Is Cement Board Over Plywood Subfloor Under Bathtub OK? - Flooring - DIY Chatroom Home Improvement Forum
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Old 04-02-2014, 07:31 PM   #1
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Is cement board over plywood subfloor under bathtub OK?


Prior to laying ceramic tile bathroom floor in 1983 house, replacing rusted enameled steel bathtub and rotted plywood subfloor consisting of two layers of 5/8" plywood (1 1/4" build).

Is it good or bad to extend new subfloor consisting of 1/2" cement board over 3/4" plywood (1 1/4" build) even under new bathtub or would 1/2" plywood over 3/4" plywood under the tub be preferable?

Thanks for your comments.

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Old 04-02-2014, 07:58 PM   #2
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Not sure I understand the question cole tell, but keep in mind that cement board is. Not normally considered to be "structural". So, a 3/4" plywood with 1/2" durock next gen cement board on t is considered to be 3/4" for purposes of figuring floor deflection, etc. ron
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Old 04-02-2014, 08:08 PM   #3
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I dont see how it would make a difference. But if you dont want to make it difficult for the next person who installs a new floor I would have a seam at the tub.
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Old 04-02-2014, 08:23 PM   #4
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There is no reason to run the Durrock under the tub---and several reasons not to.

As Ron mentioned--there is no structural strengh to the backer--so 1/4" is typically used for floors--

Often a floor is out of level---so the tub must be shimmed----the resulting gap is hidden by the backer and tile butted against the skirt of the tub----
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Old 04-02-2014, 10:30 PM   #5
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Thanks for the well thought out replies. I won't be extending the cement board under the tub, it wasn't a good idea.

I believe for the 16" joist spacing involved, 3/4" plywood subfloor is sufficient structurally for the room, so adding 1/2" plywood over the 3/4" plywood instead of the 1/2" cement board under the tub is also not necessary structurally, and to oh'mike's comment, has cosmetic disadvantages, but it would have kept the tub drain matched to the existing tub drain plumbing, but that can be handled easily enough.

My motivation for going from the existing two stacked 5/8" plywood subfloor to a 3/4" plywood subfloor is to minimize unnecessary bathroom floor height buildup when the 1/2" cement board and 3/8" ceramic tile are added and how this buildup affects handling commode seals, water line connections, door thresholds, etc.
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Old 04-03-2014, 04:56 AM   #6
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That's a good reason to use the thicker backer board----
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