Towels and a tarp or a drop cloth are about it.

If you don’t mind down and dirty and not very pretty, this will guarantee to see all the floor. Scrape up the loose tiles and clean off the floor the best you can. Then buy red guard or aqua guard. You can use a brush or roller and paint it on. Two coats is best. This is what tile installers use to seal shower walls and floors. It will never leak again. When you get ready to remodel be sure to use it on your walls and floors before you put the tile on. Remember tile and grout is no guarantee of waterproofing. Red guard waterproofs. Good luck

You're never going to get enough build-up of Redguard to waterproof on such a substrate.

I took a 10-day vacation between the time I installed Kerdi and the time I laid the tile for a customer. She used it without a problem. It is waterproof.

Flex Seal, Phil Swift says "it even works under wooder." LOL

Perhaps buy a piece of linoleum and lay it over the entire floor till you can get to the remodel. You could use a small amount of clear silicone to hold the edges down.

I don't know the age of your house, but that looks like an old tile floor that was laid on top of a bed of concrete, and they are a PIA to demo. It was common to frame a "tray" in the floor, and then pour a bed of cement to support the tile. In my house that cement bed seems to be about 3-4" thick. Some of my cast iron 2" bathroom drain pipes are embedded in the cement, so we could not replace them without tearing up the floor.

Bruce

@klem1 That Flex Seal option sounds tempting because it's waterproof, pourable (it's liquid in a can), and it's cheap. I wonder if it will stick to the floor surface, simply because of the mesh that's still there, the old concrete that's cumbled and the holes. The instructions say the surface should be "clean" and "free of dirt." Any thoughts?

@live_wire_oak I appreciate the words of caution. And I do worry about some rot ... and fingers crossed the tub won't fall through the ceiling. I do think the leak is only from water splashing out of the shower over many years, so I'm hoping it's not as bad as a constant leak from a pipe, for example. As I mentioned, we do plan on doing a complete renovation soon. But we've had trouble getting a contractor right now since we live in NYC, and most work has stopped because of Covid-19 restrictions. We can't even go to stores to pick out tiles or fixtures.

If your floor is so far gone that any amount of water that lands on it ends up going through the ceiling, then you need to STAY OUT of that bathroom. Your floor is totally rotted and could give way at any moment. Please take this seriously.

Let's not get carried away please. Lwo's measured advice is sound.

Because it seemed important to know how bad the situation is, I removed the tiles and mudboard in the problem area. Underneath is just the sheathing. You can see that most of the wood is still in tact, but it's definitely rotted in the corner by the tub. That spot is still very wet, smells earthy, and I can dig into it with my finger. In a 4" x 4" area, I could make a little 1/2 inch hole easily.

Now I'm wondering if the best solution might be to: 1) wait for wood sheathing to dry; 2) put down some kind of underlayer or mortar, and then 3) just put on some of that Flex Seal - as a temporary solution.

Or, should I put down some kind of PVC board, or something like that over the sheathing - and then put mortar on that - just in case there's reason to worry about the sheathing in that corner giving in. and the PVC board would keep us from falling through the ceiling (until we do the full renovation)!

Thanks everyone for your help!

Open the ceiling in the kitchen. It’s toast anyway. Water has run under the tub, and that corner is a big concern. You can’t see most of the damage. But if it’s “mushy” at the tub edge, that could go deeper than you think. Water goes downhill. Flex seal is junk science.

L_W_O is correct. That black-looking wood is paper mache at this point. If you can dig it out with a FINGER (ahem...who needs a screw driver!) then you have a big issue that you cannot see: the stuff underneath the tub. The weight of the tub is pushing the plywood down...just a snick. That means the water has run underneath the tub for YEARS. And years. And years.

If you can avoid using this bathtub, I suggest you do so. Until we know the level of rot (I know there is structural rot...I just don't know how much) we have to assume this is going to break through the plywood sometime soon.

Get a roll of heavy duty visqueen. Cut a piece somewhat larger than the area you need to protect. Lap it a couple of inches up onto the tub and any walls near the tub. Duct tape the turned-up edges to the walls and bathtub. The edges of the visqueen that are farther away from the area of concern can simply be duct taped to the floor. Then lay down some towels. It will look ugly until you gut things, but it will be effective and prevent further leaking through the floor. I did this once myself and it held up fine.