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I am renovating our primary bathroom in our 1958 home. I demoed the tile and decided to demo the dry pack used to float the bathroom. After getting the dry pack out it appears my bathroom slab is roughly 2 inches lower than the homes slab (anyone know why?).

I could float the small 35 sq ft bathroom area again with dry pack, but I was curious if anyone could recommend a self leveling product or cement mix that would bring the slab to the same level. Looking at a few of the self leveling products at the big box stores, I am wary about using a self leveling product and potential cracking. I will be laying tile on top. enter image description here

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  • The type of finish might give some better options. Probably a simple cement in bags might be cheaper than self levelling compound.
    – crip659
    Jan 5, 2023 at 20:24
  • with few layers of leveling concrete, letting it dry in between the layers
    – Traveler
    Jan 5, 2023 at 20:25
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    If you live in an area that gets cold, this would be the perfect opportunity to put in under floor heating.
    – Glen Yates
    Jan 5, 2023 at 21:44
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    I would ask the spouse about that. Jan 5, 2023 at 22:08
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    Looks like you have a great opportunity to fit in a floor drain (which is oddly rare in north america, but SO useful for washing the floor) and/or heating flooring
    – Alexander
    Jan 7, 2023 at 1:36

4 Answers 4

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I would suggest using concrete and leave enough depth for expected tile work.

Per https://www.cement.org/docs/default-source/fc_concrete_technology/is144-07t-resurfacing-concrete-floors.pdf

The minimum recommended thickness is 1 to 2 in. (25 to 50 mm) for a fully bonded concrete overlay placed on a base slab that is practically free of cracks and in which the concrete is sound, clean, and of good quality. The use of welded wire fabric reinforcement is usually not warranted under these conditions. In general, cracks appearing in the existing base slab can be expected to reflect through bonded overlays.

Since the existing slab is old then I recommend properly cleaning, scuffing, and applying a concrete bonding adhesive/agent to the old slab.

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  • Definitely agree with using concrete. You'd need 4 layers of the leveling cement.
    – JACK
    Jan 5, 2023 at 21:30
  • @MonkeyZeus Thank you! This is great information. I am leaning toward concrete and will find the best product for application at this thinness.
    – s3aborn
    Jan 5, 2023 at 21:53
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    @s3aborn Just make sure to take into account the thickness of any planned tile work!
    – MonkeyZeus
    Jan 6, 2023 at 12:59
  • I am struggling to find a concrete application that calls for less than 2 inches of application. Could you suggest something? It seems 1.5-1.75 inches is right in between self levelers and your typical concrete mixes.
    – s3aborn
    Jan 9, 2023 at 17:08
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    @s3aborn You have to remember that you're basically skim coating new concrete on top of your slab; it's okay if it's a little more watery as long as you remove the really large aggregates. The slab is structural, the skim coat is for flatness. If you need more inches then grab a jackhammer. Make sure to let your new concrete cure for the full 4 weeks since you plan to lay thinset directly on it. If you have enough thickness to lay down cement board then I would go for that and self-leveling compound would suffice. You can also look into GFRC (glass fiber reinforced concrete).
    – MonkeyZeus
    Jan 9, 2023 at 18:56
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What a great opportunity to do a slightly sunken walk in shower so you don’t need to have any curbs or whatever. Drop a claw foot on top that drains to the floor? Very cool.

Definitely go with a concrete and both major manufacturers have a powder latex modification additive you can use to help with adhesion. Clean and vacuum the existing concrete well.. then etch the concrete with and acid wash. Then apply your new latex modified concrete and you should be good.

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  • quite tempting to take on the curbless idea! I am struggling to find a concrete application that calls for less than 2 inches of application. Could you suggest something? It seems 1.5-1.75 inches is right in between self levelers and your typical concrete mixes.
    – s3aborn
    Jan 9, 2023 at 17:08
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The old homes built from the 40s to and early 60s used concrete as the base for a tile floor. The usual rule was a minimum of 2 inches. This is why the floor is 2 inches lower after you removed the drypack. If you were going to place tile you should have left the drypack there.

Now to correct the issue, pour concrete to about 1 and 3/4 inches. ( Leveling compound is too expensive to use for the whole 2 inches and isn't recommended for that thickness.) Then when it is dry, ( after at least 24 hrs) pour leveling compound to make everything the correct height and and smooth.

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  • I am struggling to find a concrete application that calls for less than 2 inches of application. Could you suggest something? It seems 1.5-1.75 inches is right in between self levelers and your typical concrete mixes.
    – s3aborn
    Jan 9, 2023 at 17:09
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    2 inches is the minimum thickness for a serviceable concrete pour over an existing slab, or where ground movement may be an issue. For your application there will not be any problem using that concrete mix and then leveling compound. To be sure any cracks, ( if any do develop) do not transfer to your new tile, use a decoupling agent over the leveling compound. This can be a coat of Aquaguard or similar.
    – RMDman
    Jan 9, 2023 at 17:41
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I would suggest using the 'self leveling concrete" so you do not have to worry about leveling it.

In you case you have 2 inch total.

First deduct the tile thickness (usually 1/2 inch or more)

Then deduct the mortar thickness, (usually 1/4 inch)

So now you are left with 1&1/4 inch required cement thickness

That can be easily achieved with two coats of self leveling concrete.

Surface preparation is critical, it has to be dust clean and primer coated for adhesion.

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    i've used self level compound, it doesn't self level, I can't imagine self leveling concrete actually leveling, does it? Jan 6, 2023 at 4:00

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