After tearing out the old subfloor in my half bathroom (due to water damage) I am replacing it with 3/4 in (2 cm) plywood. One side overhangs from the last joist about 9 in (22 cm) to the wall, and 11.5 in (29 cm) to the next joist. I plan to tile the floor with ceramics, so I need to brace that side somehow.
My plan was to attach three or four 11.5-inch 2x4s perpendicular to the joists and screw the plywood into those via 3-inch deck screws. My father also had a couple ideas:
- Cut a couple 4x4s and set them on the concrete below pillar-wise (unfastened), and screw the plywood into them
- Set a 2x10 on the concrete parallel to the joists, then screw the plywood into it -- this would require more of that black stuff underneath to raise it up to the other joists' level
If it's not obvious, neither of us are carpenters. I already have the 2x4s cut, but is my method unsatisfactory?
EDIT: Here are the three ideas in picture form (my idea, then my father's ideas):