We ripped up some lino on 3/4" particle board and want to replace it with tile. The tile is 3/8" and I wanted to put down 1/4" backer board. Once I tore up the floor I found a piece of tin that had been nailed down to cover this hole.
Looking for any suggestions how to fill this in appropriately so that I can lay down tile and not have to worry about it cracking the first time somebody steps here.
The joist on the left is under the wall of course, and access from below can be done, but would be a real pain.