I am installing porcelain tile in a 30 sq ftbathroom. Presently there is carpet on new plywood. No damage to the plywood. Do I need to install backerboard first or will the good plywood be OK without the backerboard?

I am installing porcelain tile in a 30 sq ftbathroom. Presently there is carpet on new plywood. No damage to the plywood. Do I need to install backerboard first or will the good plywood be OK without the backerboard?
Tile hould have a minimum of one inch and a quarter thickness of wood. More is better.
So the plywood you see is probably on top of some other sheathing. Find out exactly how thick all layers are togther . Add if necesssary. Add another layer if you have a spongy area.
Backer board in general will lead to a longer lasting job.

I have installed 1/4 inch hardi backerboard and glued down and did not realize I needed a 1/8 in gap for expansion, can I just make a gap around the walls for expansion?