1/4" is what's used on floors.
I'd play it safe and add at least 3/8 underlayment rated plywood before the tile board.
Do not have the seams line up with the seams below, fasten ever 4" on the edges and 6 to 8 in the field, do not attach it to the joist and do not use any glue.
You do know there needs to be thin set under the tile board right?
I think Joe missed the 24" o.c. part due to the malfunction. It's not a matter of suggesting, but you must go with an underlayment. The thicker the better, but need to know more.
Tell us the type and size of tiles you're planning. The type and size of the room and your location. The style of house, type of substrate, age and what's there now etc. We may need to know more, but good to start with just this info. .
Tile 4 You LLC Troy, MI - Consulting by phone/internet -PAYPAL
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