is backerboard required by code?

In all my bathrooms and kitchen, the tiles on the counterop and shower wall are attached directly to plywood or wall, i.e. there are no concrete backerboard behind the tiles. This house is 15 years old.
I understand it's better to put a backerboard (and a sheet of waterproof liner) behind the tiles. Is this optional or required by code? I'm remodeling.
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Most builders use greenboard if you're lucky. A better job is to use at least cementboard especially in areas where there is water.
You'd be suprised how much the code *doesn't* require, or how little time, effort or money it takes to do a 50-year job over a 5 or 10 year job.
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