We hired a contractor to remodel our master bathroom. For the tile installation in the shower, he started with a float-cement backing. To show quick progress, he rushed through this step and the final surface was uneven with deviations of over an inch in some places. He assured me that this was no problem and proceeded with setting the tile. To make it come out straight, he had to use well over 1 inch of thin-set in some spots:

Thin-set thickness

After pointing out to him that the instructions for the thin-set he used (MultiSet Modified Thin-Set Mortar by Custom Building Products) state that it should not be applied thicker than 1/4", he told me that he had mixed in some Portland Cement (Spec-Mix Type-S) to give it extra strength and that I shouldn't worry about it.

He had to pause installation for a bit and after 12 days of curing-time, I tried to see if I could peel off one of the tiles (a 7 1/4" x 7 1/4" piece that was crooked and needed to be redone anyways) by hand and it popped right off in one piece:

Peeled off tile

And left the wall underneath it looking like this:


Is this how it's supposed to be or did the added Portland Cement reduce the thin-set's stickiness? Are there any other potential issues with mixing cement into thin-set and applying it in this thickness?

  • Look at the voids and gaps in the thinset. Lots of crazy places for interesting moisture behavior.
    – Bryce
    Mar 16, 2014 at 5:23

3 Answers 3


Portland Cement has poor adhesive qualities by itself, which is why it is commonly mixed with a polymer. By definition, "thin-set" refers to an application thickness of under 3/16th of an inch. Other common additives to thin-sets are water retaining compounds that help them retain moisture to extend the curing time for more strength. By adding additional Portland Cement, both of these properties have been compromised in the mix. As far as curing goes, the ANSI spec for thin-set performance specifies a final set time of under 4 hours (this link has a good run-down). Tiles should be difficult to peel off within an hour.

That said, there are several other issues I can pick out from your pictures:

  • Even if the tile does adhere properly, the grout will most likely start cracking within the first couple of months. There shouldn't be any of the huge voids visible in the bottom two pictures, especially in the corners. When you float grout in, there won't be anything to support it in places other than the sides of the tiles themselves and any minimal keying from packing it into the joints.
  • In the top picture, it appears that the float-cement backing was applied over sheet rock or plaster? If this is the case, it will wick moisture into the wall. Although this will be a fairly small amount, eventually it will start bubbling around the edge.
  • Weight. This installation will weigh approximately 20 pounds per square foot more than another backing type. On a small shower, you're talking close to a ton of excess weight. If this is on a framed floor, it will deflect and exacerbate all of the other issues.
  • I don't see any indication that any structural support was incorporated into the float-cement (wire mesh, fiberglass mat, etc.), and certainly not in the "thin"-set. This means that it will be prone to cracking over time.

I think @DMoore hit the nail on the head with this one. You need to have this contractor stop work immediately. The simple fact that he can't lay tile straight (and isn't using spacers) is fire-able in my book sub-strait aside - now you're talking about what you will see when it's done. Schedule a time for a building inspector to come by and take a look at it (you did get a permit, right?) - they are generally less busy this time of the year, so will be a lot more willing to give you a good opinion. Tell them that you are concerned that the work being performed isn't up to code and that you'd like somebody to take a look before it gets covered up.

As far as how I would go about an installation like this, I would generally back everything with framing and half inch cement-board. If I was worried about deflection at all, I would thin-set a second layer of cement-board to the first layer with staggered joints. I haven't seen a shower around my neck of the woods done with float-cement backing that was installed more recently than about 1960. Even though the materials cost more, if I'm billing $40 to $50 an hour for labor it still costs half as much overall, doesn't have long down times for curing, and is a ton easier to work with.

  • Very good post. And I totally missed out on the weight possibly causing more issues. I honestly never heard of mixing cement with thinset. This is like one of those DIY don't shows but worse. What is also bad is since he is using concrete it would probably be "fine" for a few months until there was a domino of problem. Really sad. Hate seeing people go through this.
    – DMoore
    Mar 15, 2014 at 4:34
  • The drywall actually stops right where the float cement starts. The cement is built on top of a mesh that was stapled to the wood-framing underneath with roofing-paper in-between. You make a lot of good points. Thank you. We'll see how we can get this fixed...
    – Markus A.
    Mar 15, 2014 at 16:15
  • He is using spacers most of the time and most tile did come out straight. It was just this one piece that was a bit by itself and needed to be parallel to an opposing wall and at a 45 degree angle to its neighbor. He probably just eyeballed this one. The rest is pretty straight and at right angles.
    – Markus A.
    Mar 15, 2014 at 16:42
  • I loved your idea to schedule a building inspector to come by for a second opinion. But unfortunately, when I called the city, their response was "That's just a finishing job, like painting. We don't require permits for that and don't have any inspectors for this." Is there any other independent agency that I can ask for a second in-person opinion? I'd like to avoid other contractors, as they always have a financial incentive to play up the negatives to seem smart and get us to hire them for any next jobs...
    – Markus A.
    Mar 18, 2014 at 17:29
  • @MarkusA. One other option might be to get a real estate home inspector to come out and look at it. Explain that you don't need a full inspection but would like to have them take a look at your bathroom. Some home inspection companies will do this for a pretty reasonable fee if the scope of the inspection is small. Home inspectors are pretty well tuned to the types of long term problems that may crop up. They won't give you recommendations as to how to fix it, but would be able to go over concerns they would have.
    – Comintern
    Mar 18, 2014 at 22:28

Whatever he did to modify the thinset obviously does not work to the most extreme power - so it doesn't matter. Obviously he added something to the thinset or your tiles would stick much better. If it was a pure "thinset fail" then it would be pulling from concrete. There is no use arguing with the guy. It didn't work. Get your money back. Don't let him touch your house again. Sorry.

I am also gonna take a wild guess and say you paid for the "job" not materials + work separately?

Note based on author comment: Markus - I always suggest that people pay for their materials and then pay for labor separate. What you are experiencing is someone who has no idea what they are doing. Creating concrete shower walls is a bigtime skill that most good contractors don't have - sure as hell I don't and I have done 20+ showers on houses I have flipped. He is using concrete because it is super cheap and super labor intensive. The problem is that it is a major PITA to demo too. A big shower could be a be a couple thousand pounds. Most people use concrete board these days - even really bad contractors know this. I can prep a shower surround and the next day be done tiling it. Everything that he has done needs to be demo'ed. You cannot have bumpy/curvy concrete walls in your bathroom and you don't need concrete period. Every time you let him do more work you are costing yourself more money in demo and materials.

  • We bought the tile, and he's providing the thin-set and other materials as well as labor. Is it cheaper to work this way? He seems to be spending an awful lot of time trying to get the tile straight. I think the whole thing should have been much easier and faster if he had done the float cement right... And, since the thin-set is so thick, he's tiling 2 rows at a time and then letting it dry overnight before coming back for the next two rows. This can't be more efficient, can it?
    – Markus A.
    Mar 15, 2014 at 0:59
  • Wow... That's a harsh assessment! ;) When he first started with the float cement, I read up on this technique on the internet and it sounded like this was the preferred method over backer-board as it basically creates a one-piece shower box to tile on rather than consist of sections that come together in the corners. Supposedly this helps prevent leaks and cracking when the walls behind the cement/backer-board shift, which, since we live in earthquake-rich California, sounded like a good idea to me. Did I misunderstand something there?
    – Markus A.
    Mar 15, 2014 at 4:06
  • Also, I'm not sure he's going this route because it's super labor intensive. We (maybe made the mistake to have) hired him to do the entire remodel and when he gave us the quote, we didn't even talk about how he was going to do the shower. So as far as his profits are concerned, the less labor and material cost he generates now, the better off he will be.
    – Markus A.
    Mar 15, 2014 at 4:09
  • I would take a concrete shower too before backerboard. The problem is that doing this is truly a lost art. I totally don't understand the thinking on the earthquake situation but maybe others can ring in. Concrete cracks easily too. I would think concrete board plus kerdi would be preferred in your area. He thought about profits originally thinking wow if I use concrete and bust this thing out I make big $$$. Well once you start it, there is really no turning back. I am giving you honest advice. This isn't a gray area.
    – DMoore
    Mar 15, 2014 at 4:26
  • @MarkusA. If you're worried about earthquake performance, remember that concrete has all of its strength in compression and almost none in tension. It isn't the concrete that gives strength against movement - it is the reinforcing that is in the concrete. The structural danger is when concrete tries to flex. This is why they put fiberglass meshes in products like Durock. A 4" thick non-reinforced slab has a fraction of the tensile strength of a decent backer board.
    – Comintern
    Mar 15, 2014 at 4:50

5 days of curing time (concrete does not dry, it cures), is not enough. That said, the installation is janky.

  • Actually, I just checked the time-stamps on my photographs again. It's been 12 days already! Should that be enough curing time?
    – Markus A.
    Mar 15, 2014 at 1:11
  • According to the TCA Handbook for Ceramic Tile Installation: the cure period can fluctuate from 14 days to over 60 days, depending on the geographic location, the climatic conditions, and whether the installation is interior or exterior. But really the entire job has to come out, it has multiple flaws.
    – Bryce
    Mar 15, 2014 at 5:33

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.