Need help solving subfloor issues in bathroom

F350

I'm doing a reno on the upstairs hallway bathroom.

I have run into a real bad problem with the subfloor in the bathroom. Towards the rear end there is a large dip in it, from an asshat making a notch for a duct for the master bathroom behind the wall. That weakened the subfloor in that area over the years. The subfloor gives when you step on it. What can be done to strengthen it? What's worse is that two pieces of subfloor is joined up there.

Towards the center of the bathroom, the subfloor rises significantly because four subfloor portions all meet here (think Four Corners).

I threw out the old vanity and sink. The toilet is being stored elsewhere. The tub stays, it's a cast iron tub built onto the wall on right.

The bathroom is approximately 30 square feet. Shape is square wall to wall on three sides, tub as the fourth. I plan to lay 12x12 porcelain tiles.

Suggestions?

CplDevilDog

Pictures?

The joist is notched on the top? To what depth? Might have a structural issue there.

OSB sheathing layed out wrong and a cast iron tub? Renovated previously? When was the house built?

I would expect a plank subfloor with a cast iron tub.

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F350

The subfloor was notched, but I don't know if a joist was compromised when the duct was put in. I can't see the direction the joists run from the opening.

I don't think it's OSB (never seen them personally) just plain old plywood. You can tell here.

No renovations ever, it was built in 1980. I took a look at the tub access. No planks. Just regular plywood, and I see a piece of 2x6 supporting the bottom of the tub on three joists sistered together.

CplDevilDog

I would lay your hardi backer right over it. By the time you get your baseboard on there's going to be such a small gap that can actually be stepped on as to not be a concern.

Unless its a lot softer than it looks (rot?).

Of bigger concern (IMO) is the high spot in the "Four Corners". picture of that area?

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F350

The stuff on the plywood is the backing from the old mosaic tile that was on there. I haven't sanded down or anything. Just scrapped off the loose stuff. I don't see any water damage, but it does look 'dry' to me- best I could describe the condition of the plywood subfloor.

What's a hardi-backer? A product?

The second picture is the area. The 4th line is just out of area on the bottom since my digital camera can't go that wide. It's about 1/4" higher than any other area. Wavy could describe the bathroom floor. The house has settled significantly since it was built.

CplDevilDog

Hardi-backer is a trademark for Cement Backer Board. You will definitely need this between your tile and your plywood.

Best if you settle any "wavy" issues before you start your cement board. If the plywood is delaminating due to the glue failing in the moist bathroom, you'll need to screw it all down to the joists.

Sanding and self leveling floor compound also.

Tile will crack (especially at 12x12) if you just lay it on plywood.

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F350

I see - I wasn't aware. I thought I just needed to level it and use the orange stuff you lay down before the tile. Good thing I came on here.

The subfloor is nailed down the joists. Should I remove them and replace with screws? I like screws better. Might give me a chance to glue them down the joists also.

Yes, I will use self-leveling cement. What grit should I sand the subfloor? 36?

Is this the correct order to lay down tile: self-leveling cement, then cement board, then thinset. Should there be cement between the plywood/self-leveling cement and the cement board?