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?