Cement Board

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Old 03-13-06, 05:53 AM
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Cement Board

I have some questions, I am remodeling a small master bath 5'x10', it is already gutted. I have removed the first subfloor of 1/2" plywood and plan on replacing it with 1/2" light grey cement board. I plan on running it under the shower base perpendicular to my floor joists, staggering /taping seems and screwing butted ends to the joists as well as screwing at the 6" marks. My confusion is the thin set under the cement board. Is it the same that I would use for the tile? Should I use a 1/4" trowl? Also is keeping the cement board level with the thin set under it a concern? I just want to make sure that I am on the right track.
Thanks, Jeff
 
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Old 03-13-06, 07:08 AM
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Thinset under the backer is required. Use unmodified thinset under the backer w/ a 1/4x1/4 square notch trowel. It will follow the floor as you fasten it down and the thinset compresses. Any leveling gets done over the backer.

But...before you get ahead of yourself, with the underlayment layer gone, now please post back with the actual subfloor make up-ply, plank, how thick? Drilling a small hole will let you fine out. Also, what are your joist size, spacing, and unsupported spans? Nailing pattern of the subfloor will show the spacing, dropping a piece of coat hangar wire or a dowel into the hole you drill will show the size of the joist, and measuring the ceiling of the room below will show the unsupported span. Just throwing down a layer of cbu doesn't make a floor "tile ready" as it offers no strength to the floor, it mearly serves as a bonding surface.
 
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Old 03-13-06, 08:30 AM
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Cement Board

Tilebri Thanks,
The subfloor is 1/2" plywood also, my joists are 2/10's -16" on
center. Distance from ceiling to floor is 8'. I think that I'll be ok.
Do you think that the weight of the shower base on the cement board that I will set in mortar will be any issue?

Thanks,
Jeff
 
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Old 03-13-06, 08:58 AM
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So to clarify.

At present the plan is to put 1/2 cbb over top of one layer of 1/2 inch plywood?
 
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Old 03-13-06, 09:00 AM
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1/2" is way too thin. Add a layer of 5/8" bc exterior grade ply, screwed only to the first layer, then use 1/4" cbu or a membrane. You can run the backer under the shower pan if you wish, but there's no need to do so. Normally, it's only run up to the pan, leaving a gap for expansion/contraction which later would be caulked when the tile is set.

I asked about joist span, as measured from below. Doesn't matter to me how high the ceilings are
 
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Old 03-13-06, 09:49 AM
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Yes,
To clarify, that was my plan, 1/2" cbb over 1/2" ply. my reasoning was that the previous owners had installed tile directly over the 1/2" underlayment that I removed. It had not cracked, they just did a crappy job. I guess I need to reconsider the basic foundation to start with.

Jeff
 
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Old 03-13-06, 01:36 PM
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Jeff

Add the 5/8" plywood. You can run that under the pan. Use 1/4" cement board - (1/2" is for walls and it sounds like you dont need the added height). The finished height of your new floor will only be about 1/2" higher than it was. You could use a membrane instead of the cement board. which would reduce that height a little bit.
 
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Old 03-14-06, 02:17 PM
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Guys, I made a stupid measuring mistake, the underlayment that I took out was 5/8" instead of 1/2". So if I go with the ply, Is any exterior ply ok? Or is it called something special. What is the story on the suggested membrain? I haven't seen it, but if it works it sounds good to minimize the floor height. Thanks again for all of the input!
Jeff
 
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Old 03-14-06, 02:55 PM
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ac or bc exterior plywood is good. For a membrane you can use Schluter Ditra. It will add a little more than 1/8" to the floor height. Check out the link below.

http://www.schluter.com/english/prod...601-index.html
 
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Old 03-15-06, 12:29 PM
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Membrane

The Idea of the membrane sounds interesting,neat video. Do you have to do anything with the seems of the 5/8" ply if you go over it with membrane? Can you do thinset & mesh tape on the plywood seems?

Thanks again, Jeff
 
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Old 03-15-06, 01:43 PM
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No need to do anything to the seams of the plywood.
 
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