Flooring Tile - Ceramic in 2nd floor bathroom

Joists are 2x8, 16" OC, 11' max unsupported.
Subfloor is single layer 5/8" T&G waferboard.
House is <20 years old.
Hallway is 1/4" above the subfloor.

Can I just 1/4" CBU over the subfloor and tile or MUST I put down a layer of plywood? My concern is the height between the two floors.

If plywood, can I get away with 3/8" or must it be thicker?

Without plywood, I'm looking at 1/4" + 2 layers of thinset (another 1/8" + 1/8"?) = 1/2" above the hallway.

With 3/8" plywood, I'm looking at 1/4" + 3/8" + 1/8" + 1/8" = 7/8" above.

With 5/8" plywood, that'll be 1/4" + 5/8" + 1/8" + 1/8" = 1 1/8" above. I would have to plane the door at this point.

Your at the bare minimum now with what you have, that's assuming everything is in great shape, I don't like one layer of wood, 3/4" maybe, if it's solid, 5/8", I would add another layer of 1/2" ext. grade A/C or B/C grade plywood, then 1/4" cementboard or a membrane if heights a problem.:)

You can install a marble saddle in the doorway for the height transition.:)

Been busy...not much progress. Just have the 1/2" ply down.

When trowelling down thinset for the CBU, do I avoid the 1/8" gap between the sheets of plywood or do I go right over them as if they weren't there?

Better to just caulk these gaps with a any cheap caulking to keep thinset out.:)