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We are currently redoing our shower and would like to use float-cement to carry the tile. The wall behind it is a bit crooked, so, to get it nice and level, the float-cement would have to be a bit thick (>1") in some areas.

Is there a maximum thickness that float-cement can have, for example before it starts cracking in ways that could damage the tiles attached to it?

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I am not aware of any maximum thickness requirements, only minimum. Many tile setters do not like going over a cement base that is thinner than 1 1/2" for fear of cracking, although there are additives of one sort or another to get it thinner, though not much. To have more concrete on one side or another to account for an existing condition, will not adversely affect the concrete. The concrete needs to be sloped to the drain anyway so no shower water sets.

The concern you have with a thicker concrete base is confusing to me. The only way that a thicker base can crack is to put too much water in the mix, when the water dissipates, the concrete will crack from shrinkage. But the amount of water to be used in that situation is to be only enough to get the concrete to set up. The term used with this type of concrete setting is "dry pack" at least it is what the tile setters called it. The amount of water needed to get the concrete damp enough to stay in a ball when you pick up a handful and squeeze. No squishing, no water dripping. Just getting it damp enough to hold together. With this, there is no maximum, though 2"-2 1/2" is the thickest ever needed to make the base for tile. You could add reinforcing wire to it although I have never seen it done for showers. mainly because of the rust potential, I think. Concrete and galvanized don't get along too well, let alone regular wire mesh with no rust proofing.

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A friend of ours with a background in construction was telling me that "Basically, float-cement is the same as stucco, and if you make it too thick, it will crack". I'm just trying to verify his assessment. Do you know if there are different "cements" that can be used for this job and if they come with different restrictions? –  Markus A. Feb 17 at 23:28
    
Stucco is applied wet. If you placed the concrete base in the shower that wet, yes it would crack. Adding just enough to get it damp solves that problem. –  Jack Feb 17 at 23:30
    
There may be other cements, but I have always seen Portland cement and sand used for this. I does need to be a strong mix, this is it... Stucco base will be the same, if I recall right that is on the shelves of the big stores. I myself use sand mix concrete. –  Jack Feb 17 at 23:32
    
We're using the Spec Mix Pre-Blended Mortar Type-S, which I believe is a Portland cement and sand mix also –  Markus A. Feb 17 at 23:33
    
So, if I understand you correctly, we can make it as thick as we need to, if we make sure to keep it dry enough? –  Markus A. Feb 17 at 23:34

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