Hardi backer board over concrete in bathroom?

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Old 04-26-08, 09:04 PM
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Hardi backer board over concrete in bathroom?

So, I've already cut all the hardi board to put on the floor and I got ready to mix the thinset and thought I would double check my procedure on the good 'ol inter-web... Glad I did because it made me really question what I am doing.

The jist of my situation is that I am renovating my 1920's home and I tore out a closet that stuck into the bathroom to make it bigger. (actually just usable.) The floor in the bathroom is concrete and the closet floor is wood. I poured concrete and then a top layer of self-leveling compound to fill the gap between the wood and concrete and my original theory was to stick 1/4 inch hardi board over the entire floor to bridge the seam between the old closet and original bath.

The problem is I can't exactly screw the hardi down to the concrete floor so I was considering using glue instead to be sure it wouln't get loose some day. But then I started thinking... Is the hardi really even necessary? What do you guys think?

Here's a few pics in case anyone is interested:

before:
http://i55.photobucket.com/albums/g1...e/IMG_3715.jpg

during:
http://i55.photobucket.com/albums/g1...e/IMG_3801.jpg

Thanks,

GM
 
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Old 04-27-08, 06:29 AM
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You cant install the hardi over concrete, thats correct. When you say concrete floor is this a slab or is this a mud bed over a framed floor that you removed the tile from?
 
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Old 04-27-08, 09:14 AM
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HeresJonnie is correct, and it's BTW not intended in lieu of correct sub-flooring either. Here's the cite and link we provide in our inspection reports:

"The minimum subfloor specified is 5/8" exterior grade plywood or 23/32'' exterior grade OSB subflooring. HardieBacker board must be installed in accordance with local building codes and the floor must be engineered not to exceed the L/360 deflection criteria, including live and dead design loads, for the specific joist spacing used. HardieBacker board is not designed for use over concrete. "

- HARDIEBACKER™ BACKERBOARD - FAQ

There's a lot of useful information in the FAQ, it's well worth reading through from top to bottom if you plan on installing HardieBacker board.
 

Last edited by Michael Thomas; 04-27-08 at 09:49 AM.
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Old 04-27-08, 09:29 AM
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I would use Schluter Ditra in this area.

What was the wood structure comprised of before you poured concrete?
 
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Old 04-27-08, 06:20 PM
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Thanks for all the help guys! I'm really glad I asked now.

The concrete in the bathroom area is about 4 inches thick and was poured over a framed floor during the original construction of the house.

The wood floor in what used to be a closet is the original tongue and groove hardwood floor (apx 3/4" to 1") that is installed without sub flooring directly on the joists. The closet was only about 3x3 so there is a rather fair amount of support underneath.

I filled in the gap between the concrete and the wood by bracing to the framing on both sides and building a "box" to pour high strength concrete in. I filled it within an inch or so of the floor level, let it cure and then poured the self leveling compound on top to level it all out.

What would be the correct way to do this? Will I be ok just leveling out the whole area and laying the tile right on top? I don't want to be yet another home owner that half a$$es the job.

Thanks again!
 
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Old 04-27-08, 06:30 PM
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Making sure you have a flat floor is important, so pour your SLC.

If your budget permits, install Ditra as an uncoupling membrane. If your budget can't handle this, (about $83, 54sqft) then use a high quality modified thinset like Flexbond.

You would thinset Ditra down to the subfloor, then tile.
 
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Old 04-28-08, 01:15 PM
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So you have 4" of mud over a framed floor. That sounds real good. If the floor has some small craters in it, you can fill them in with thinset prior to tiling to get the floor nice and flat. Ditra would be insurance, but in my opinion, not needed here.
 
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Old 04-28-08, 08:52 PM
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well, most of the floor has the 4" of concrete except for the 3x3 square in the corner that used to be a closet that's still wood. I leveled the floor but I didn't pour SLC on the wood because it was just a little higher than the rest of the floor and I didn't think 1/16 to 1/8" of SLC would be very good over the wood floor. Should I use mastic to stick the tile to the wood? (I feel like I'm half a$$ing it again...
 
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Old 04-29-08, 06:59 AM
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Looks like the plywood and mud bed or slab are in plane with one another. Being so, I think a membrane such as Noble CIS or Ditra would be in order here, but I'd go one step further and install a movement joint between the two surfaces by installing a soft joint (color texture matched caulk on the plywood side of the tiles that bridge the joint. Seriously consider replacing that window with a vinyl clad window as well and waterproof the cbu around the window with a liquid waterproofing membrane over the jams, sill and about 6" around the walls surrounding the windows.
 
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Old 05-18-08, 10:30 AM
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Same but different

I too have a problem of concrete over wood framing in my foyer, but my problem is I need to raise the floor at least 1/2" to 3/4" to make a decent transition from tile to carpet. It originally had tile with 1/2" concrete board nailed into the concrete. This is an area of about 10' by 10'
The concrete I'm dealing with is only a couple of inches thick and when it was originally poured, they must not have had any trowels with them because it was absolutely pathetic. It was off 1" from one end to the other. I poured leveler and got it pretty close to the point where I could install 1/2" Hardi over and make it ready for tile. But now I'm reading to find out I can't use. I have been in the building and remodeling business all my life and I'm at a stand still on this one. Any help would be appreciated!
 
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Old 05-18-08, 10:43 AM
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Hardiebacker does not install over concrete.

hardiebacker is to be installed over wood subfloor or studs per manufacturer.

What are your joists sizes and spacing, along with unsupported span?

Only option I see is to install a mud bed to the desired height as long as your currently joists meet deflection.
 
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Old 05-18-08, 11:25 AM
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Joists are 16" O,C. but are only 2x8's with a 2x4 laid flat on top, 3/4" sturdi floor and then 2" concrete.
 
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Old 05-19-08, 07:03 AM
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esti

You can install the tile over the slc. As an added precaution you could use an isolation membrane over the slc. BUT DO NOT INSTALL HARDI OVER THE SLC.
 
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Old 05-24-08, 10:47 AM
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How do you suggest I raise the level of my floor 1/2 to 3/4" without using board? I don't think my floor joists can handle that much weight if I use concrete.
 
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Old 05-24-08, 03:17 PM
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You could install Schluter Ditra which is 1/8" high, plus the thinset and tile should give you a little over 1/2" height. Ditra also has a XL version which is 1/4" thick, but not readily available.

Ditra is expensive so have you thought about a mud bed like I suggested earlier?
 
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