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Old 09-11-2009, 01:02 PM   #1
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Thickness of cement board on plywood ceiling


Hi

I'm building a steam shower in my basement in place of the old tub enclosure (part of redoing the bathroom).

I've ripped everything out and re-framed the walls. On the ceiling I have solid 8" joists a little irregularily spaced due to a fireplace above (It's a 1920's home). To prepare for the tiled ceiling, I screwed 3/4" plywood under the joists, and planned to put permabase cement board on that. Originally I planned for 1/2", but now I'm wondering if 1/4" is enough given that I am attaching it to 3/4" plywood. Normally ceilings are supposed to be 1/2" cement but with a plywood backing, would 1/4" not be enough? Alternately I could use 1/2" plywood and 1/2" cement board, but plywood adds more strength than cement board.

My motivation is that the ceiling is a little on the low side. Not sure if it is weakening things for 1/4" gain in height. I plan to kerdi over the whole thing, so I am wondering if that wouldn't be enough or should I stick to my original plan?
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Old 09-11-2009, 01:17 PM   #2
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Thickness of cement board on plywood ceiling


Provided you set the backerboard in thinset to the plywood (as you should) and secure it to the plywood with plenty of screws, I don't see any reason that 1/4" would be a problem.
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Old 09-11-2009, 01:32 PM   #3
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Thickness of cement board on plywood ceiling


How would you recommend applying the thinset? The plywood is on the ceiling now. Should I apply it to the back of the cement before screwing it in? How thick should the layer of thinset be?

Thanks!
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Old 09-11-2009, 05:07 PM   #4
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Thickness of cement board on plywood ceiling


Use a square-notched trowel...I'd say 3/8" notches would be fine. You can apply it to the ceiling or to the sheets I suppose.
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Old 09-12-2009, 08:46 AM   #5
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Thickness of cement board on plywood ceiling


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Originally Posted by hacksepe View Post
How would you recommend applying the thinset? The plywood is on the ceiling now. Should I apply it to the back of the cement before screwing it in? How thick should the layer of thinset be?

Thanks!
I'd apply it to the plywood. It would add weight to the cement board.
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Old 09-13-2009, 07:07 PM   #6
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Thickness of cement board on plywood ceiling


I applied it to the cement board and it was pretty easy to put up. My wife and son lifted into place and I screwed in the concrete screw. It worked out just great, Thanks!
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