On Jul 17, 10:35*pm, metspitzer wrote:
I have a bathroom where, due to poor calking, the floor has rotted.
The floor is tile and still looks fine, but when I go in the basement
and look at the floor above the floor is rotted completely thru and I
can see the bottom of the tile. *The amount of rot is about 5x5
inches.

What I want to do, for now, is use some filler material and fill in
the rotted wood. *Then take a piece of plywood and toenail it into the
floor joists, or use a couple of blocks to screw to the floor joists.

Any recommendations on what to use for filler?

No, you can't patch on rot. The only fix is to remover the tile
until you get to solid wood then replace the wood, maybe the framing
as well, and put the tile back.

I have to agree with Smitty, and you may well have a serious
safety issue.