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Old 09-17-2019, 09:41 AM   #1
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Cement Board Screw Length


I'm putting 1/2" CBU over 3/4" T&G subfloor. I need to use 1-1/4" screws instead of the standard 1-5/8" screws with 1/2" CBU since I am running hydronic heat under the subfloor and don't want to puncture pex tubing.

Should I be worried about using the 1-1/4" screw with the 1/2" CBU? There will be thinset between the CBU and subfloor.

Thanks,
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Old 09-17-2019, 09:49 AM   #2
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Re: Cement Board Screw Length


How thick is the sub floor?
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Old 09-17-2019, 09:50 AM   #3
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Re: Cement Board Screw Length


Where is it going to go with 1" scrs?
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Old 09-17-2019, 10:00 AM   #4
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Re: Cement Board Screw Length


3/4" T&G subfloor.

Good point senior, I get a little carried away with details a lot of the time.
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Old 09-17-2019, 10:20 AM   #5
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Re: Cement Board Screw Length


If you can glue hardwood to concrete why not glue the CBU down.
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Old 09-17-2019, 10:23 AM   #6
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Re: Cement Board Screw Length


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Originally Posted by TheBigPig View Post
3/4" T&G subfloor.

Good point senior, I get a little carried away with details a lot of the time.
Sorry I missed the thickness the first time around.
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Old 09-17-2019, 11:44 AM   #7
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Re: Cement Board Screw Length


Quote:
Originally Posted by TheBigPig View Post
I'm putting 1/2" CBU over 3/4" T&G subfloor.

Should I be worried about using the 1-1/4" screw with the 1/2" CBU? There will be thinset between the CBU and subfloor.

Thanks,
The Pig
Check the CBU manufacturer requirements for the wood floor underlayment installation.

For example this is USG Durock specs:

Wood framing and countertops: fasten with 1-1/2" 11-gauge hotdipped galvanized roofing nails, or USG Durock™ Brand Tile Backer Screws for wood framing or 1-1/4" USG Durock™ Tile Backer Screws for wood framing or equivalent.
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Old 09-17-2019, 12:40 PM   #8
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Re: Cement Board Screw Length


tdiss, thanks, per the req'ts, I'm good with the shorter screw.
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Old 09-17-2019, 04:43 PM   #9
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Re: Cement Board Screw Length


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tdiss, thanks, per the req'ts, I'm good with the shorter screw.

Better yet, use Schluter Ditra. No screws. I didn't use it for my first bathroom since it seemed confusing (its not) but I used it for my 2nd and I would use it again.
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Old 09-17-2019, 04:55 PM   #10
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Re: Cement Board Screw Length


1/2" cbu is for walls. 1/4" is for floors. I agree with using Ditra, since all your support is in the framing and subflooring. It would help for us to know the size and spacing of your framing and the thickness of all subflooring strata. If all you have is 3/4" it may not be enough. CBU offers no structural substance. If there was a 1% chance of hitting your hydro under the subflooring, I would not use nails or screws, period.
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Old 09-17-2019, 05:38 PM   #11
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Re: Cement Board Screw Length


Agree on Ditra. I used it in my bathrooms. Great product! The only problem with it is if you want to match the adjacent room floor elevation (wood floor for example). You can use a thicker Ditra XL but’s THREE times more expensive than CBU.
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Old 09-17-2019, 06:47 PM   #12
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Quote:
Originally Posted by tdisss View Post
Agree on Ditra. I used it in my bathrooms. Great product! The only problem with it is if you want to match the adjacent room floor elevation (wood floor for example). You can use a thicker Ditra XL but’s THREE times more expensive than CBU.
I had the opposite problem. my hardwood floors are the cheap 5/16" thick top nailed planks. I also needed to add 1/2" plywood to reinforce my subfloor for tile. I was above the height of the adjacent room before I even put down the ditra so the thinner the better.
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Old 09-19-2019, 02:54 PM   #13
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Re: Cement Board Screw Length


I know 1/2" CBU is typically for walls, but I've done the research and it is allowed on floors as well. I tore out the old 1927 hardwood and subfloor (which was under 2 other layers of flooring) and installed 3/4" T&G advantech subfloor glued and screwed to the floor joists per the manufacturers recommendation. If I use ditra or 1/4" CBU, the finished kitchen floor will be lower than the hardwoods in the adjacent rooms, which I am trying to avoid.

Advantech subfloor is installed over joists spaced 16" OC, which is suitable for a CBU and tile over top, structurally. I'll be dropping the CBU into a bed of thinset between the subfloor and the CBU. I was really only concerned with the screw length to be used through the CBU, but per the manufacturer, I'm good with the 1-1/4" screw which shouldn't puncture the pex floor heat I plan on installing below.

Thanks for all the help and suggestions.
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Old 09-19-2019, 03:45 PM   #14
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Re: Cement Board Screw Length


Before you proceed with your plan please double check the actual overall depth of your underlayment. For example the thickness of ½” Hardibacker is actually 13/32” and ¾” plywood is typ. 23/32” in thickness.
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Old 09-19-2019, 05:18 PM   #15
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Thanks tdiss. That’s why I like to use permabase, which is a true 1/2”. Also, floor heat won’t be installed until later so I’ll be able to work around or grind down any screw protrusions.
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