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Unread 11-01-2004, 04:08 PM   #1
lruiz
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Join Date: Oct 2004
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Red face No mud on subfloor under backer board - risky?

Hi Guys,

I wanted to know if I would come across a trouble if I don't apply mud on the sub-floor when installing the Hardibacker board? I've screwed down the boards in every marker spot.

Am I being risky or would I be ok?

This Hardi-backer board is really hard to cut other then the water resistant drywall I'm using for the wall tiling Would a Rotozip cutter work ok and less then a noise maker. Live in a condo w/ rules on working in the home and my hand is tried using the utility knife and hand saw - ugh.

thanks,
Lou
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Unread 11-01-2004, 04:13 PM   #2
jadnashua
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Tile doesn't like flex. CBU is hard, but will bend and conform to the minor imperfections on your floor. The CBU acts like a decoupling layer (even though it is screwed down!). If it isn't fully supported underneath, you have a high likelyhood of failure. The thinset under the cbu fills in the minor voids and gives you a solid unflexing layer. If the floor is not stiff enough, regardless, you would likely have a failure. Did you check the status of your floor by using the Deflecto link above? You really need to pull it up and put down some thinset. BUT, before you get it all reinstalled, verify that your floor is sufficient to support a reliable tile install. IT may need some other work prior to being able to support tile.
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Unread 11-01-2004, 04:24 PM   #3
lruiz
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Talking

Just went to the Deflecto link I'm in good shape to use ceremic or stone tile.

That subfloor is from 1/2 - 3/4 inch plywood.

Before, underneath the old tile was some 1/2 plaster-kind of gesso material almost rock material. The material is really water absorded as well, so that had to be removed.

Thanks for the advice I'll apply thinset and lay the Hardi-backerboards after.

Last edited by lruiz; 11-01-2004 at 04:31 PM.
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Unread 11-01-2004, 04:59 PM   #4
jadnashua
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Double-check with the pros, but I think the minimum plywood layer needs to be 5/8" and more is better. Think of it this way, the subfloor can deflect as you stand on the part directly over the joist, but, now stand exactly inbetween the joists. That 1/2" plywood would flex too much. So, depending on where you are standing, it will flex a different amount. I think you probably need more subflooring. CBU is NOT structural. So, for a floor, it doesn't make any difference whether you use 1/2" or 1/4" stuff, except to change the finished height of the floor. Eitehr one on a floor will be an adequate decoupling layer (you'd want to use the 1/2" stuff on a wall, though). If it is 3/4" and you've check the deflection specs, then you should be okay. If it is thinner, you probably will have problems.
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Unread 11-01-2004, 05:41 PM   #5
Raymond S
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Hey, Lou
Where are you putting that greenboard?
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Unread 11-01-2004, 05:50 PM   #6
jadnashua
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You threw me on putting thinset then 'after' the cbu. Just to be clear, apply thinset, set the board on it, then screw or nail it down while it is still soft (no dried skin). If you've already got it screwed down once, take it out one sheet at a time, put down some thinset, place it back down, and rescrew it in. You'll likely get in the same holes and it will be much easier!
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