Ontario building code 188.8.131.52. moisture Resistant Backing
(1) Ceramic and plastic tile installed on walls around bathtubs or showers shall be applied over moisture resistant backing.
Does this mean cement board, kerdi board are a overkill?
I always used Kerdi board. But i have also seen plain drywall and drywall with kerdi membrane
Not overkill. It means exactly that. Before installing the tile in a wet area, you must first install a waterproofing membrane over the walls or a waterproof board such as kerdiboard instead of drywall. Cement board alone will still allow water to weep.
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