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Unread 09-06-2020, 10:36 PM   #1
hardieboy
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flatten pre-cementboard install

Facts:
3/4 plywood sub

small bathroom, area is 5.5ft wide by 7.25 ft long

bit of a high spot in middle of the length of room spanning the width causes 1/8-1/4 inch flatness problem (I can make the 6 ft level spanning length rock back and forth).

no joist problems.

looking to put down 1/4 inch Hardie Cementboard, then 18"x18" ceramic tiles.
I've done my homework, I know the substrate needs to be REALLY flat for tiles this size. For whatever reason I want to think by laying the first two Cementboard pieces the length of room will result in a flat surface, but I don't want to put it down and go hit my head against a wall.

Any recommendations for flattening out pre-cementboard install?
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Unread 09-06-2020, 11:08 PM   #2
Tool Guy - Kg
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Welcome to the forum, WS!

The cement board will exactly follow the contour of the subfloor, irrespective of the Hardibacker orientation.

Folks are often tempted to patch the bare subfloor before installing the Hardie, thinking they will have a nice flat surface to tile over. But the patching material has a nasty habit of “volcano’ing-up” when screws are fastened down through the Hardie. And you don’t want little hard piles of crumbling patching material preventing your Hardie from being installed all the way down into the combed mortar bed.

You can install the Hardie, per instructions, then apply a cementitious patch to the top of the Hardie on the low ends of the room to flatten it all out.

Or you could use self leveling cement, but working with the stuff is finicky when you are trying to pour such a thin layer or when trying to work it to a feathered edge. If you do decide to use self leveling cement, I’d suggest researching brands, as they all have their own installation instructions and some cement board plays nicer with self leveling cement than Hardiebacker. Or you might consider using Ardex Liquid Backerboard in place of the cement board/self leveling cement combo.

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Unread 09-06-2020, 11:14 PM   #3
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Thanks for the timely reply.

I have seen some recommendations for self leveling (which I was trying to avoid altogether, but understand that may not be possible) over cement board.

What about self leveling the subfloor and then installing cement board over that? My own logic tells me that’s a no-no.
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Unread 09-07-2020, 12:26 AM   #4
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That's what he was explaining in the second paragraph of his post. You don't want to be installing nails or screws through a cement material like self-leveling compound.
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Unread 09-07-2020, 10:29 AM   #5
hardieboy
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Thanks, I see that now.

So after doing some more homework (looks like Ardex and Henry are the same company), I’m looking at using Henry 542 liquid backerboard.

Prior to application, I plan to patch seems and nicks in subfloor with Henry 547 universal patch and skimcoat with 546 additive (which is in instructions for the liquid backerboard), then prime with Henry 554 for wood.

Also using edgeban for the perimeter.

Then I intend to apply red guard waterproofing and crack prevention membrane prior tiling, as well as use a mortar meant for large tiles.

Anyone see a problem with this strategy?

I guess my only concern is having to access some plumbing or subfloor in following years, or my wife wanting to replace the tile in 15 years. If I’m looking at this right, I would have to replace the subfloor in such a situation.
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Unread 09-07-2020, 10:39 AM   #6
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What's the purpose of using Redgard over the liquid backer board?
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Unread 09-07-2020, 11:11 AM   #7
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For waterproofing and crack prevention.
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Unread 09-07-2020, 01:14 PM   #8
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Having laid down about 20 or so bags of self leveler in my past DIY projects I agree with Tool Guy that it is very finicky. That said, it is the easiest way to go - especially for such a small space. I'd lay down self leveler before putting down the backer board. The screws should have no problem going through a thin layer. If there is a thick spot it is easy enough to pre-drill.

You should have a spiked roller to break the surface tension and spiked shoe covers to give you access to the whole room. Also, despite it being "self leveling" you should screed along two parallel rails to achieve the best result. After curing you can carefully fine tune with a cup grinder (with a dust shroud and vac). Don't use rapid set self leveling compound unless you are very experienced and don't forget the primer.
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Unread 09-07-2020, 03:26 PM   #9
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Welcome, WS.

I'm gonna disagree with Tom here. You do not want to apply a SLC over a wood framed floor over which you intend to install a CBU. The danger is not in not being able to penetrate the SLC with your fasteners, that's quite easy. One of the dangers is in breaking up the cured SLC and creating pieces that will keep your CBU from installing tight against the subfloor as intended.

The procedure is not recommended by the manufacturer of the CBU nor the manufacturer of the SLC.

The industry recommendation, along with the product manufacturers' recommendations, is that for CBU you do your flattening or leveling after the CBU is installed; for sheet-type substrates, you do all your flattening or leveling before installing the membrane.

My opinion; worth price charged.
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Unread 09-07-2020, 05:28 PM   #10
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Thanks again to all, the advice is very much appreciated.

I do not intend to install the CBU over the SLU. I’ve ordered everything needed to execute my plan as stated above, using Henry Luquid Backerboard in place of cement backerboard.

CX, anything seem off on my plan from earlier post?
Quote:

Prior to application, I plan to patch seems and nicks in subfloor with Henry 547 universal patch and skimcoat with 546 additive (which is in instructions for the liquid backerboard), then prime with Henry 554 for wood.

Also using edgeban for the perimeter.

Then I intend to apply red guard waterproofing and crack prevention membrane prior tiling, as well as use a mortar meant for large tiles.
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Unread 09-21-2020, 09:03 AM   #11
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So, this did not go great. It's definitely smooth, but it is not flat. And the far end of the room still drops off a quarter of an inch. I also have a low spot on the left side of the room where I did not before. Any advice? Can I add more Henry Liquid Backerboard to the far end of the room to get to a feathered edge?
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Unread 09-21-2020, 09:54 AM   #12
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Not sure just what Henry's policy on re-coating might be, WS, and I don't see it in their online installation instructions. You might wanna contact them with the question.

My opinion; worth price charged.
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Unread 09-21-2020, 10:06 AM   #13
hardieboy
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I called them and they said I had two options:

1) reprime and re-coat with liquid backerboard.

2) use Henry 549 in low spots to build up.

Seems like option 2 has been recommended against in this forum. Am I reading this wrong?
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Unread 09-21-2020, 12:34 PM   #14
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Where did you see that it's not recommended?
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Unread 09-21-2020, 01:50 PM   #15
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I guess it was more a building up of low spots with the thin set that I read was not recommended.
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