No thinset under cement board

21 Replies

Just went to inspect some work and noticed they weren't using thinset to lay down the cement board under porcelain tile.

When I asked the contractor, he asked his flooring guy who had been doing tile for 25 years and "never puts thinset under cement board".

Our agreement was to put down cement board and tile.

Nothing was specifically said about thinset. The manufacturer dictates to use thinset between subfloor and cement board.

It is 1/2" durock over 1" nominal thick plank.

For the record, this is my first time really contracting anything. It's been all me and cheap DIY/Craigslist hacks before this, so I am willing to take pointers on how I could have/should have done something different.

Should I have been more specific in work instructions?

Should I demand he redo the job to what I think is the right way?

Should I meet him in the middle and pay a bit more?

Most of the 375sq ft of tile is already down.

What would you do?

I've never used thinset under cement board, for what it's worth.

I'm not sure you really have standing to make him do it the way you think is right...you say yourself you're a DIY guy and he's the pro...tread carefully

Yeah just talked to my friend who does flooring for a living. He said he doesn't put thinset under.

The manufacturers all require it.

I'll probably not make a big deal of this.

I do whatever manufacture recommends on my jobs so I can gain their warranties and I can warranty my work properly. If my guys did that I would have them pull it up because I can't warranty it.

I did not use to put thin set on before I knew about it and haven't had a call back.

I probably would leave it since you have adequate subfloor and either ask is next time or find another tiler. Theres plenty of people out there that do good work that will take direction.

I have never used thinset under the cement board either. The critical issue is when there is a gap between the subfloor and the underlayment. It will bounce and then the grout tends to crack. I always make sure I use the appropriate screws for the installation and make sure it is screwed down REALLY well. Special screws are made for this application and are coated and the threads are course for the best grip.

I have had problems with guys not putting enough screws in and trying to save money and the other is using the thinnest cement board over spongey framing. The grout will crack up right away.

Putting a lot of screws in can negate the need for the thin set. Thin set is probably only essential for thin strips of hardie backer or thin sub flooring. If they are screwed down with the proper screws every spot on the hardie backer, (6"??) it should be fine.

Both durock and hardibacker specify it should be attached to the floor with thinset. You should have addressed this with your contract, because its a commonly skipped step.

You say "It is 1/2" durock over 1" nominal thick plank". Are you referring to 1x8 boards that run across the joists diagonally? Or something similar? I have that in my residence. I've seen it in other houses. You cannot put a cement backer board over that. You need to put a layer or plywood, minimum 5/8 IIRC, over those boards, then mortar the backer to the plywood. Those 1x8 boards won't give a solid foundation and will flex. Over time, that will cause tile to pop loose or crack.

Thinset is absolutely required on all hardiboard applications. As well as recommended by manufacturer. All sub floors have movement cementing the hardiboard will not allow the hardiboard to move seperate from the subfloor, it will strenghten the subfloor as intebded by the manufacturer. I have done a number of tearouts and reinstallations cayse of constant grout cracking and loose tiles entirely because the hardiboard was not installed properly. The fact that self proclaimed (tile experts) decide that they know better than the manufacturer and take it upon themselves do change installation requirements boggles y mind. Not to mention the number of times ive seen people use durarock on the floor instead of hardiboard is incomprehensible. Instructions are there for  reason people. That being said keep doing  you are its nothing but job security for myself.

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The purpose of backer board on a floor is to create a monolithic substrate... one continuous mass that moves as one, rather than  individual sheets that can expand and contract individually. The backer is set with at least 1/8" gaps between sheets to allow enough room for thinset to 'glue' edges while fiberglass tape on joints knits them together. By itself this joint is strong, however it is the thinsetting of the backer board to the subfloor that locks the joints together creating the monolithic substrate.

Something to remember is that just because numerous people do something a given way, doesn't make it right. Ive learned bad habits as well. Not everything a manufacturer specifies is absolutely necessary or even makes sense, but if one is in business, they owe it to their customers to follow manufacturer specifications to ensure that in the event of problems, warranties are in effect.

I ALWAYS thinset my backer board down! I don't understand why a tile setter would bypass this step... its so quick and easy to do. I would hold contractor responsible. It matters not if this step was laid out in the contract. It is reasonable to expect a tradesman will follow industry standard and manufacturer specifications. If we had to spell out every step for every application on a job, the contracts would be as thick as the Obamacare bill! You deserve to have the job done right!

3rd generation tile guy here. Many years of experience.

Typically I will follow manufacturers recommendations but I will not thin-set under cement backer board. If you have ever tried removing thin-set installed backer board then you will know why. You will be taking up the floor down to the floor joists if you ever want to remodel. 

Just screw down and tape joints like manufacturer recommends. Install a quality porcelain tile with a "flex" type of thin-set.

All the problems I've seen have been when a cheap ceramic tile was used on a dimensional lumber sub-floor. Doesn't matter how well it's installed, if you use cheap ceramic over a wood subfloor you will more than likely get cracks in the tile.

@Michael Herr  @Jon Holdman  beat me to it, but I would be less concerned about the lack of thinset that I would be about putting the CBU over 1x planks without at least 5/8" plywood over the planks.

As Jon said the 1x planks will almost certainly deflect more than the tile allow resulting in cracked tiled over time.   CBU does not provide any increased structural support, you need the plywood.

if you do not have the tile guy rip it up, is there a flexible grout you can use?  they are grouting vinyl tile now to make it look like real tile, that type grout would be likely be more flexible than cementous grout.  the grout is what will crack unless the tile is so fragile that it breaks instead of the grout cracking.

@Michael Herr  

Thinset is absolutely required beneath backer-board on a floor. 

Also, backer-board provides no structural strength at all. You can not install it directly over a 1x subfloor; that is way too thin and will deflect.   It does not sound as if you have a quality-minded contractor.  Just because someone has been doing something a certain way for a long time, does not mean it's right. 

You need to take it up and start over. If you don't, you will wish you had. 

Art Allen 

I think that's bad advice. 

If you look at the time stamp on this thread the job is done and the contractor is long gone. 

What's the worst that can happen? My tiles will pop and grout will fail,  which will require ripping it up and started over. So I should go ahead and rip it up and start over, guaranteeing that worst case scenario? 

It was not done right, but it may or may not be a problem. Plenty of experienced people said it will be fine. I wouldn't bet money on my grout not cracking, but I'm definitely going to wait it out and see if it's a problem before I go ripping out and redoing tile.  

I will make sure to include "must follow manufacturers specifications" in my contracts from now on. 

Art Allen 

Edit: Now that I think about it more, your advice is decent advice had I done that in March, just not now that it's done.  

@Michael Herr  

Sorry, I didn't realize I was responding to such an old thread.  A BP rookie mistake!  :)

@Michael Herr   I know it is an old thread but I wouldn't be too worried. I do this for a living, and while I *always* use thinset underneath I've actually seen the majority of the guys don't. Their work isn't failing left and right, either. In fact, a friend of mine having a rental done in So Cal asked this question to me ~2 years ago after the contractor had already installed it. So far, 2 years later, it is still fine. So long as they screw it in as often as recommended (thats what those marks are for), you should be okay. Most grouts have a latex additive anyways which will keep them just flexible enough for tiny movement. If the house settles a bit more, you might get some movement--but thinset under the backerboard wouldn't have fixed that. Just re-grout if need be, and I'm sure you'll be fine. 

Not to resurrect old threads, but this one popped up while I was searching this morning to prove to a guy I've hired recently to help with a backlog of several homes I need to get finished why he MUST use a layer of thinset BAG mortar under the backer board I have him installing on the floor of a bathroom we will be tiling TOGETHER, since he's not a full time tile pro and I not only love using ceramic or porcelain tile for bathrooms (or natural stone for my higher end projects) but I'm also picky as hell about the finished product and from what I've seen, other than the full time tile pros, 90% of installs done by either someone who's hired or DIY are done either poorly or just flat out wrong.

There is a GREAT resource on the net though, honestly as good as Biggerpockets is for a wide variety of real estate related questions, this site "The John Bridge Tile Forum" is for ANY sort of tiling related questions. It is free to join and anyone can go on there and ask anything from a simple question that gets a simple answer, all the way to people who all the time will start a thread titled something like "Mary's first ever master bathroom gut and renewal" where the thread may end up taking 15 pages, as she begins with the planning stages for her DIY project (or maybe she's hiring someone for the physical work) and you can see it progress all the way to when she puts the fuzzy cover on her new toilet seat cover in her brand new, totally redone and retiled bathroom! Their many moderators who are real, full time tile guys/gals will usually sort of assign themselves to a thread like that, where that one person will follow it all the way through, but usually others will chime in as well. Like I said, if you can see the value people get here from BP, you'll see that same incredible wealth of info there for info on ANY aspect of PROPERLY installing tile.

As far as this particular thread goes, glad to hear there haven't been any issues and to be honest, as long as it is properly screwed down AND you have a proper subfloor (something else that I see so many "I've been doing this for years" type guys be clueless about) it should be fine for years, but it won't have the lifespan of a properly installed job with the thinset bed under the backer board.

Three other very common MAJOR errors I've seen time and time again, even from the "years of experience" crowd are the subfloor issue, which I won't go into detail on here, but that truly involves the deflection of the floor, whch involves the size, span and type of material of the joists and then what is used for a subfloor on top of them (and using any natural stone vs ceramic/porcelain requires a MUCH stronger floor) 

Second is the mixing of the thinset mortar. (and if "Handyman Pete who keeps being a harda** about his "years of experience" tiling shows up with a bucket of that pre-mixed mortar for anything other than a backsplash, push him out the door and lock it immediately!) On any bag of thinset will be the part where it says usually to mix it for several minutes, then let it stand for usually 10 min or so, then mix it a second time for maybe 3 to 5 minutes. For whatever reason this is, according to the REAL pros and the manufacturers, it is VERY important to follow those directions, even though if you simply mix it for a few minutes and skip the rest, the mortar seems no different to work with than if you follow the directions correctly, even if it seems like a waste of time. Apparently though, those extra steps are crucial to getting the right results, which should be a tile job you can come back to in 10, 20 or even 50 years and it still looks great! 

Third is properly waterproofing a tub surround or shower area, which considering the hysteria so many people have over mold, which yes can be deadly but I'm sure many here have seen what I've seen where even a tiny bit of of simple mildew or ANY kind of mold, not just the "bad mold" sends buyers running for their lives, seems like a stupid step to skip. Yet, I'd bet half of the Handyman Pete type characters have never heard of Red Guard, Kerdi or any other waterproofing method!

I've seen dozens dozens of tile floors go in over the past year, and notice the installers ONLY use lots of hardieboard screws!

NO thinset!

I use it to help level as well as glue seams, but seems nobody else does. They say the screws does the job better than the glue.  I tend to be skimpy on the screws, though.

On concrete and over commercial tiles, I use no cementboard, just mortar then tiles.

So what about the instructions to put down a vapor barrier under Durock?  USG wants the installer to use their brand of vapor barrier between the subfloor and backer, or tar paper.  What would be the purpose of thinset over the tar paper? It certainly wouldn't create a more solid, less moveable bond, since the vapor barrier rejects any water based substance.  Lots of screws in a 6 to 8 inch grid should stop movement between plywood and backer, unless King Kong visits and uses the bathroom.

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