curtain correctly. 3. the house is ~ 20 years old and we've been here 7, so who knows what else occurred before. The boys are older now, and are much more responsible for that area.
So, finally my question. Do I need to move the tub, tear up the walls, cut away the flooring and replace it at the beams? Or, can I dry out the floor, clean up the wood with bleach & natures miracle, then lay some new hardi board over the whole thing? I plan to tile and seal the floor when I am done. If I go the easy route, is there a wood treatment I can put down to level the wood in that rough spot?
Ugh. Thanks in advance.
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Paulaner wrote:

No, you said the plywood underneath is solid so leave it in place. Even if you did decide to cut it out you could put blocking between the floor joists to support the edges of the new smaller patch. You are right to go with the backerboard or cement board. Plus, nobody could walk back there once the toilet is in place except an elf and I hear they don't weigh much.
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snipped-for-privacy@iname.com says...

I had a similar situation, except it was under a sink in a 3x5 1/2 bath. A pipe sprang a pin hole and leaked just enough water to ruin the subfloor but not be detected until I pulled up the vinyl floor. Not even enough water to stain the sheetrock in the kitchen below. Unfortunately, both layers of subfloor had to come out.

DIYer_Alert;
I agree with RayV, if the subfloor is solid, leave it. You *could* use some leveling compound on the subfloor, but I think it's a waste of time since you're going to use thinset for the hardi- backer anyway. If it's really rough, you might want to level it with thinset before installing the Hardibacker so the thinset for the backer is easier to get troweled on right. I'd screw down the subfloor to the joists before putting down the hardi-backer just to make sure nothing is going to move.
Yes, some bleach to kill any mold and perhaps a week with a fan running to make sure everything is perfectly dry is a good idea.
/DIYer_Alert;
--
Keith

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