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Unread 01-11-2021, 09:43 AM   #35
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Join Date: Jul 2005
Location: Fairfax, Va
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Matt, CBU doesn't technically add any stiffness so 1/2" CBU isn't necessary unless you just need/want to add height to the floor.

Some people do like to add water proofing to the main floor. Note, however, "water proofing" only the floor, without also sealing the floor to wall joints isn't, in my mind, water proofing. Any small amount of water that may get on the floor won't hurt the tile, mortar, or CBU. If you believe enough water will get on the main floor to soak the tile, mortar, and CBU I respectfully suggest you've got other problems. If you want to add a membrane (sheet or liquid) for crack isolation then go for it.

Exterior glue plywood is recommended any time you are applying a product mixed with water to the plywood. The moisture can soften the glue in regular plywood, resulting in possible delamination. Exterior glue resists that.
If I recall correctly my memory is excellent, but my ability to access it is intermittent.
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