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I’m building an ambitious wet room style shower/bath in a 1911 basement. It happens to be in a corner with 7’ concrete walls on both sides. I need cement board on all sides, top, and bottom. I can’t afford to lose the space on the outer walls taken by framing a stud wall on top the concrete to hang cement board. I need to maximize space for a freestanding tub.

I want to find a way to fasten and level multiple sheets of cement board on the concrete wall without a deep structure underneath it. Like any old foundation, there’s at least 1 inch variance +/- at any point in the wall.

My first throught is some kind of Tapcon style fastener that could by turning left or right, adjust the distance of the cement board from the wall, but I can’t seem to find anything like this already done. It also seems a little bit ridiculous given how many fasteners cement board needs.

I welcome any zero or ultra low profile ideas! Those 2-3” really mean a ton.

As always, thanks for your time!

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    Does the pic show what you're working with or does it show what you're aiming for? I don't see anything which looks like "at least 1 inch variance +/- at any point in the wall" there.
    – brhans
    Commented Jul 3, 2023 at 7:31
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    If you are open to suggestions, just coat the walls with an epoxy, like that used on garage floors. The unevenness becomes an accent feature.
    – RMDman
    Commented Jul 3, 2023 at 11:36
  • How are you planning to finish the walls? The picture doesn't show tile, as far as I can see. Some kind of stucco-like coating?
    – Huesmann
    Commented Jul 3, 2023 at 12:15
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    You've asked something of an XY question. Why do you think you need to use cement board? Is that a crutch for poor troweling skills?
    – isherwood
    Commented Jul 3, 2023 at 14:20
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    Why do you think you need cementboard? If you're not going to insulate the walls (a space-saving decision which you may come to regret) bare concrete is good for anything cementboard is good for, functionally.
    – Ecnerwal
    Commented Jul 3, 2023 at 15:54

5 Answers 5

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The thinnest possible solution is to straighten the wall with mortar and a mesh. Then apply thinset directly to the mortar.

Prepare the surface of the wall by scraping any loose paint and scarring it. Make type N mortar, which is best for working with foundation repair. Decide which wall will be the reference for squaring the shower -- assuming the foundation surface is not perfectly square. Use a laser and/or lines to guide the new layer of mortar. With a trowel, apply in thin layers. On any thick sections, use masonry mesh, sold at any hardware store. It helps keep the mortar from falling as you apply it. But you can also just apply it in thin layers. Once you are done, no need to use the cement boards. Cement boards are a substitute for mortar, and you are already doing it the old-fashioned way.

Regarding other answers suggesting grinding down a foundation: Really? The cost of the disk is more than all of the mortar you could ever use. It's also never a good idea to grind a foundation. Instead of spending $300 in an angle grinder and disk, invest in a good $20 trowel + $20 worth of materials, and learn a useful skill.

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  • Ah. Maybe "scarifying".
    – isherwood
    Commented Jul 3, 2023 at 19:07
  • A silicon carbide block is all of $10, and remarkably effective. Smoothing minor projections on a foundation wall has no structural impact.
    – Ecnerwal
    Commented Jul 4, 2023 at 12:11
  • The OP mentioned a 1911 house with 7' basement. It's fair to assume a fieldstone foundation. I don't think that you are going to wear down 1" of granite stones by hand. But some people have a lot of time, so it might be worth it to them.
    – Cheery
    Commented Jul 4, 2023 at 13:34
  • The OP mentioned concrete walls, so assuming fieldstone is quite a stretch, actually.
    – Ecnerwal
    Commented Jul 4, 2023 at 14:16
  • I've never seen a pre-war concrete foundation in residential construction, but I suppose they exist.
    – Cheery
    Commented Jul 4, 2023 at 16:54
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Grab your dust mask!

Get a diamond cup (or two or three?) for your angle grinder and grind the walls flat. Heck, you will even gain that 1" of floor space due to removing the unevenness instead of filling the gaps.

Yes, it will be slow. Yes, it will make a mess. Yes, it will be very time consuming. But, you'll end up with a nice flat surface to attach your cement board to without loosing any space.

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  • There are self-cleaning vacs around, that are made to deal with the dust during concrete grinding.
    – Martin
    Commented Jul 3, 2023 at 15:04
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    That's true, @Martin, and it will help, but it will still make a mess. This is actually an appropriate use of a dust mask. And a good opportunity to use some 6 mil plastic sheeting to curtain off the area to minimize the dust spread to the rest of the house.
    – FreeMan
    Commented Jul 3, 2023 at 15:16
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Why not just tile the walls? You can get tiles with concrete appearance.
This solves the problem of taking minimal space while not being labor intensive. It is also waterproof. Any tile guy can work with wall having 1" of bump, tile glue has a wide range tolerance (they will probably pre-glue pieces of cement board in the holes to even out the surface).

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Most small variations can be dealt with by adding lath where needed,using a line to level to. There are laser line levels or you can go old school and simply use a taut string, as if taut it Will be straight line. There is also a product named Schluter Ditra which may bond directly to the concrete, if, as one answer previous suggested---you can live with the cave like natural variations of this old poured concrete. I find that hexagonal tiles tolerate variations well. I'd go to a tile place and ask questions there. Not home depot or the like. A real place patronised by trades. You will get good pragmatic answers and suggestions from either the staff or from people who do this day in and day out and admire your chutzpah. I don't like the idea of you grinding concrete. That's a job for professionals. It's dangerous and foolhardy in two ways. Best of luck with your project! (The product I mentioned is a decoupling membrane and minimises tile cracking.It looks odd to my generation ---old school-- used to the heavy old cement board, but seems to work well.)

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    An edit for formatting would improve readability of this answer. While you're at it, you could include some tips on how to apply the lath to get a level surface. There's nothing old-school about a string - masons use them all the time. You also complain about two ways that grinding is "dangerous and foolhardy" but don't expound on either of them. It would be more appropriate to put them as comments on the "grind it" answer, but since you don't have enough rep, at least list them here...
    – FreeMan
    Commented Jul 3, 2023 at 16:05
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I want to find a way to fasten and level multiple sheets of cement board on the concrete wall without a deep structure underneath it.

Is this a requirement or just an explanation of the current situation?

Anyways, create a temporary wall to hold the cement board vertically as close to the foundation as possible.

Now you have a few options:

  • Fill the void with concrete
    • Once dry, then fasten the cement board
  • Spot-fill the planned screw locations with canned expanding foam

Considerations:

Cement board does flex so consider using thicker or multiple sheets so that your tiling work doesn't turn to garbage as soon as someone leans on the shower wall. This consideration is specific to the "spot-fill" solution.

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