Depends on the tub. if an acrylic tib they are sometimes set into mortar. It
creates a firm base and eliminates flex. The tub install instructions will
guide you.

RT*M

"e patashnikov" wrote in message
om...
From what I've read, cement board is placed between the plywood
underlayment and the floor tile in the bathroom. What is placed
between the subfloor and the bathtub?