Cracked floor tile due to improper substrate preparationI know there are some people out there (not my regular readers like you!) that read what I write and think to themselves ‘okay, but I’m sure that won’t happen with my installation’. So periodically I’m gonna post things like this that show exactly what happens when things aren’t built correctly. And yes, it will happen to yours, too, if the proper steps aren’t taken. If you care to see more train wrecks you can check out my ‘flawed‘ page wherein I post photos of absolutely horrible tile installations which I’ve torn out and replaced.

See that crack in the tile right there? (The line down the center is not a grout line – it’s a crack. You can click on it for a larger version) That bathroom floor is less than eight months old. It was installed with hardibacker over the subfloor and thinset. At least that part is correct, but that was about it. There was no thinset beneath the hardi and the seams between the sheets were not taped and thinsetted. To a lesser extent the correct screws were not used in the hardi – they committed the cardinal sin of using drywall screws in the backerboard. Yeah. Wrong.

So, back to the crack. (Never thought I’d ever type that!) As soon as I walked in and saw it I knew exactly what was wrong and I knew why. The crack was in an absolutely straight line – a dead giveaway that the crack is likely over a seam in the backerboard which wasn’t taped. If you read my post about how to correctly install flooring backerboards you will see that there needs to be thinset below the boards, the seams need to be taped, and the proper screws need to be used. None of which was done.

And here’s what was beneath it: Improperly prepared substrate beneath cracked floor tile

If you click on that photo (I dare you!) you can see the crack follows the seam of the backerboard exactly. Without the tape on the seams the individual boards may move in different directions and, without the support beneath from thinset, they will move independently and eventually crack your tile or, more commonly, your grout lines first.

When you tape and thinset your backerboard seams it will lock the two separate sheets together and any movements in the substrate (seasonal micro-movements, completely normal) will all move as one and in the same direction. This won’t cause any stress on your tile.

I simply pulled up all those cracked pieces and chipped the old thinset out of there, installed proper screws along the seam, taped and mudded the seam (when I say ‘mudded’ the seams I mean thinset) and reset new tiles and grouted it up. Once that grout cures it will lighten and it will look brand new.

Repaired floor tileSo all these little things like ‘tape and mud your backerboard seams’ that I throw out there may seem like it’s just overkill or taking extra precautions which aren’t really necessary – well, they are necessary. And this is why. This will also happen on a shower wall if your seams are not taped and mudded. If the boards move differently it causes uneven stress on your tile – it needs to release somewhere. Ninety seconds worth of work to tape the seam to begin with would have prevented this – just do it! (damnit)

If preventing cracked tile isn’t enough motivation for you maybe this will: all of my regular readers know what happens if your tile or grout crack – your dog will burst into flames! So if you don’t do it for your tile, do it for your pooch. Not only are flaming dogs bad for, well, the dogs, but they tend to run around and catch other stuff on fire too! You don’t want that, do you?


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  • Nadine

    So, I have this exact problem. My subway tile that was installed in the fall of 2016, cracked vertically along the backerboard seams the very next spring. My tile contractor came back and replaced them all free of charge and insists it must be a structural issue. Since replacing the tiles I have a new vertical crack through at least 10 tiles and a horizontal crack along another seam. Here’s the thing…I took pictures throughout the project and all the seams are taped with mesh tape and mudded! I don’t know what kind of screws he used but they’re spaced well apart. Could it be more of a structural issue or are the tiles garbage that they won’t take the stress?

    • Roger

      Hi Nadine,

      It isn’t the tiles, it’s always prep. Tiles have nothing to do with it.

      It COULD be a structural issue, but I’ve never seen that. Doesn’t mean it can’t happen, just means I haven’t seen it.

      Did he use alkali-resistant mesh tape? Regular drywall mesh tape is not interwoven and the glue can be broken down by the alkali in cement products (like thinset). Alkali-resistant mesh tape is not broken down by alkali and it interwoven so it doesn’t come apart. Regular drywall mesh tape is not interwoven, once the glue holding it together is compromise it will fall apart, negating the use of it to begin with. Regular drywall mesh tape would be my guess.

  • Jordan

    Hi. I had three ceramic floor tiles that were moving/disbonded at the grout seams. I pulled them up and scraped the mortar off the backerboard. THEN! It was revealed that it was off course a backerboard seam! One backerboard side of the seam is about 1/4″ higher than the other side. Two questions… 1) should I cut the backerboard seam out and install new flush backerboard (creating more seams, which I would tape)… or 2) just put a bunch more screws in the high side and thinset/mortar over the seam to level? Anything helps! I wish I had a picture.

    • Roger

      Hi Jordan,

      You can do either one, they both SHOULD eliminate the issue. However, the fact that it was installed like that there could very well mean you’ll have issues elsewhere down the road. It may be higher because that seam may be installed directly over the subfloor seam beneath it. 1/4″ is A LOT.

      • Jordan

        Thanks Roger. I actually went ahead and screwed in the high side backerboard which brought the seam edge almost flush, cleaned up the removed tile and mortar, and put it all back. It turned out great, and now I’m noticing more spots throughout the house that need the same treatment! ehh

  • Regina

    Hi! I just had an upstairs full bathroom tiled. I am worried because everything I’ve read says that you need hardibacker over second story wood subfloor. My installer laid the tiles directly over the wood subfloor. When I questioned him about it and mentioned potential cracks, He said that hardibacker wasn’t necessary since there was no moisture source from below, or between the floor and the tiles. He also mentioned that it would have raised the height of my floor. To me, it sounded like a story from someone who didn’t want to do it the right way. I also worry about mold building and that if we ever have to replace tiles, it will destroy my subfloor. Please tell me that he was right and I am wrong, that the hardibacker is not necessarily needed. thank you.

    • Regina

      One more note: He also sprayed undiluted bleach from a spray bottle directly on my wood subfloor to eliminate urine smell in another room (Not a bathroom.) Is this harmful to the strength/integrity of the wood foundation? I thought that bleach would weaken wood, but he told me that he “does this all the time.” I genuinely hope that I’m wrong on both points and worrying for nothing. Thanks for your input! Great article.

      • Roger

        Unless he soaked it with the bleach it shouldn’t be an issue.

    • Roger

      Hi Regina,

      Sorry, he is wrong. The hardi has absolutely nothing to do with a moisture ‘barrier’. You need a proper bonding substrate for tile, the plywood floor is not it. Also, when exposed to water the plywood will soak in water and swell, any proper tile substrate will not. This is not a guarantee of failure, but it certainly increases the chances of failure greatly.

  • Ben

    Hi, Im currently skimming seams on my hardibackernoard with thinset however on my last two seams I had to open another bag of product and soon realized that it was behaving differently. I looked closer at the back and found it is actually mud bed mortar. To I need to cut these seams out and scrape the mortar off or will it be fine underneath my final thinset coat used to adhere the tiles? Thanks!

    • Roger

      Hi Ben,

      You need to remove it. deck mud isn’t made to actually bond to anything, it isn’t the same at thinset or mortar. On the upside it will be easy to scrape off.

  • SEAN

    I want to re-use my current shower door. if i go with one backerboard then i’ll have .5 inch gap between the door and the frame, it seems like the only way to reuse the door is either to double up backerboard or have plywood then backerboard then tile? Which method is best? Thanks

    • Roger

      Hi Sean,

      I would double up the backerboard. You really don’t want plywood behind your wall, there’s way too much movement in plywood for vertical installations.

      • SEAN

        Thanks for your reply. also, do you recommend to add any adhesive between the backer board? Thanks

        • Roger

          Nope, just screwing them to the studs will be fine.

  • Rick

    We installed 1/2″ wonderboard over 3/4″ plywood using Versabond. We also taped and mudded the seams using the Versabond. The problem is, we didn’t put in the screws while the thinset was wet since it standed not to walk on it for 24 hours. Is it too late to screw the wonderboard down and does it have to come up and be redone? Or, can we still screw down the wonderboard. It has been down about 2 weeks now.

    • Rick

      Update…I attempted to screw down the wonderboard and it seemed to work well. It is still solid and nothing cracked.

    • Roger

      Hi Rick,

      You can screw it down whenever you want. It can be down for 6 months if it has to be. Not walking over it only means for a day until the thinset beneath it is cured so it doesn’t shift around before you screw it down.

  • Dana

    Hi floor elf,

    We bought a new home and this exact same thing has happened to our master bathroom twice. They have come back and did a complete demo and redid the floors… 1 month later it looks worse than it did before… cracks all over the place running in straight lines just like this. We just sent this article to our developer.. thank you so much! We want him to know that we know whats up, and stop halfway fixing the problem. My only question is should they be staggering the backer board so it doesn’t line up in straight line piece after piece? We want to make sure we know exactly what needs to be done so we can micro manage this the third time around. So much for trusting subs to know how to do a professional job. We are so frustrated because this has completely displaced us from our bedroom and make our house a dusty mess with all this construction over and over in a brand new home only 1 year old!!! :(

    • Roger

      Hi Dana,

      Yes, they should absolutely be staggered.

  • Goldy

    Hi. Installer is putting hardibacker 1/4 over wobbly wood floors. 100 year old interlocking 2″ wood strips. Ceramic Tile is 24×48. I am arguing he needs wonderboard but he is refusing because of the extra height. This is for an entrance and I’m worried the tiles will crack. He jumps and it shakes with the hardibacker in place. The floor squeaks with hardibacker on.
    What do you think?

    • Roger

      Hi Goldy,

      I think you need a new installer, and do more research. The size or type of backerboard has nothing to do with the deflection ratio (how sturdy your floor is) of your floor. It is a structural issue, at the very least you need a layer of plywood over those boards, then the tile substrate. Even at that it may not be enough, you need someone who knows what all this means, it’s apparent that your current contractor does not.

  • Pam

    Hi Roger,
    For a non-wet location like a backsplash, I’ve seen that you can install a small tile directly onto drywall. Do I need to tape and mud those seams also? If so, is it for the same reason mentioned in this article, to prevent cracking? Thanks!

    • Pam

      And if tape and mud are needed on the drywall, do I use drywall tape and mud, or mesh tape and thinset? Thanks again!

      • Roger

        Mesh tape and thinset.

    • Roger

      Hi Pam,

      Yes you can, and yes, the same reason.

  • Glen Barras

    Can nylon webbing be used over the seam in hardie board on the floor?

    • Roger

      Hi Glenn,

      What do you mean by nylon webbing? You want something thin enough so that it doesn’t interfere with your tile. It should be no thicker than fiberglass drywall mesh.

  • Bill

    Hi –
    I’m redoing a kitchen floor and I just noticed one of the hardie boards dips slightly at the seam when it’s stepped on. It happens about half way down the horizontal seam. There is not movement on any other seams, or any other boards. I used tape and mud on the seam, but it seems like the seam has “cracked” between the two boards. Unfortunately, this board is located in the middle of the kitchen, so it’s not easily accessible.

    How can I go about fixing this board? Does this require replacing that section of hardie board, or can thinset be placed within the seam and on top of the board to compensate for the movement?

    • Roger

      Hi Bill,

      If you can force thinset into that seam so it gets below the edges of the backer that will shore it up enough to solve the problem, provided the problem is space between the backer and subfloor and not a weak spot in the subfloor itself.

  • Bridget

    We are tearing our tile up in the kitchen. It held up well for about 10 years. Then, one by one, tiles began to crack. Last count was over 30 tiles before I started ripping them up. We are not going with tile again. Probably laminate. My question to you is, do we have to rip up the backer board, or can we somehow repair it? Could we lay another underlay on top for more soundproofing as well? I will try to post a picture. Thanks!

    • Bridget

      You can see everything better on this picture.

    • Roger

      Hi Bridget,

      The problem wasn’t the tile, it was the installation. Notice how there is very little thinset on the backer? It’s just a crappy install. You’ll need to scrape off the thinset that is left, otherwise it will eventually crack and you’ll hear grinding beneath your laminate. But yes, you can install over that.

      • Bridget

        THanks! Greatly appreciated!

      • Bridget

        But do you see the big crack. Can we still lay our new flooring over that or do we need to replace or repair it first? We are installing EVP

  • J walls

    Regarding a Kerdi waterproofing system over Hardi in a shower—
    My Hardi is in, but not exactly right. Specifically:
    1) The inside corners in some places have up to 1/4″ gap (instead of the recommended 1/16 to 1/8). Can I just silicone that and proceed with the Kerdi, or should I replace the Hardi?
    2) Some of the in-plane gaps between Hardi are up to 1/4″. Can I just fill that with thinset as the Kerdi is installed or should I replace the Hardi?

    Thanks for your guidance.

    • Roger

      Hi j,

      1. Just silicone and kerdi.
      2. Yes, fill with thinset and kerdi over it.

  • Mary

    I have a question. I applied thin set, I applied backer board nailed with proper nails staggered seems and taped. However, I started to tile and notice the area at the taped seems have a little bump, or swelled or something. It is not flat!!!!
    Crap, what can I do?

    • Roger

      Hi Mary,

      The little bump shouldn’t be more than 1/16″. If it is you can scrape it down to get it flat, or sand it. If it is that small then the amount of thinset from your trowel should more than compensate for going over 1/16″. If it does not you need to use a larger trowel.

  • Brian

    Hi Roger,

    I’m going to install porcelain 12″ x 12″ tile in the kitchen. I just put down 1 and a 1/2 sheets so far of hardibacker board over Mapei Porcelain tile morter Polymer enriched. The problem is that the Rock-On screws (for Durock and other cement backerboards) are not countersinking. The screw head is slightly raised or is this normal? I’m using a large wall powered hammer drill on regular driver setting (no hammer) and stripped one of them trying to get it down. I managed to get them down pretty close. I’m thinking of buying an Impact driver would this be better or maybe the specific Hardibacker screws you show above?

    As for the screws already installed If an Impact driver is a solution is it ok to countersink them in the dried thinset or is it too late? Or to float the tiles over the screw heads can I use a larger notched trowel? I have a 10mm x 10mm (13/32″) and a 1/2″ x 1/2″. Or is there another solution?
    Sorry for the long winded question.
    Thanks Brian

    • Brian

      Here is one of them:

    • Roger

      Hi Brian,

      The hardibacker screws are the ones you want. And an impact drill works best as well. You can use a larger trowel over the screws, but you really do want to get them countersunk. Yes, you can do it through cured thinset, no issues.

      • Brian

        Thank you Roger!

  • Jim

    Hi Roger,
    I am replacing tile in a shower. When I tore down the old tile and cement board I found a vapor barrier tacked to furring strips which are attached to the outside walls, the house is block construction.
    The vapor barrier is not intact where it meets the cement at the floor. I assume it needs to be replaced. Does it need to be wrapped over the concrete at the bottom.
    See photo
    Also, when the floor tiles were removed some of the mud bed came up, maybe about 3/4″ in some areas, 80% of the floor is fine. Can this be patched with a product like Mapie 4:1 mud bed mix or can quickrete cement patch be used?
    Thank you for this great information source

    • Roger

      Hi Jim,

      That depends. From your photo I cannot tell what, if any, type of waterproofing you have for your shower floor. The vapor barrier needs to overlap the shower floor waterproofing. You can use the mud bed mix for a patch, but you’ll need to bond it to the old with thinset. Cement patch can be used for an area that size.

  • Steve

    Good point. If you thinset down and nail every 8 inches the fiber mesh is unnecessary and adds no strength. I find it hard to believe fiber mesh cured any problem with the floor

  • Antoine Brathwaite

    Hi! I’m doing a remodel on my bathroom and installing backerboard around my tub. The issue that I’m having is that the drywall is 5/8 but I can’t find 5/8 backerboard. The closest I have found is 1/2. What are my options ?

    • Roger

      Hi Antoine,
      You’ll need to shim out your studs by 1/8″ behind the backerboards.

  • maria witt

    Tiling a shower and our niche box is about 1/4 inch thicker than our backer board. We are planning 4×16″ subway tile hung vertically, how do we correct for this hump?

    • Roger

      Hi Maria,

      Most premade niches are 1/2″, is yours not? Or did you actually use 1/4″ backer in your shower, because if you did you need to remove that ad replace it with 1/2″ backer.

  • FloorOgre

    What about using galvanized staples (a lot of them) in your floor hardiboard? I’ve torn up a few and it’s not easy to do. The staples stay behind when the underlayment comes up. I’m talking 18 ga. Shot it with a pneumatic stapler. Really fast.

    • Roger

      I wouldn’t, but it’s your floor – do what you want. :D

  • Justun

    What to do with a real high corner with backer board

    • Roger

      You mean a corner that sticks out more than the surrounding substrate? You need to figure out why it does that. Either remove some of the stud which is causing it to do that or shim out the surrounding substrate to meet up flush with the ‘high’ backer.

  • Michael Scott

    Hello Roger,

    Thank you so much for having the formost resource on the web for all us homeowners that are trying to diy tile with more energy than knowledge! I tilt my beer to you and will keep an eye out to make sure my dog does not catch fire.

    I have a (wall) corner of hardibacker 500 that has cracked slightly around both the original screw and the second one I put in after adding some blocking (by sliding underneath the board) to the stud to try to gain a spot for another screw. The rest of the board is solid, and this corner is going to be behind a vanity where it should not be bumped.

    The board is horizontally mounted, and this corner is an inside corner.

    Can I use thinset and tape to try to strengthen this board, or do I need to take the whole thing down?

    I tried to upload a picture, not sure if it worked.

    Thank you so much,


    • Roger

      Hi Mike,

      Yes, just thinset and tape and it’ll be just fine.

      • Michael Scott

        Awesome! Thank you sir. I may have also screwed up another aspect of this horizontal board and the one below it. I noticed a slight lip between the upper board and the lower board.

        Closer examination revealed a slight hump on the middle stud by the upper edge of the lower board. This is indicated by the gap between the level and the upper board in the picture. Is this something I could overcome when I thinset/tape the seam by feathering out the thinset? I was planning to use smaller tile or mesh mosaic on this wall anyway, If that helps any. I can try to get an exact measurement, but I know it was so small, I was having difficulty reading it due to the metal end of the tape measure. I didn’t know if you had any general rules for out of plumb backer board joints vs tile size?

        Thank you again,

        • Roger

          The smaller the tile the more difficult it is to overcome. With smaller mosaics you do want to tape and mud that, then feather the thinset out so it at least gives the appearance of being flat.

          • Michael Scott

            Thanks Roger, I really appreciate it. Might go with something larger then since everything is still up in the air for design. Will be purchasing your pre-made shower pan / topical membrane guide once my cast iron shower pan arrives (working on opposite side of room now).

  • Jeff

    How many times am I required to mud the tape joints

    • Roger

      Hi Jeff,

      Just once.

  • Carolyn

    How important is the 1/8″ spacing between boards in a tub surround?

    Also, can you place one board directly over another horizontally i. e. two boards 3’0″ x 5’0″ makes up the entire surface to be tiled, or do you have to cut boards and stagger to allow for vertical movement?

    • Roger

      Hi Carolyn,

      It’s fairly important. What’s the issue?

      Yes, you can place one directly over the other, that’s why they’re sized like that. You’re only required to stagger when a vertical seam is necessary.

  • Annie

    Hi! My husband I moved into our new house in summer. We knew that grout of our bathroom floor was getting loose in parts. He finally got around to re-grout this week and discovered that whoever tiled the floor did it right over the subfloor. (He could see wood when he removed the old grout.) We were terrified and started searching the net frantically to see how bad our situation is and came across your page. So my question is, should we rip up the floor and start all over again? Or is there any other way?

    • Roger

      Hi Annie,

      Yes, you should rip it out and properly install a floor. You can regrout it, but you’ll end up having the same issue shortly down the road. Wood always expands and contracts A LOT. Without a proper substrate between the wood and tile you’ll always have issues.