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Old 08-21-2010, 10:18 PM   #1
mertivsvyna
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Do I need to use hardibacker &/or Redgard on a bathroom floor?

We're building a new bathroom and want to install 12"x12" marble tiles on the floor. At present the floor consists of 3/4" exterior grade flakeboard on top of 3/4" tongue-in-groove pine on top of a 3/4" pine plank subfloor mounted on 4"x10" joists. Mechanically it is absolutely SOLID.

Our tile installer suggested putting down a layer of hardibacker on top of the present floor and then laying the tile on top of that with thinset.

I was concerned that this approach would mean there was no vapor barrier between the wood and tile layers so I started looking into using Redgard.
The Redgard instructions however state that thinset can be applied directly to it so I now have two questions:

1) Should we be applying Redgard or another vapor barrier either below the hardibacker or between the hardibacker and thinset?

2) Do we even need the Hardibacker? If the floor is solid, the redgard will bond to it and provide a vapor barrier, and the tile and thinset can be applied to the redgard - then what's the point of the hardibacker layer?

- Chris
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Old 08-21-2010, 10:52 PM   #2
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Welcome, Chris.

Too many layers of the wrong stuff to suit me.

But first, let's clarify the joist structure. You indicate you have 4x10" joists. What it the type of wood, the spacing, and the unsupported span of the joists?

For the subflooring, the MIA requires two layers of plywood or OSB, properly installed, for subflooring under a natural stone installation.

On top of that there must also be a proper tiling substrate of some kind. It can be a fiber/cement board like Hardibacker, any of a number of other CBUs, or an uncoupling or crack isolation membrane. You gotta have one of'em.

Without knowing just what it is you're calling "flakeboard," but even if it's OSB I say it's gotta come up, along with the pine flooring.

The original board subflooring, if in good condition, can remain and be topped with a minimum of half-inch plywood, depending upon the spacing of your joists.

There is no requirement for a vapor barrier anywhere in that subfloor package as far as tile installation is concerned. If you have a moisture problem it should be dealt with in the crawl space or basement or whatever else might be below your floor.

Not what you wanna hear, I know, but that would be my recommendation.

My opinion; worth price charged.
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Old 08-22-2010, 11:50 AM   #3
mertivsvyna
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The top layer is OSB and, as I said, deflection is NOT an issue. The building is a turn of the century warehouse and former machine shop. The 4"x10"x12' joists on 12" centers do not flex and the 3 layers of 3/4" are all tied together with a multitude of 2" exterior grade screws.

I'm not concerned about dampness from below but about protecting the OSB from dampness from above since it is a bathroom floor.
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Old 08-22-2010, 02:21 PM   #4
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We don't normally worry about waterproofing bathroom floors, but if you know you'll have a problem with a lotta moisture on yours, by all means waterproof it if you like.

I'll stand by my comments on the subflooring. Too much stuff in there to move about, some of it (the second layer of pine) installed using a method that is intended to allow some movement. Whether that will transfer up through your layer of OSB or not, I can't say.

And it's your house and your stone tile and your dinero and you can certainly put down a tiling substrate over what you have and install your tiles. Might work out fine for you.

Might not.

My opinion; worth price charged.
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Old 08-25-2010, 01:49 PM   #5
mertivsvyna
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Thanks for the opinion and info but my real question wasn't regarding the floor deflection but was:

Why use Hardibacker? If the floor is solid, the redgard will bond to it, and the tile and thinset can be applied to the redgard - then what's the point of the hardibacker layer?

Note: The best solution I've seen so far is the Ditra system:
- it can be bonded to OSB
- it's only 1/4" thick
- it provides a vapor barrier
- it ties together the tiles and mortar but lets this unified layer float to some degree over the subfloor
- it's light weight, easy to carry and cut

The only problem is that it's THREE TIMES THE PRICE OF HARDIBACKER !?!?!
What the heck....?
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Old 08-25-2010, 01:54 PM   #6
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you get what you pay for really. Ditra I think is only 1/8 the XL i think would be 1/4.

it does "float" on your underlayment per se and it is much easier to cut and work with than hardibacker or other CBU.

you dont' need to screw or tape and mud any joints, you cut it with scissors and u thinset it straight to your underlayment.

but like you said it is much more $$ so that's where you weigh out your differences is the $$ worth the time savings and other benefits, your call.

a properly installed CBU tile installation will work just as well too.
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