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Unread 01-25-2022, 08:28 AM   #1
kamaitland
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Leveling Bathroom Floor

We are remodeling a bathroom and have pulled up old tile floor down to the original subfloor boards. We installed new 3/4" tongue and groove ply wood and then hardiebacker board on top of it screwing each layer down. We have a nice flat surface but noticed that the beam in the middle of room (only ~6' x ~6') is a little lower causing about a 1/8-1/4" dip in that area. Looking for the best way to level. We were just going to use leveling compound on top of the hardiebacker board to build up that area and then tile on top (will probably be using 12"x24" tiles).
1. Is this the best approach?
2. Should we use self leveling compound or just regular leveling compound?

Any other expert advice is welcome.

TIA for the advice.
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Unread 01-25-2022, 09:28 AM   #2
cx
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Welcome Kerry.

You don't need to level your floor at all to make your tiles happy, they care only about flat. The larger the tiles, the more they care.

If your floor is already level, using a self-leveling material to fill in the dip may work well enough, but if the floor is not level, the SLC may cause more trouble than it's likely to fix. A cementitious patching compound may be your best bet there.

But first, did you put a bed of thinset mortar over your new plywood subfloor before attaching your Hardiebacker?

And did you evaluate your joists structure to determine if it qualified for a ceramic tile installation before you installed your new subfloor?

My opinion; worth price charged.
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Unread 01-25-2022, 10:10 AM   #3
kamaitland
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Thank you for the reply CX. We did not put a layer of thinset between the plywood and hardibacker, is that a must? What purpose does it serve and what are the cons if not doing it?

As mentioned the one joist is apparently a little lower than the rest so it causes the gradual dip from the two joists on either side (16" on C) with the max depth of about 1/4 inch. The rest of the floor appear level as measured by using a 4' level across the floor.

In terms of evaluating the joists for ceramic tile, I'm not sure what you mean. If you are suggesting evaluating for load, I can tell you that the previous floor had the original sub-floor boards, a couple of layers of 1/4" lauan plywood in pieces and parts patching then at least 1" of cement and mortar and a thick 1970s mosaic tile on top of that (see pic for reference).
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Unread 01-25-2022, 04:41 PM   #4
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Well, if you had tile before and you're comfortable that your structure is sound enough to have tile on a CBU installation rather than a mortar bed, that's up to you. We got away with a lot back in the day when all floor tile was set over a mortar bed. Doesn't always translate well to the newer methods.

If you'll look near the top of the page in the dark blue bar you'll see a link to our Deflectometer. Putting your joist information into that calculator will give you a quick go/no-go indication on the joists.

As for the mortar bed under the Hardiebacker being a must, I'd say it's rather an absolute must. And the manufacturer requires it. You did, of course, read the manufacturer's installation instructions before you installed the product, right?

It's really important, and it's especially important with Hardiebacker because of the density of the board and the abrupt, rigid edges. The purpose of the thinset mortar is to create a 100 percent footprint for the CBU panel and allow no vertical movement at all. The mortar holds it up, the mechanical fasteners hold it down.

My opinion; worth price charged.
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Unread 01-25-2022, 06:25 PM   #5
jadnashua
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Please, people, READ THE INSTRUCTIONS!

If you have questions on interpreting them, feel free to ask here, or call the manufacturer for guidance.

Also note, the instructions can change as experience or research exposes an issue, so the instructions get changed so you'll have a reliable end result. Second guessing them, with a reputable company anyway, is throwing away lots of expert advice. IOW, what might have been may no longer be. They don't change things just for the hell of it.

In addition to bedding the cbu on the floor, prior to laying tile, you need to use their alkali resistant mesh tape on the seams. This helps to make the assembly act like a monolithic structure rather than lots of smaller parts that can move independently.
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Unread 01-25-2022, 06:34 PM   #6
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Why, Jim. You sound just like a crotchety ol' engineer.
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Unread 01-26-2022, 12:43 AM   #7
jadnashua
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That sounds about right...

Some of these products are quite highly engineered and if you don't understand the instructions or follow them the results can be less than you want. The good thing is that they sometimes have some overkill built in, so (slight) errors in application MIGHT not produce a problem. Following the instructions with a quality engineered product pretty much guarantees a reliable outcome. You MIGHT get lucky, but then, you might not!

Personally, I like to understand the 'why' they want things done a certain way, but that's not necessary. It IS necessary, if you decide you need or want to deviate, and only then can you make an assessment as to whether it's likely to work or not in your situation.

The other hassle is that some things can take a special set of circumstance to fail. That may not occur for years. Some things progressively, slowly degrade when you cut corners, and may not show up for maybe even a decade or more.

That 300-pound mother-in-law in spiked high heels makes some significant point loads...that can ruin your day!

You go for ages with a max of one or two people in the bathroom at the same time, then, you're showing off, and there's 5-6 people in there for a bit. That extra load can cause something to fail that worked just fine with one or two. Throw a party and have 20 people, all of a sudden things don't work right and tile crack or the bond breaks.
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